Significantly improve formatter trait docs

Each formatting trait now has an example of implementation, as well as a
fuller description of what it's supposed to output.

It also contains a link to the module-level documentation which

Fixes #25765

src/libcore/fmt/mod.rs

@@ -267,11 +267,16 @@ fn fmt(&self, fmt: &mut Formatter) -> Result {
     }
 }
 
-/// Format trait for the `?` character. Useful for debugging, all types
-/// should implement this.
+/// Format trait for the `?` character.
+///
+/// `Debug` should format the output in a programmer-facing, debugging context.
 ///
 /// Generally speaking, you should just `derive` a `Debug` implementation.
 ///
+/// For more information on formatters, see [the module-level documentation][module].
+///
+/// [module]: ../index.html
+///
 /// # Examples
 ///
 /// Deriving an implementation:
@@ -327,8 +332,39 @@ pub trait Debug {
     fn fmt(&self, &mut Formatter) -> Result;
 }
 
-/// When a value can be semantically expressed as a String, this trait may be
-/// used. It corresponds to the default format, `{}`.
+/// 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]: ../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 = "`{Self}` cannot be formatted with the default \
                             formatter; try using `:?` instead if you are using \
                             a format string"]
@@ -339,7 +375,43 @@ pub trait Display {
     fn fmt(&self, &mut Formatter) -> Result;
 }
 
-/// Format trait for the `o` character
+/// Format trait for the `o` character.
+///
+/// The `Octal` trait should format its output as a number in base-8.
+///
+/// For more information on formatters, see [the module-level documentation][module].
+///
+/// [module]: ../index.html
+///
+/// # Examples
+///
+/// Basic usage with `i32`:
+///
+/// ```
+/// let x = 42; // 42 is '52' in octal
+///
+/// assert_eq!(format!("{:o}", x), "52");
+/// ```
+///
+/// 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.
@@ -347,7 +419,43 @@ pub trait Octal {
     fn fmt(&self, &mut Formatter) -> Result;
 }
 
-/// Format trait for the `b` character
+/// Format trait for the `b` character.
+///
+/// The `Binary` trait should format its output as a number in binary.
+///
+/// For more information on formatters, see [the module-level documentation][module].
+///
+/// [module]: ../index.html
+///
+/// # Examples
+///
+/// Basic usage with `i32`:
+///
+/// ```
+/// let x = 42; // 42 is '101010' in binary
+///
+/// assert_eq!(format!("{:b}", x), "101010");
+/// ```
+///
+/// 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);
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Binary {
     /// Formats the value using the given formatter.
@@ -355,7 +463,44 @@ pub trait Binary {
     fn fmt(&self, &mut Formatter) -> Result;
 }
 
-/// Format trait for the `x` character
+/// Format trait for the `x` character.
+///
+/// The `LowerHex` trait should format its output as a number in hexidecimal, with `a` through `f`
+/// in lower case.
+///
+/// For more information on formatters, see [the module-level documentation][module].
+///
+/// [module]: ../index.html
+///
+/// # Examples
+///
+/// Basic usage with `i32`:
+///
+/// ```
+/// let x = 42; // 42 is '2a' in hex
+///
+/// assert_eq!(format!("{:x}", x), "2a");
+/// ```
+///
+/// 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.
@@ -363,7 +508,44 @@ pub trait LowerHex {
     fn fmt(&self, &mut Formatter) -> Result;
 }
 
-/// Format trait for the `X` character
+/// Format trait for the `X` character.
+///
+/// The `UpperHex` trait should format its output as a number in hexidecimal, with `A` through `F`
+/// in upper case.
+///
+/// For more information on formatters, see [the module-level documentation][module].
+///
+/// [module]: ../index.html
+///
+/// # Examples
+///
+/// Basic usage with `i32`:
+///
+/// ```
+/// let x = 42; // 42 is '2A' in hex
+///
+/// assert_eq!(format!("{:X}", x), "2A");
+/// ```
+///
+/// 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.
@@ -371,7 +553,44 @@ pub trait UpperHex {
     fn fmt(&self, &mut Formatter) -> Result;
 }
 
-/// Format trait for the `p` character
+/// Format trait for the `p` character.
+///
+/// The `Pointer` trait should format its output as a memory location. This is commonly presented
+/// as hexidecimal.
+///
+/// For more information on formatters, see [the module-level documentation][module].
+///
+/// [module]: ../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.
@@ -379,7 +598,42 @@ pub trait Pointer {
     fn fmt(&self, &mut Formatter) -> Result;
 }
 
-/// Format trait for the `e` character
+/// Format trait for the `e` character.
+///
+/// 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]: ../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.
@@ -387,7 +641,42 @@ pub trait LowerExp {
     fn fmt(&self, &mut Formatter) -> Result;
 }
 
-/// Format trait for the `E` character
+/// Format trait for the `E` character.
+///
+/// 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]: ../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.