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    // Copyright 2013-2015 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Utilities for formatting and printing strings.

    #![stable(feature = "rust1", since = "1.0.0")]

    use cell::{UnsafeCell, Cell, RefCell, Ref, RefMut};
    use marker::PhantomData;
    use mem;
    use num::flt2dec;
    use ops::Deref;
    use result;
    use slice;
    use str;

    mod float;
    mod num;
    mod builders;

    #[stable(feature = "fmt_flags_align", since = "1.28.0")]
    /// Possible alignments returned by `Formatter::align`
    #[derive(Debug)]
    pub enum Alignment {
        #[stable(feature = "fmt_flags_align", since = "1.28.0")]
        /// Indication that contents should be left-aligned.
        Left,
        #[stable(feature = "fmt_flags_align", since = "1.28.0")]
        /// Indication that contents should be right-aligned.
        Right,
        #[stable(feature = "fmt_flags_align", since = "1.28.0")]
        /// Indication that contents should be center-aligned.
        Center,
    }

    #[stable(feature = "debug_builders", since = "1.2.0")]
    pub use self::builders::{DebugStruct, DebugTuple, DebugSet, DebugList, DebugMap};

    #[unstable(feature = "fmt_internals", reason = "internal to format_args!",
               issue = "0")]
    #[doc(hidden)]
    pub mod rt {
        pub mod v1;
    }

    /// The type returned by formatter methods.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::fmt;
    ///
    /// #[derive(Debug)]
    /// struct Triangle {
    ///     a: f32,
    ///     b: f32,
    ///     c: f32
    /// }
    ///
    /// impl fmt::Display for Triangle {
    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    ///         write!(f, "({}, {}, {})", self.a, self.b, self.c)
    ///     }
    /// }
    ///
    /// let pythagorean_triple = Triangle { a: 3.0, b: 4.0, c: 5.0 };
    ///
    /// println!("{}", pythagorean_triple);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub type Result = result::Result<(), Error>;

    /// The error type which is returned from formatting a message into a stream.
    ///
    /// This type does not support transmission of an error other than that an error
    /// occurred. Any extra information must be arranged to be transmitted through
    /// some other means.
    ///
    /// An important thing to remember is that the type `fmt::Error` should not be
    /// confused with [`std::io::Error`] or [`std::error::Error`], which you may also
    /// have in scope.
    ///
    /// [`std::io::Error`]: ../../std/io/struct.Error.html
    /// [`std::error::Error`]: ../../std/error/trait.Error.html
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::fmt::{self, write};
    ///
    /// let mut output = String::new();
    /// match write(&mut output, format_args!("Hello {}!", "world")) {
    ///     Err(fmt::Error) => panic!("An error occurred"),
    ///     _ => (),
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Copy, Clone, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
    pub struct Error;

    /// A collection of methods that are required to format a message into a stream.
    ///
    /// This trait is the type which this modules requires when formatting
    /// information. This is similar to the standard library's [`io::Write`] trait,
    /// but it is only intended for use in libcore.
    ///
    /// This trait should generally not be implemented by consumers of the standard
    /// library. The [`write!`] macro accepts an instance of [`io::Write`], and the
    /// [`io::Write`] trait is favored over implementing this trait.
    ///
    /// [`write!`]: ../../std/macro.write.html
    /// [`io::Write`]: ../../std/io/trait.Write.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Write {
        /// Writes a slice of bytes into this writer, returning whether the write
        /// succeeded.
        ///
        /// This method can only succeed if the entire byte slice was successfully
        /// written, and this method will not return until all data has been
        /// written or an error occurs.
        ///
        /// # Errors
        ///
        /// This function will return an instance of [`Error`] on error.
        ///
        /// [`Error`]: struct.Error.html
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fmt::{Error, Write};
        ///
        /// fn writer<W: Write>(f: &mut W, s: &str) -> Result<(), Error> {
        ///     f.write_str(s)
        /// }
        ///
        /// let mut buf = String::new();
        /// writer(&mut buf, "hola").unwrap();
        /// assert_eq!(&buf, "hola");
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn write_str(&mut self, s: &str) -> Result;

        /// Writes a [`char`] into this writer, returning whether the write succeeded.
        ///
        /// A single [`char`] may be encoded as more than one byte.
        /// This method can only succeed if the entire byte sequence was successfully
        /// written, and this method will not return until all data has been
        /// written or an error occurs.
        ///
        /// # Errors
        ///
        /// This function will return an instance of [`Error`] on error.
        ///
        /// [`char`]: ../../std/primitive.char.html
        /// [`Error`]: struct.Error.html
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fmt::{Error, Write};
        ///
        /// fn writer<W: Write>(f: &mut W, c: char) -> Result<(), Error> {
        ///     f.write_char(c)
        /// }
        ///
        /// let mut buf = String::new();
        /// writer(&mut buf, 'a').unwrap();
        /// writer(&mut buf, 'b').unwrap();
        /// assert_eq!(&buf, "ab");
        /// ```
        #[stable(feature = "fmt_write_char", since = "1.1.0")]
        fn write_char(&mut self, c: char) -> Result {
            self.write_str(c.encode_utf8(&mut [0; 4]))
        }

        /// Glue for usage of the [`write!`] macro with implementors of this trait.
        ///
        /// This method should generally not be invoked manually, but rather through
        /// the [`write!`] macro itself.
        ///
        /// [`write!`]: ../../std/macro.write.html
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fmt::{Error, Write};
        ///
        /// fn writer<W: Write>(f: &mut W, s: &str) -> Result<(), Error> {
        ///     f.write_fmt(format_args!("{}", s))
        /// }
        ///
        /// let mut buf = String::new();
        /// writer(&mut buf, "world").unwrap();
        /// assert_eq!(&buf, "world");
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn write_fmt(&mut self, args: Arguments) -> Result {
            // This Adapter is needed to allow `self` (of type `&mut
            // Self`) to be cast to a Write (below) without
            // requiring a `Sized` bound.
            struct Adapter<'a,T: ?Sized +'a>(&'a mut T);

            impl<'a, T: ?Sized> Write for Adapter<'a, T>
                where T: Write
            {
                fn write_str(&mut self, s: &str) -> Result {
                    self.0.write_str(s)
                }

                fn write_char(&mut self, c: char) -> Result {
                    self.0.write_char(c)
                }

                fn write_fmt(&mut self, args: Arguments) -> Result {
                    self.0.write_fmt(args)
                }
            }

            write(&mut Adapter(self), args)
        }
    }

    #[stable(feature = "fmt_write_blanket_impl", since = "1.4.0")]
    impl<'a, W: Write + ?Sized> Write for &'a mut W {
        fn write_str(&mut self, s: &str) -> Result {
            (**self).write_str(s)
        }

        fn write_char(&mut self, c: char) -> Result {
            (**self).write_char(c)
        }

        fn write_fmt(&mut self, args: Arguments) -> Result {
            (**self).write_fmt(args)
        }
    }

    /// A struct to represent both where to emit formatting strings to and how they
    /// should be formatted. A mutable version of this is passed to all formatting
    /// traits.
    #[allow(missing_debug_implementations)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Formatter<'a> {
        flags: u32,
        fill: char,
        align: rt::v1::Alignment,
        width: Option<usize>,
        precision: Option<usize>,

        buf: &'a mut (dyn Write+'a),
        curarg: slice::Iter<'a, ArgumentV1<'a>>,
        args: &'a [ArgumentV1<'a>],
    }

    // NB. Argument is essentially an optimized partially applied formatting function,
    // equivalent to `exists T.(&T, fn(&T, &mut Formatter) -> Result`.

    struct Void {
        _priv: (),
        /// Erases all oibits, because `Void` erases the type of the object that
        /// will be used to produce formatted output. Since we do not know what
        /// oibits the real types have (and they can have any or none), we need to
        /// take the most conservative approach and forbid all oibits.
        ///
        /// It was added after #45197 showed that one could share a `!Sync`
        /// object across threads by passing it into `format_args!`.
        _oibit_remover: PhantomData<*mut dyn Fn()>,
    }

    /// This struct represents the generic "argument" which is taken by the Xprintf
    /// family of functions. It contains a function to format the given value. At
    /// compile time it is ensured that the function and the value have the correct
    /// types, and then this struct is used to canonicalize arguments to one type.
    #[derive(Copy)]
    #[allow(missing_debug_implementations)]
    #[unstable(feature = "fmt_internals", reason = "internal to format_args!",
               issue = "0")]
    #[doc(hidden)]
    pub struct ArgumentV1<'a> {
        value: &'a Void,
        formatter: fn(&Void, &mut Formatter) -> Result,
    }

    #[unstable(feature = "fmt_internals", reason = "internal to format_args!",
               issue = "0")]
    impl<'a> Clone for ArgumentV1<'a> {
        fn clone(&self) -> ArgumentV1<'a> {
            *self
        }
    }

    impl<'a> ArgumentV1<'a> {
        #[inline(never)]
        fn show_usize(x: &usize, f: &mut Formatter) -> Result {
            Display::fmt(x, f)
        }

        #[doc(hidden)]
        #[unstable(feature = "fmt_internals", reason = "internal to format_args!",
                   issue = "0")]
        pub fn new<'b, T>(x: &'b T,
                          f: fn(&T, &mut Formatter) -> Result) -> ArgumentV1<'b> {
            unsafe {
                ArgumentV1 {
                    formatter: mem::transmute(f),
                    value: mem::transmute(x)
                }
            }
        }

        #[doc(hidden)]
        #[unstable(feature = "fmt_internals", reason = "internal to format_args!",
                   issue = "0")]
        pub fn from_usize(x: &usize) -> ArgumentV1 {
            ArgumentV1::new(x, ArgumentV1::show_usize)
        }

        fn as_usize(&self) -> Option<usize> {
            if self.formatter as usize == ArgumentV1::show_usize as usize {
                Some(unsafe { *(self.value as *const _ as *const usize) })
            } else {
                None
            }
        }
    }

    // flags available in the v1 format of format_args
    #[derive(Copy, Clone)]
    enum FlagV1 { SignPlus, SignMinus, Alternate, SignAwareZeroPad, DebugLowerHex, DebugUpperHex }

    impl<'a> Arguments<'a> {
        /// When using the format_args!() macro, this function is used to generate the
        /// Arguments structure.
        #[doc(hidden)] #[inline]
        #[unstable(feature = "fmt_internals", reason = "internal to format_args!",
                   issue = "0")]
        pub fn new_v1(pieces: &'a [&'a str],
                      args: &'a [ArgumentV1<'a>]) -> Arguments<'a> {
            Arguments {
                pieces,
                fmt: None,
                args,
            }
        }

        /// This function is used to specify nonstandard formatting parameters.
        /// The `pieces` array must be at least as long as `fmt` to construct
        /// a valid Arguments structure. Also, any `Count` within `fmt` that is
        /// `CountIsParam` or `CountIsNextParam` has to point to an argument
        /// created with `argumentusize`. However, failing to do so doesn't cause
        /// unsafety, but will ignore invalid .
        #[doc(hidden)] #[inline]
        #[unstable(feature = "fmt_internals", reason = "internal to format_args!",
                   issue = "0")]
        pub fn new_v1_formatted(pieces: &'a [&'a str],
                                args: &'a [ArgumentV1<'a>],
                                fmt: &'a [rt::v1::Argument]) -> Arguments<'a> {
            Arguments {
                pieces,
                fmt: Some(fmt),
                args,
            }
        }

        /// Estimates the length of the formatted text.
        ///
        /// This is intended to be used for setting initial `String` capacity
        /// when using `format!`. Note: this is neither the lower nor upper bound.
        #[doc(hidden)] #[inline]
        #[unstable(feature = "fmt_internals", reason = "internal to format_args!",
                   issue = "0")]
        pub fn estimated_capacity(&self) -> usize {
            let pieces_length: usize = self.pieces.iter()
                .map(|x| x.len()).sum();

            if self.args.is_empty() {
                pieces_length
            } else if self.pieces[0] == "" && pieces_length < 16 {
                // If the format string starts with an argument,
                // don't preallocate anything, unless length
                // of pieces is significant.
                0
            } else {
                // There are some arguments, so any additional push
                // will reallocate the string. To avoid that,
                // we're "pre-doubling" the capacity here.
                pieces_length.checked_mul(2).unwrap_or(0)
            }
        }
    }

    /// This structure represents a safely precompiled version of a format string
    /// and its arguments. This cannot be generated at runtime because it cannot
    /// safely be done, so no constructors are given and the fields are private
    /// to prevent modification.
    ///
    /// The [`format_args!`] macro will safely create an instance of this structure.
    /// The macro validates the format string at compile-time so usage of the
    /// [`write`] and [`format`] functions can be safely performed.
    ///
    /// You can use the `Arguments<'a>` that [`format_args!`] returns in `Debug`
    /// and `Display` contexts as seen below. The example also shows that `Debug`
    /// and `Display` format to the same thing: the interpolated format string
    /// in `format_args!`.
    ///
    /// ```rust
    /// let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
    /// let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
    /// assert_eq!("1 foo 2", display);
    /// assert_eq!(display, debug);
    /// ```
    ///
    /// [`format_args!`]: ../../std/macro.format_args.html
    /// [`format`]: ../../std/fmt/fn.format.html
    /// [`write`]: ../../std/fmt/fn.write.html
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Copy, Clone)]
    pub struct Arguments<'a> {
        // Format string pieces to print.
        pieces: &'a [&'a str],

        // Placeholder specs, or `None` if all specs are default (as in "{}{}").
        fmt: Option<&'a [rt::v1::Argument]>,

        // Dynamic arguments for interpolation, to be interleaved with string
        // pieces. (Every argument is preceded by a string piece.)
        args: &'a [ArgumentV1<'a>],
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a> Debug for Arguments<'a> {
        fn fmt(&self, fmt: &mut Formatter) -> Result {
            Display::fmt(self, fmt)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a> Display for Arguments<'a> {
        fn fmt(&self, fmt: &mut Formatter) -> Result {
            write(fmt.buf, *self)
        }
    }

    /// `?` formatting.
    ///
    /// `Debug` should format the output in a programmer-facing, debugging context.
    ///
    /// Generally speaking, you should just `derive` a `Debug` implementation.
    ///
    /// When used with the alternate format specifier `#?`, the output is pretty-printed.
    ///
    /// For more information on formatters, see [the module-level documentation][module].
    ///
    /// [module]: ../../std/fmt/index.html
    ///
    /// This trait can be used with `#[derive]` if all fields implement `Debug`. When
    /// `derive`d for structs, it will use the name of the `struct`, then `{`, then a
    /// comma-separated list of each field's name and `Debug` value, then `}`. For
    /// `enum`s, it will use the name of the variant and, if applicable, `(`, then the
    /// `Debug` values of the fields, then `)`.
    ///
    /// # Examples
    ///
    /// Deriving an implementation:
    ///
    /// ```
    /// #[derive(Debug)]
    /// struct Point {
    ///     x: i32,
    ///     y: i32,
    /// }
    ///
    /// let origin = Point { x: 0, y: 0 };
    ///
    /// println!("The origin is: {:?}", origin);
    /// ```
    ///
    /// Manually implementing:
    ///
    /// ```
    /// use std::fmt;
    ///
    /// struct Point {
    ///     x: i32,
    ///     y: i32,
    /// }
    ///
    /// impl fmt::Debug for Point {
    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    ///         write!(f, "Point {{ x: {}, y: {} }}", self.x, self.y)
    ///     }
    /// }
    ///
    /// let origin = Point { x: 0, y: 0 };
    ///
    /// println!("The origin is: {:?}", origin);
    /// ```
    ///
    /// This outputs:
    ///
    /// ```text
    /// The origin is: Point { x: 0, y: 0 }
    /// ```
    ///
    /// There are a number of `debug_*` methods on [`Formatter`] to help you with manual
    /// implementations, such as [`debug_struct`][debug_struct].
    ///
    /// `Debug` implementations using either `derive` or the debug builder API
    /// on [`Formatter`] support pretty printing using the alternate flag: `{:#?}`.
    ///
    /// [debug_struct]: ../../std/fmt/struct.Formatter.html#method.debug_struct
    /// [`Formatter`]: ../../std/fmt/struct.Formatter.html
    ///
    /// Pretty printing with `#?`:
    ///
    /// ```
    /// #[derive(Debug)]
    /// struct Point {
    ///     x: i32,
    ///     y: i32,
    /// }
    ///
    /// let origin = Point { x: 0, y: 0 };
    ///
    /// println!("The origin is: {:#?}", origin);
    /// ```
    ///
    /// This outputs:
    ///
    /// ```text
    /// The origin is: Point {
    ///     x: 0,
    ///     y: 0
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_on_unimplemented(
        on(crate_local, label="`{Self}` cannot be formatted using `{{:?}}`",
                        note="add `#[derive(Debug)]` or manually implement `{Debug}`"),
        message="`{Self}` doesn't implement `{Debug}`",
        label="`{Self}` cannot be formatted using `{{:?}}` because it doesn't implement `{Debug}`",
    )]
    #[doc(alias = "{:?}")]
    #[lang = "debug_trait"]
    pub trait Debug {
        /// Formats the value using the given formatter.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fmt;
        ///
        /// struct Position {
        ///     longitude: f32,
        ///     latitude: f32,
        /// }
        ///
        /// impl fmt::Debug for Position {
        ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        ///         write!(f, "({:?}, {:?})", self.longitude, self.latitude)
        ///     }
        /// }
        ///
        /// assert_eq!("(1.987, 2.983)".to_owned(),
        ///            format!("{:?}", Position { longitude: 1.987, latitude: 2.983, }));
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn fmt(&self, f: &mut Formatter) -> Result;
    }

    /// Format trait for an empty format, `{}`.
    ///
    /// `Display` is similar to [`Debug`][debug], but `Display` is for user-facing
    /// output, and so cannot be derived.
    ///
    /// [debug]: trait.Debug.html
    ///
    /// For more information on formatters, see [the module-level documentation][module].
    ///
    /// [module]: ../../std/fmt/index.html
    ///
    /// # Examples
    ///
    /// Implementing `Display` on a type:
    ///
    /// ```
    /// use std::fmt;
    ///
    /// struct Point {
    ///     x: i32,
    ///     y: i32,
    /// }
    ///
    /// impl fmt::Display for Point {
    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    ///         write!(f, "({}, {})", self.x, self.y)
    ///     }
    /// }
    ///
    /// let origin = Point { x: 0, y: 0 };
    ///
    /// println!("The origin is: {}", origin);
    /// ```
    #[rustc_on_unimplemented(
        message="`{Self}` doesn't implement `{Display}`",
        label="`{Self}` cannot be formatted with the default formatter",
        note="in format strings you may be able to use `{{:?}}` \
              (or {{:#?}} for pretty-print) instead",
    )]
    #[doc(alias = "{}")]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Display {
        /// Formats the value using the given formatter.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fmt;
        ///
        /// struct Position {
        ///     longitude: f32,
        ///     latitude: f32,
        /// }
        ///
        /// impl fmt::Display for Position {
        ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        ///         write!(f, "({}, {})", self.longitude, self.latitude)
        ///     }
        /// }
        ///
        /// assert_eq!("(1.987, 2.983)".to_owned(),
        ///            format!("{}", Position { longitude: 1.987, latitude: 2.983, }));
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn fmt(&self, f: &mut Formatter) -> Result;
    }

    /// `o` formatting.
    ///
    /// The `Octal` trait should format its output as a number in base-8.
    ///
    /// For primitive signed integers (`i8` to `i128`, and `isize`),
    /// negative values are formatted as the two’s complement representation.
    ///
    /// The alternate flag, `#`, adds a `0o` in front of the output.
    ///
    /// For more information on formatters, see [the module-level documentation][module].
    ///
    /// [module]: ../../std/fmt/index.html
    ///
    /// # Examples
    ///
    /// Basic usage with `i32`:
    ///
    /// ```
    /// let x = 42; // 42 is '52' in octal
    ///
    /// assert_eq!(format!("{:o}", x), "52");
    /// assert_eq!(format!("{:#o}", x), "0o52");
    ///
    /// assert_eq!(format!("{:o}", -16), "37777777760");
    /// ```
    ///
    /// Implementing `Octal` on a type:
    ///
    /// ```
    /// use std::fmt;
    ///
    /// struct Length(i32);
    ///
    /// impl fmt::Octal for Length {
    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    ///         let val = self.0;
    ///
    ///         write!(f, "{:o}", val) // delegate to i32's implementation
    ///     }
    /// }
    ///
    /// let l = Length(9);
    ///
    /// println!("l as octal is: {:o}", l);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Octal {
        /// Formats the value using the given formatter.
        #[stable(feature = "rust1", since = "1.0.0")]
        fn fmt(&self, f: &mut Formatter) -> Result;
    }

    /// `b` formatting.
    ///
    /// The `Binary` trait should format its output as a number in binary.
    ///
    /// For primitive signed integers ([`i8`] to [`i128`], and [`isize`]),
    /// negative values are formatted as the two’s complement representation.
    ///
    /// The alternate flag, `#`, adds a `0b` in front of the output.
    ///
    /// For more information on formatters, see [the module-level documentation][module].
    ///
    /// # Examples
    ///
    /// Basic usage with [`i32`]:
    ///
    /// ```
    /// let x = 42; // 42 is '101010' in binary
    ///
    /// assert_eq!(format!("{:b}", x), "101010");
    /// assert_eq!(format!("{:#b}", x), "0b101010");
    ///
    /// assert_eq!(format!("{:b}", -16), "11111111111111111111111111110000");
    /// ```
    ///
    /// Implementing `Binary` on a type:
    ///
    /// ```
    /// use std::fmt;
    ///
    /// struct Length(i32);
    ///
    /// impl fmt::Binary for Length {
    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    ///         let val = self.0;
    ///
    ///         write!(f, "{:b}", val) // delegate to i32's implementation
    ///     }
    /// }
    ///
    /// let l = Length(107);
    ///
    /// println!("l as binary is: {:b}", l);
    /// ```
    ///
    /// [module]: ../../std/fmt/index.html
    /// [`i8`]: ../../std/primitive.i8.html
    /// [`i128`]: ../../std/primitive.i128.html
    /// [`isize`]: ../../std/primitive.isize.html
    /// [`i32`]: ../../std/primitive.i32.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Binary {
        /// Formats the value using the given formatter.
        #[stable(feature = "rust1", since = "1.0.0")]
        fn fmt(&self, f: &mut Formatter) -> Result;
    }

    /// `x` formatting.
    ///
    /// The `LowerHex` trait should format its output as a number in hexadecimal, with `a` through `f`
    /// in lower case.
    ///
    /// For primitive signed integers (`i8` to `i128`, and `isize`),
    /// negative values are formatted as the two’s complement representation.
    ///
    /// The alternate flag, `#`, adds a `0x` in front of the output.
    ///
    /// For more information on formatters, see [the module-level documentation][module].
    ///
    /// [module]: ../../std/fmt/index.html
    ///
    /// # Examples
    ///
    /// Basic usage with `i32`:
    ///
    /// ```
    /// let x = 42; // 42 is '2a' in hex
    ///
    /// assert_eq!(format!("{:x}", x), "2a");
    /// assert_eq!(format!("{:#x}", x), "0x2a");
    ///
    /// assert_eq!(format!("{:x}", -16), "fffffff0");
    /// ```
    ///
    /// Implementing `LowerHex` on a type:
    ///
    /// ```
    /// use std::fmt;
    ///
    /// struct Length(i32);
    ///
    /// impl fmt::LowerHex for Length {
    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    ///         let val = self.0;
    ///
    ///         write!(f, "{:x}", val) // delegate to i32's implementation
    ///     }
    /// }
    ///
    /// let l = Length(9);
    ///
    /// println!("l as hex is: {:x}", l);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait LowerHex {
        /// Formats the value using the given formatter.
        #[stable(feature = "rust1", since = "1.0.0")]
        fn fmt(&self, f: &mut Formatter) -> Result;
    }

    /// `X` formatting.
    ///
    /// The `UpperHex` trait should format its output as a number in hexadecimal, with `A` through `F`
    /// in upper case.
    ///
    /// For primitive signed integers (`i8` to `i128`, and `isize`),
    /// negative values are formatted as the two’s complement representation.
    ///
    /// The alternate flag, `#`, adds a `0x` in front of the output.
    ///
    /// For more information on formatters, see [the module-level documentation][module].
    ///
    /// [module]: ../../std/fmt/index.html
    ///
    /// # Examples
    ///
    /// Basic usage with `i32`:
    ///
    /// ```
    /// let x = 42; // 42 is '2A' in hex
    ///
    /// assert_eq!(format!("{:X}", x), "2A");
    /// assert_eq!(format!("{:#X}", x), "0x2A");
    ///
    /// assert_eq!(format!("{:X}", -16), "FFFFFFF0");
    /// ```
    ///
    /// Implementing `UpperHex` on a type:
    ///
    /// ```
    /// use std::fmt;
    ///
    /// struct Length(i32);
    ///
    /// impl fmt::UpperHex for Length {
    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    ///         let val = self.0;
    ///
    ///         write!(f, "{:X}", val) // delegate to i32's implementation
    ///     }
    /// }
    ///
    /// let l = Length(9);
    ///
    /// println!("l as hex is: {:X}", l);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait UpperHex {
        /// Formats the value using the given formatter.
        #[stable(feature = "rust1", since = "1.0.0")]
        fn fmt(&self, f: &mut Formatter) -> Result;
    }

    /// `p` formatting.
    ///
    /// The `Pointer` trait should format its output as a memory location. This is commonly presented
    /// as hexadecimal.
    ///
    /// For more information on formatters, see [the module-level documentation][module].
    ///
    /// [module]: ../../std/fmt/index.html
    ///
    /// # Examples
    ///
    /// Basic usage with `&i32`:
    ///
    /// ```
    /// let x = &42;
    ///
    /// let address = format!("{:p}", x); // this produces something like '0x7f06092ac6d0'
    /// ```
    ///
    /// Implementing `Pointer` on a type:
    ///
    /// ```
    /// use std::fmt;
    ///
    /// struct Length(i32);
    ///
    /// impl fmt::Pointer for Length {
    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    ///         // use `as` to convert to a `*const T`, which implements Pointer, which we can use
    ///
    ///         write!(f, "{:p}", self as *const Length)
    ///     }
    /// }
    ///
    /// let l = Length(42);
    ///
    /// println!("l is in memory here: {:p}", l);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Pointer {
        /// Formats the value using the given formatter.
        #[stable(feature = "rust1", since = "1.0.0")]
        fn fmt(&self, f: &mut Formatter) -> Result;
    }

    /// `e` formatting.
    ///
    /// The `LowerExp` trait should format its output in scientific notation with a lower-case `e`.
    ///
    /// For more information on formatters, see [the module-level documentation][module].
    ///
    /// [module]: ../../std/fmt/index.html
    ///
    /// # Examples
    ///
    /// Basic usage with `i32`:
    ///
    /// ```
    /// let x = 42.0; // 42.0 is '4.2e1' in scientific notation
    ///
    /// assert_eq!(format!("{:e}", x), "4.2e1");
    /// ```
    ///
    /// Implementing `LowerExp` on a type:
    ///
    /// ```
    /// use std::fmt;
    ///
    /// struct Length(i32);
    ///
    /// impl fmt::LowerExp for Length {
    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    ///         let val = self.0;
    ///         write!(f, "{}e1", val / 10)
    ///     }
    /// }
    ///
    /// let l = Length(100);
    ///
    /// println!("l in scientific notation is: {:e}", l);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait LowerExp {
        /// Formats the value using the given formatter.
        #[stable(feature = "rust1", since = "1.0.0")]
        fn fmt(&self, f: &mut Formatter) -> Result;
    }

    /// `E` formatting.
    ///
    /// The `UpperExp` trait should format its output in scientific notation with an upper-case `E`.
    ///
    /// For more information on formatters, see [the module-level documentation][module].
    ///
    /// [module]: ../../std/fmt/index.html
    ///
    /// # Examples
    ///
    /// Basic usage with `f32`:
    ///
    /// ```
    /// let x = 42.0; // 42.0 is '4.2E1' in scientific notation
    ///
    /// assert_eq!(format!("{:E}", x), "4.2E1");
    /// ```
    ///
    /// Implementing `UpperExp` on a type:
    ///
    /// ```
    /// use std::fmt;
    ///
    /// struct Length(i32);
    ///
    /// impl fmt::UpperExp for Length {
    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    ///         let val = self.0;
    ///         write!(f, "{}E1", val / 10)
    ///     }
    /// }
    ///
    /// let l = Length(100);
    ///
    /// println!("l in scientific notation is: {:E}", l);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait UpperExp {
        /// Formats the value using the given formatter.
        #[stable(feature = "rust1", since = "1.0.0")]
        fn fmt(&self, f: &mut Formatter) -> Result;
    }

    /// The `write` function takes an output stream, and an `Arguments` struct
    /// that can be precompiled with the `format_args!` macro.
    ///
    /// The arguments will be formatted according to the specified format string
    /// into the output stream provided.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::fmt;
    ///
    /// let mut output = String::new();
    /// fmt::write(&mut output, format_args!("Hello {}!", "world"))
    ///     .expect("Error occurred while trying to write in String");
    /// assert_eq!(output, "Hello world!");
    /// ```
    ///
    /// Please note that using [`write!`] might be preferable. Example:
    ///
    /// ```
    /// use std::fmt::Write;
    ///
    /// let mut output = String::new();
    /// write!(&mut output, "Hello {}!", "world")
    ///     .expect("Error occurred while trying to write in String");
    /// assert_eq!(output, "Hello world!");
    /// ```
    ///
    /// [`write!`]: ../../std/macro.write.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn write(output: &mut dyn Write, args: Arguments) -> Result {
        let mut formatter = Formatter {
            flags: 0,
            width: None,
            precision: None,
            buf: output,
            align: rt::v1::Alignment::Unknown,
            fill: ' ',
            args: args.args,
            curarg: args.args.iter(),
        };

        let mut pieces = args.pieces.iter();

        match args.fmt {
            None => {
                // We can use default formatting parameters for all arguments.
                for (arg, piece) in args.args.iter().zip(pieces.by_ref()) {
                    formatter.buf.write_str(*piece)?;
                    (arg.formatter)(arg.value, &mut formatter)?;
                }
            }
            Some(fmt) => {
                // Every spec has a corresponding argument that is preceded by
                // a string piece.
                for (arg, piece) in fmt.iter().zip(pieces.by_ref()) {
                    formatter.buf.write_str(*piece)?;
                    formatter.run(arg)?;
                }
            }
        }

        // There can be only one trailing string piece left.
        if let Some(piece) = pieces.next() {
            formatter.buf.write_str(*piece)?;
        }

        Ok(())
    }

    impl<'a> Formatter<'a> {
        fn wrap_buf<'b, 'c, F>(&'b mut self, wrap: F) -> Formatter<'c>
            where 'b: 'c, F: FnOnce(&'b mut (dyn Write+'b)) -> &'c mut (dyn Write+'c)
        {
            Formatter {
                // We want to change this
                buf: wrap(self.buf),

                // And preserve these
                flags: self.flags,
                fill: self.fill,
                align: self.align,
                width: self.width,
                precision: self.precision,

                // These only exist in the struct for the `run` method,
                // which won’t be used together with this method.
                curarg: self.curarg.clone(),
                args: self.args,
            }
        }

        // First up is the collection of functions used to execute a format string
        // at runtime. This consumes all of the compile-time statics generated by
        // the format! syntax extension.
        fn run(&mut self, arg: &rt::v1::Argument) -> Result {
            // Fill in the format parameters into the formatter
            self.fill = arg.format.fill;
            self.align = arg.format.align;
            self.flags = arg.format.flags;
            self.width = self.getcount(&arg.format.width);
            self.precision = self.getcount(&arg.format.precision);

            // Extract the correct argument
            let value = match arg.position {
                rt::v1::Position::Next => { *self.curarg.next().unwrap() }
                rt::v1::Position::At(i) => self.args[i],
            };

            // Then actually do some printing
            (value.formatter)(value.value, self)
        }

        fn getcount(&mut self, cnt: &rt::v1::Count) -> Option<usize> {
            match *cnt {
                rt::v1::Count::Is(n) => Some(n),
                rt::v1::Count::Implied => None,
                rt::v1::Count::Param(i) => {
                    self.args[i].as_usize()
                }
                rt::v1::Count::NextParam => {
                    self.curarg.next().and_then(|arg| arg.as_usize())
                }
            }
        }

        // Helper methods used for padding and processing formatting arguments that
        // all formatting traits can use.

        /// Performs the correct padding for an integer which has already been
        /// emitted into a str. The str should *not* contain the sign for the
        /// integer, that will be added by this method.
        ///
        /// # Arguments
        ///
        /// * is_nonnegative - whether the original integer was either positive or zero.
        /// * prefix - if the '#' character (Alternate) is provided, this
        ///   is the prefix to put in front of the number.
        /// * buf - the byte array that the number has been formatted into
        ///
        /// This function will correctly account for the flags provided as well as
        /// the minimum width. It will not take precision into account.
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn pad_integral(&mut self,
                            is_nonnegative: bool,
                            prefix: &str,
                            buf: &str)
                            -> Result {
            let mut width = buf.len();

            let mut sign = None;
            if !is_nonnegative {
                sign = Some('-'); width += 1;
            } else if self.sign_plus() {
                sign = Some('+'); width += 1;
            }

            let mut prefixed = false;
            if self.alternate() {
                prefixed = true; width += prefix.chars().count();
            }

            // Writes the sign if it exists, and then the prefix if it was requested
            let write_prefix = |f: &mut Formatter| {
                if let Some(c) = sign {
                    f.buf.write_str(c.encode_utf8(&mut [0; 4]))?;
                }
                if prefixed { f.buf.write_str(prefix) }
                else { Ok(()) }
            };

            // The `width` field is more of a `min-width` parameter at this point.
            match self.width {
                // If there's no minimum length requirements then we can just
                // write the bytes.
                None => {
                    write_prefix(self)?; self.buf.write_str(buf)
                }
                // Check if we're over the minimum width, if so then we can also
                // just write the bytes.
                Some(min) if width >= min => {
                    write_prefix(self)?; self.buf.write_str(buf)
                }
                // The sign and prefix goes before the padding if the fill character
                // is zero
                Some(min) if self.sign_aware_zero_pad() => {
                    self.fill = '0';
                    self.align = rt::v1::Alignment::Right;
                    write_prefix(self)?;
                    self.with_padding(min - width, rt::v1::Alignment::Right, |f| {
                        f.buf.write_str(buf)
                    })
                }
                // Otherwise, the sign and prefix goes after the padding
                Some(min) => {
                    self.with_padding(min - width, rt::v1::Alignment::Right, |f| {
                        write_prefix(f)?; f.buf.write_str(buf)
                    })
                }
            }
        }

        /// This function takes a string slice and emits it to the internal buffer
        /// after applying the relevant formatting flags specified. The flags
        /// recognized for generic strings are:
        ///
        /// * width - the minimum width of what to emit
        /// * fill/align - what to emit and where to emit it if the string
        ///                provided needs to be padded
        /// * precision - the maximum length to emit, the string is truncated if it
        ///               is longer than this length
        ///
        /// Notably this function ignores the `flag` parameters.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fmt;
        ///
        /// struct Foo;
        ///
        /// impl fmt::Display for Foo {
        ///     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        ///         formatter.pad("Foo")
        ///     }
        /// }
        ///
        /// assert_eq!(&format!("{:<4}", Foo), "Foo ");
        /// assert_eq!(&format!("{:0>4}", Foo), "0Foo");
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn pad(&mut self, s: &str) -> Result {
            // Make sure there's a fast path up front
            if self.width.is_none() && self.precision.is_none() {
                return self.buf.write_str(s);
            }
            // The `precision` field can be interpreted as a `max-width` for the
            // string being formatted.
            let s = if let Some(max) = self.precision {
                // If our string is longer that the precision, then we must have
                // truncation. However other flags like `fill`, `width` and `align`
                // must act as always.
                if let Some((i, _)) = s.char_indices().skip(max).next() {
                    // LLVM here can't prove that `..i` won't panic `&s[..i]`, but
                    // we know that it can't panic. Use `get` + `unwrap_or` to avoid
                    // `unsafe` and otherwise don't emit any panic-related code
                    // here.
                    s.get(..i).unwrap_or(&s)
                } else {
                    &s
                }
            } else {
                &s
            };
            // The `width` field is more of a `min-width` parameter at this point.
            match self.width {
                // If we're under the maximum length, and there's no minimum length
                // requirements, then we can just emit the string
                None => self.buf.write_str(s),
                // If we're under the maximum width, check if we're over the minimum
                // width, if so it's as easy as just emitting the string.
                Some(width) if s.chars().count() >= width => {
                    self.buf.write_str(s)
                }
                // If we're under both the maximum and the minimum width, then fill
                // up the minimum width with the specified string + some alignment.
                Some(width) => {
                    let align = rt::v1::Alignment::Left;
                    self.with_padding(width - s.chars().count(), align, |me| {
                        me.buf.write_str(s)
                    })
                }
            }
        }

        /// Runs a callback, emitting the correct padding either before or
        /// afterwards depending on whether right or left alignment is requested.
        fn with_padding<F>(&mut self, padding: usize, default: rt::v1::Alignment,
                           f: F) -> Result
            where F: FnOnce(&mut Formatter) -> Result,
        {
            let align = match self.align {
                rt::v1::Alignment::Unknown => default,
                _ => self.align
            };

            let (pre_pad, post_pad) = match align {
                rt::v1::Alignment::Left => (0, padding),
                rt::v1::Alignment::Right |
                rt::v1::Alignment::Unknown => (padding, 0),
                rt::v1::Alignment::Center => (padding / 2, (padding + 1) / 2),
            };

            let mut fill = [0; 4];
            let fill = self.fill.encode_utf8(&mut fill);

            for _ in 0..pre_pad {
                self.buf.write_str(fill)?;
            }

            f(self)?;

            for _ in 0..post_pad {
                self.buf.write_str(fill)?;
            }

            Ok(())
        }

        /// Takes the formatted parts and applies the padding.
        /// Assumes that the caller already has rendered the parts with required precision,
        /// so that `self.precision` can be ignored.
        fn pad_formatted_parts(&mut self, formatted: &flt2dec::Formatted) -> Result {
            if let Some(mut width) = self.width {
                // for the sign-aware zero padding, we render the sign first and
                // behave as if we had no sign from the beginning.
                let mut formatted = formatted.clone();
                let old_fill = self.fill;
                let old_align = self.align;
                let mut align = old_align;
                if self.sign_aware_zero_pad() {
                    // a sign always goes first
                    let sign = unsafe { str::from_utf8_unchecked(formatted.sign) };
                    self.buf.write_str(sign)?;

                    // remove the sign from the formatted parts
                    formatted.sign = b"";
                    width = if width < sign.len() { 0 } else { width - sign.len() };
                    align = rt::v1::Alignment::Right;
                    self.fill = '0';
                    self.align = rt::v1::Alignment::Right;
                }

                // remaining parts go through the ordinary padding process.
                let len = formatted.len();
                let ret = if width <= len { // no padding
                    self.write_formatted_parts(&formatted)
                } else {
                    self.with_padding(width - len, align, |f| {
                        f.write_formatted_parts(&formatted)
                    })
                };
                self.fill = old_fill;
                self.align = old_align;
                ret
            } else {
                // this is the common case and we take a shortcut
                self.write_formatted_parts(formatted)
            }
        }

        fn write_formatted_parts(&mut self, formatted: &flt2dec::Formatted) -> Result {
            fn write_bytes(buf: &mut dyn Write, s: &[u8]) -> Result {
                buf.write_str(unsafe { str::from_utf8_unchecked(s) })
            }

            if !formatted.sign.is_empty() {
                write_bytes(self.buf, formatted.sign)?;
            }
            for part in formatted.parts {
                match *part {
                    flt2dec::Part::Zero(mut nzeroes) => {
                        const ZEROES: &'static str = // 64 zeroes
                            "0000000000000000000000000000000000000000000000000000000000000000";
                        while nzeroes > ZEROES.len() {
                            self.buf.write_str(ZEROES)?;
                            nzeroes -= ZEROES.len();
                        }
                        if nzeroes > 0 {
                            self.buf.write_str(&ZEROES[..nzeroes])?;
                        }
                    }
                    flt2dec::Part::Num(mut v) => {
                        let mut s = [0; 5];
                        let len = part.len();
                        for c in s[..len].iter_mut().rev() {
                            *c = b'0' + (v % 10) as u8;
                            v /= 10;
                        }
                        write_bytes(self.buf, &s[..len])?;
                    }
                    flt2dec::Part::Copy(buf) => {
                        write_bytes(self.buf, buf)?;
                    }
                }
            }
            Ok(())
        }

        /// Writes some data to the underlying buffer contained within this
        /// formatter.
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn write_str(&mut self, data: &str) -> Result {
            self.buf.write_str(data)
        }

        /// Writes some formatted information into this instance.
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn write_fmt(&mut self, fmt: Arguments) -> Result {
            write(self.buf, fmt)
        }

        /// Flags for formatting
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_deprecated(since = "1.24.0",
                           reason = "use the `sign_plus`, `sign_minus`, `alternate`, \
                                     or `sign_aware_zero_pad` methods instead")]
        pub fn flags(&self) -> u32 { self.flags }

        /// Character used as 'fill' whenever there is alignment.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fmt;
        ///
        /// struct Foo;
        ///
        /// impl fmt::Display for Foo {
        ///     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        ///         let c = formatter.fill();
        ///         if let Some(width) = formatter.width() {
        ///             for _ in 0..width {
        ///                 write!(formatter, "{}", c)?;
        ///             }
        ///             Ok(())
        ///         } else {
        ///             write!(formatter, "{}", c)
        ///         }
        ///     }
        /// }
        ///
        /// // We set alignment to the left with ">".
        /// assert_eq!(&format!("{:G>3}", Foo), "GGG");
        /// assert_eq!(&format!("{:t>6}", Foo), "tttttt");
        /// ```
        #[stable(feature = "fmt_flags", since = "1.5.0")]
        pub fn fill(&self) -> char { self.fill }

        /// Flag indicating what form of alignment was requested.
        ///
        /// # Examples
        ///
        /// ```
        /// extern crate core;
        ///
        /// use std::fmt::{self, Alignment};
        ///
        /// struct Foo;
        ///
        /// impl fmt::Display for Foo {
        ///     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        ///         let s = if let Some(s) = formatter.align() {
        ///             match s {
        ///                 Alignment::Left    => "left",
        ///                 Alignment::Right   => "right",
        ///                 Alignment::Center  => "center",
        ///             }
        ///         } else {
        ///             "into the void"
        ///         };
        ///         write!(formatter, "{}", s)
        ///     }
        /// }
        ///
        /// fn main() {
        ///     assert_eq!(&format!("{:<}", Foo), "left");
        ///     assert_eq!(&format!("{:>}", Foo), "right");
        ///     assert_eq!(&format!("{:^}", Foo), "center");
        ///     assert_eq!(&format!("{}", Foo), "into the void");
        /// }
        /// ```
        #[stable(feature = "fmt_flags_align", since = "1.28.0")]
        pub fn align(&self) -> Option<Alignment> {
            match self.align {
                rt::v1::Alignment::Left => Some(Alignment::Left),
                rt::v1::Alignment::Right => Some(Alignment::Right),
                rt::v1::Alignment::Center => Some(Alignment::Center),
                rt::v1::Alignment::Unknown => None,
            }
        }

        /// Optionally specified integer width that the output should be.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fmt;
        ///
        /// struct Foo(i32);
        ///
        /// impl fmt::Display for Foo {
        ///     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        ///         if let Some(width) = formatter.width() {
        ///             // If we received a width, we use it
        ///             write!(formatter, "{:width$}", &format!("Foo({})", self.0), width = width)
        ///         } else {
        ///             // Otherwise we do nothing special
        ///             write!(formatter, "Foo({})", self.0)
        ///         }
        ///     }
        /// }
        ///
        /// assert_eq!(&format!("{:10}", Foo(23)), "Foo(23)   ");
        /// assert_eq!(&format!("{}", Foo(23)), "Foo(23)");
        /// ```
        #[stable(feature = "fmt_flags", since = "1.5.0")]
        pub fn width(&self) -> Option<usize> { self.width }

        /// Optionally specified precision for numeric types.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fmt;
        ///
        /// struct Foo(f32);
        ///
        /// impl fmt::Display for Foo {
        ///     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        ///         if let Some(precision) = formatter.precision() {
        ///             // If we received a precision, we use it.
        ///             write!(formatter, "Foo({1:.*})", precision, self.0)
        ///         } else {
        ///             // Otherwise we default to 2.
        ///             write!(formatter, "Foo({:.2})", self.0)
        ///         }
        ///     }
        /// }
        ///
        /// assert_eq!(&format!("{:.4}", Foo(23.2)), "Foo(23.2000)");
        /// assert_eq!(&format!("{}", Foo(23.2)), "Foo(23.20)");
        /// ```
        #[stable(feature = "fmt_flags", since = "1.5.0")]
        pub fn precision(&self) -> Option<usize> { self.precision }

        /// Determines if the `+` flag was specified.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fmt;
        ///
        /// struct Foo(i32);
        ///
        /// impl fmt::Display for Foo {
        ///     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        ///         if formatter.sign_plus() {
        ///             write!(formatter,
        ///                    "Foo({}{})",
        ///                    if self.0 < 0 { '-' } else { '+' },
        ///                    self.0)
        ///         } else {
        ///             write!(formatter, "Foo({})", self.0)
        ///         }
        ///     }
        /// }
        ///
        /// assert_eq!(&format!("{:+}", Foo(23)), "Foo(+23)");
        /// assert_eq!(&format!("{}", Foo(23)), "Foo(23)");
        /// ```
        #[stable(feature = "fmt_flags", since = "1.5.0")]
        pub fn sign_plus(&self) -> bool { self.flags & (1 << FlagV1::SignPlus as u32) != 0 }

        /// Determines if the `-` flag was specified.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fmt;
        ///
        /// struct Foo(i32);
        ///
        /// impl fmt::Display for Foo {
        ///     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        ///         if formatter.sign_minus() {
        ///             // You want a minus sign? Have one!
        ///             write!(formatter, "-Foo({})", self.0)
        ///         } else {
        ///             write!(formatter, "Foo({})", self.0)
        ///         }
        ///     }
        /// }
        ///
        /// assert_eq!(&format!("{:-}", Foo(23)), "-Foo(23)");
        /// assert_eq!(&format!("{}", Foo(23)), "Foo(23)");
        /// ```
        #[stable(feature = "fmt_flags", since = "1.5.0")]
        pub fn sign_minus(&self) -> bool { self.flags & (1 << FlagV1::SignMinus as u32) != 0 }

        /// Determines if the `#` flag was specified.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fmt;
        ///
        /// struct Foo(i32);
        ///
        /// impl fmt::Display for Foo {
        ///     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        ///         if formatter.alternate() {
        ///             write!(formatter, "Foo({})", self.0)
        ///         } else {
        ///             write!(formatter, "{}", self.0)
        ///         }
        ///     }
        /// }
        ///
        /// assert_eq!(&format!("{:#}", Foo(23)), "Foo(23)");
        /// assert_eq!(&format!("{}", Foo(23)), "23");
        /// ```
        #[stable(feature = "fmt_flags", since = "1.5.0")]
        pub fn alternate(&self) -> bool { self.flags & (1 << FlagV1::Alternate as u32) != 0 }

        /// Determines if the `0` flag was specified.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fmt;
        ///
        /// struct Foo(i32);
        ///
        /// impl fmt::Display for Foo {
        ///     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        ///         assert!(formatter.sign_aware_zero_pad());
        ///         assert_eq!(formatter.width(), Some(4));
        ///         // We ignore the formatter's options.
        ///         write!(formatter, "{}", self.0)
        ///     }
        /// }
        ///
        /// assert_eq!(&format!("{:04}", Foo(23)), "23");
        /// ```
        #[stable(feature = "fmt_flags", since = "1.5.0")]
        pub fn sign_aware_zero_pad(&self) -> bool {
            self.flags & (1 << FlagV1::SignAwareZeroPad as u32) != 0
        }

        // FIXME: Decide what public API we want for these two flags.
        // https://github.com/rust-lang/rust/issues/48584
        fn debug_lower_hex(&self) -> bool { self.flags & (1 << FlagV1::DebugLowerHex as u32) != 0 }

        fn debug_upper_hex(&self) -> bool { self.flags & (1 << FlagV1::DebugUpperHex as u32) != 0 }

        /// Creates a [`DebugStruct`] builder designed to assist with creation of
        /// [`fmt::Debug`] implementations for structs.
        ///
        /// [`DebugStruct`]: ../../std/fmt/struct.DebugStruct.html
        /// [`fmt::Debug`]: ../../std/fmt/trait.Debug.html
        ///
        /// # Examples
        ///
        /// ```rust
        /// use std::fmt;
        /// use std::net::Ipv4Addr;
        ///
        /// struct Foo {
        ///     bar: i32,
        ///     baz: String,
        ///     addr: Ipv4Addr,
        /// }
        ///
        /// impl fmt::Debug for Foo {
        ///     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        ///         fmt.debug_struct("Foo")
        ///             .field("bar", &self.bar)
        ///             .field("baz", &self.baz)
        ///             .field("addr", &format_args!("{}", self.addr))
        ///             .finish()
        ///     }
        /// }
        ///
        /// assert_eq!(
        ///     "Foo { bar: 10, baz: \"Hello World\", addr: 127.0.0.1 }",
        ///     format!("{:?}", Foo {
        ///         bar: 10,
        ///         baz: "Hello World".to_string(),
        ///         addr: Ipv4Addr::new(127, 0, 0, 1),
        ///     })
        /// );
        /// ```
        #[stable(feature = "debug_builders", since = "1.2.0")]
        pub fn debug_struct<'b>(&'b mut self, name: &str) -> DebugStruct<'b, 'a> {
            builders::debug_struct_new(self, name)
        }

        /// Creates a `DebugTuple` builder designed to assist with creation of
        /// `fmt::Debug` implementations for tuple structs.
        ///
        /// # Examples
        ///
        /// ```rust
        /// use std::fmt;
        /// use std::marker::PhantomData;
        ///
        /// struct Foo<T>(i32, String, PhantomData<T>);
        ///
        /// impl<T> fmt::Debug for Foo<T> {
        ///     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        ///         fmt.debug_tuple("Foo")
        ///             .field(&self.0)
        ///             .field(&self.1)
        ///             .field(&format_args!("_"))
        ///             .finish()
        ///     }
        /// }
        ///
        /// assert_eq!(
        ///     "Foo(10, \"Hello\", _)",
        ///     format!("{:?}", Foo(10, "Hello".to_string(), PhantomData::<u8>))
        /// );
        /// ```
        #[stable(feature = "debug_builders", since = "1.2.0")]
        pub fn debug_tuple<'b>(&'b mut self, name: &str) -> DebugTuple<'b, 'a> {
            builders::debug_tuple_new(self, name)
        }

        /// Creates a `DebugList` builder designed to assist with creation of
        /// `fmt::Debug` implementations for list-like structures.
        ///
        /// # Examples
        ///
        /// ```rust
        /// use std::fmt;
        ///
        /// struct Foo(Vec<i32>);
        ///
        /// impl fmt::Debug for Foo {
        ///     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        ///         fmt.debug_list().entries(self.0.iter()).finish()
        ///     }
        /// }
        ///
        /// // prints "[10, 11]"
        /// println!("{:?}", Foo(vec![10, 11]));
        /// ```
        #[stable(feature = "debug_builders", since = "1.2.0")]
        pub fn debug_list<'b>(&'b mut self) -> DebugList<'b, 'a> {
            builders::debug_list_new(self)
        }

        /// Creates a `DebugSet` builder designed to assist with creation of
        /// `fmt::Debug` implementations for set-like structures.
        ///
        /// # Examples
        ///
        /// ```rust
        /// use std::fmt;
        ///
        /// struct Foo(Vec<i32>);
        ///
        /// impl fmt::Debug for Foo {
        ///     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        ///         fmt.debug_set().entries(self.0.iter()).finish()
        ///     }
        /// }
        ///
        /// // prints "{10, 11}"
        /// println!("{:?}", Foo(vec![10, 11]));
        /// ```
        ///
        /// [`format_args!`]: ../../std/macro.format_args.html
        ///
        /// In this more complex example, we use [`format_args!`] and `.debug_set()`
        /// to build a list of match arms:
        ///
        /// ```rust
        /// use std::fmt;
        ///
        /// struct Arm<'a, L: 'a, R: 'a>(&'a (L, R));
        /// struct Table<'a, K: 'a, V: 'a>(&'a [(K, V)], V);
        ///
        /// impl<'a, L, R> fmt::Debug for Arm<'a, L, R>
        /// where
        ///     L: 'a + fmt::Debug, R: 'a + fmt::Debug
        /// {
        ///     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        ///         L::fmt(&(self.0).0, fmt)?;
        ///         fmt.write_str(" => ")?;
        ///         R::fmt(&(self.0).1, fmt)
        ///     }
        /// }
        ///
        /// impl<'a, K, V> fmt::Debug for Table<'a, K, V>
        /// where
        ///     K: 'a + fmt::Debug, V: 'a + fmt::Debug
        /// {
        ///     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        ///         fmt.debug_set()
        ///         .entries(self.0.iter().map(Arm))
        ///         .entry(&Arm(&(format_args!("_"), &self.1)))
        ///         .finish()
        ///     }
        /// }
        /// ```
        #[stable(feature = "debug_builders", since = "1.2.0")]
        pub fn debug_set<'b>(&'b mut self) -> DebugSet<'b, 'a> {
            builders::debug_set_new(self)
        }

        /// Creates a `DebugMap` builder designed to assist with creation of
        /// `fmt::Debug` implementations for map-like structures.
        ///
        /// # Examples
        ///
        /// ```rust
        /// use std::fmt;
        ///
        /// struct Foo(Vec<(String, i32)>);
        ///
        /// impl fmt::Debug for Foo {
        ///     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        ///         fmt.debug_map().entries(self.0.iter().map(|&(ref k, ref v)| (k, v))).finish()
        ///     }
        /// }
        ///
        /// // prints "{"A": 10, "B": 11}"
        /// println!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)]));
        /// ```
        #[stable(feature = "debug_builders", since = "1.2.0")]
        pub fn debug_map<'b>(&'b mut self) -> DebugMap<'b, 'a> {
            builders::debug_map_new(self)
        }
    }

    #[stable(since = "1.2.0", feature = "formatter_write")]
    impl<'a> Write for Formatter<'a> {
        fn write_str(&mut self, s: &str) -> Result {
            self.buf.write_str(s)
        }

        fn write_char(&mut self, c: char) -> Result {
            self.buf.write_char(c)
        }

        fn write_fmt(&mut self, args: Arguments) -> Result {
            write(self.buf, args)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Display for Error {
        fn fmt(&self, f: &mut Formatter) -> Result {
            Display::fmt("an error occurred when formatting an argument", f)
        }
    }

    // Implementations of the core formatting traits

    macro_rules! fmt_refs {
        ($($tr:ident),*) => {
            $(
            #[stable(feature = "rust1", since = "1.0.0")]
            impl<'a, T: ?Sized + $tr> $tr for &'a T {
                fn fmt(&self, f: &mut Formatter) -> Result { $tr::fmt(&**self, f) }
            }
            #[stable(feature = "rust1", since = "1.0.0")]
            impl<'a, T: ?Sized + $tr> $tr for &'a mut T {
                fn fmt(&self, f: &mut Formatter) -> Result { $tr::fmt(&**self, f) }
            }
            )*
        }
    }

    fmt_refs! { Debug, Display, Octal, Binary, LowerHex, UpperHex, LowerExp, UpperExp }

    #[unstable(feature = "never_type", issue = "35121")]
    impl Debug for ! {
        fn fmt(&self, _: &mut Formatter) -> Result {
            *self
        }
    }

    #[unstable(feature = "never_type", issue = "35121")]
    impl Display for ! {
        fn fmt(&self, _: &mut Formatter) -> Result {
            *self
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Debug for bool {
        #[inline]
        fn fmt(&self, f: &mut Formatter) -> Result {
            Display::fmt(self, f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Display for bool {
        fn fmt(&self, f: &mut Formatter) -> Result {
            Display::fmt(if *self { "true" } else { "false" }, f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Debug for str {
        fn fmt(&self, f: &mut Formatter) -> Result {
            f.write_char('"')?;
            let mut from = 0;
            for (i, c) in self.char_indices() {
                let esc = c.escape_debug();
                // If char needs escaping, flush backlog so far and write, else skip
                if esc.len() != 1 {
                    f.write_str(&self[from..i])?;
                    for c in esc {
                        f.write_char(c)?;
                    }
                    from = i + c.len_utf8();
                }
            }
            f.write_str(&self[from..])?;
            f.write_char('"')
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Display for str {
        fn fmt(&self, f: &mut Formatter) -> Result {
            f.pad(self)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Debug for char {
        fn fmt(&self, f: &mut Formatter) -> Result {
            f.write_char('\'')?;
            for c in self.escape_debug() {
                f.write_char(c)?
            }
            f.write_char('\'')
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Display for char {
        fn fmt(&self, f: &mut Formatter) -> Result {
            if f.width.is_none() && f.precision.is_none() {
                f.write_char(*self)
            } else {
                f.pad(self.encode_utf8(&mut [0; 4]))
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Pointer for *const T {
        fn fmt(&self, f: &mut Formatter) -> Result {
            let old_width = f.width;
            let old_flags = f.flags;

            // The alternate flag is already treated by LowerHex as being special-
            // it denotes whether to prefix with 0x. We use it to work out whether
            // or not to zero extend, and then unconditionally set it to get the
            // prefix.
            if f.alternate() {
                f.flags |= 1 << (FlagV1::SignAwareZeroPad as u32);

                if let None = f.width {
                    f.width = Some(((mem::size_of::<usize>() * 8) / 4) + 2);
                }
            }
            f.flags |= 1 << (FlagV1::Alternate as u32);

            let ret = LowerHex::fmt(&(*self as *const () as usize), f);

            f.width = old_width;
            f.flags = old_flags;

            ret
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Pointer for *mut T {
        fn fmt(&self, f: &mut Formatter) -> Result {
            Pointer::fmt(&(*self as *const T), f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: ?Sized> Pointer for &'a T {
        fn fmt(&self, f: &mut Formatter) -> Result {
            Pointer::fmt(&(*self as *const T), f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: ?Sized> Pointer for &'a mut T {
        fn fmt(&self, f: &mut Formatter) -> Result {
            Pointer::fmt(&(&**self as *const T), f)
        }
    }

    // Implementation of Display/Debug for various core types

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Debug for *const T {
        fn fmt(&self, f: &mut Formatter) -> Result { Pointer::fmt(self, f) }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Debug for *mut T {
        fn fmt(&self, f: &mut Formatter) -> Result { Pointer::fmt(self, f) }
    }

    macro_rules! peel {
        ($name:ident, $($other:ident,)*) => (tuple! { $($other,)* })
    }

    macro_rules! tuple {
        () => ();
        ( $($name:ident,)+ ) => (
            #[stable(feature = "rust1", since = "1.0.0")]
            impl<$($name:Debug),*> Debug for ($($name,)*) where last_type!($($name,)+): ?Sized {
                #[allow(non_snake_case, unused_assignments, deprecated)]
                fn fmt(&self, f: &mut Formatter) -> Result {
                    let mut builder = f.debug_tuple("");
                    let ($(ref $name,)*) = *self;
                    $(
                        builder.field(&$name);
                    )*

                    builder.finish()
                }
            }
            peel! { $($name,)* }
        )
    }

    macro_rules! last_type {
        ($a:ident,) => { $a };
        ($a:ident, $($rest_a:ident,)+) => { last_type!($($rest_a,)+) };
    }

    tuple! { T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Debug> Debug for [T] {
        fn fmt(&self, f: &mut Formatter) -> Result {
            f.debug_list().entries(self.iter()).finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl Debug for () {
        #[inline]
        fn fmt(&self, f: &mut Formatter) -> Result {
            f.pad("()")
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Debug for PhantomData<T> {
        fn fmt(&self, f: &mut Formatter) -> Result {
            f.pad("PhantomData")
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Copy + Debug> Debug for Cell<T> {
        fn fmt(&self, f: &mut Formatter) -> Result {
            f.debug_struct("Cell")
                .field("value", &self.get())
                .finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + Debug> Debug for RefCell<T> {
        fn fmt(&self, f: &mut Formatter) -> Result {
            match self.try_borrow() {
                Ok(borrow) => {
                    f.debug_struct("RefCell")
                        .field("value", &borrow)
                        .finish()
                }
                Err(_) => {
                    // The RefCell is mutably borrowed so we can't look at its value
                    // here. Show a placeholder instead.
                    struct BorrowedPlaceholder;

                    impl Debug for BorrowedPlaceholder {
                        fn fmt(&self, f: &mut Formatter) -> Result {
                            f.write_str("<borrowed>")
                        }
                    }

                    f.debug_struct("RefCell")
                        .field("value", &BorrowedPlaceholder)
                        .finish()
                }
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'b, T: ?Sized + Debug> Debug for Ref<'b, T> {
        fn fmt(&self, f: &mut Formatter) -> Result {
            Debug::fmt(&**self, f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'b, T: ?Sized + Debug> Debug for RefMut<'b, T> {
        fn fmt(&self, f: &mut Formatter) -> Result {
            Debug::fmt(&*(self.deref()), f)
        }
    }

    #[stable(feature = "core_impl_debug", since = "1.9.0")]
    impl<T: ?Sized + Debug> Debug for UnsafeCell<T> {
        fn fmt(&self, f: &mut Formatter) -> Result {
            f.pad("UnsafeCell")
        }
    }

    // If you expected tests to be here, look instead at the run-pass/ifmt.rs test,
    // it's a lot easier than creating all of the rt::Piece structures here.

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