Document proper usage of fmt::Error and fmt()'s Result.

Documentation of these properties previously existed in a lone paragraph
in the `fmt` module's documentation:
<https://doc.rust-lang.org/1.78.0/std/fmt/index.html#formatting-traits>
However, users looking to implement a formatting trait won't necessarily
look there. Therefore, let's add the critical information (that
formatting per se is infallible) to all the involved items.

library/alloc/src/fmt.rs

@@ -403,7 +403,7 @@
 //! is, a formatting implementation must and may only return an error if the
 //! passed-in [`Formatter`] returns an error. This is because, contrary to what
 //! the function signature might suggest, string formatting is an infallible
-//! operation. This function only returns a result because writing to the
+//! operation. This function only returns a [`Result`] because writing to the
 //! underlying stream might fail and it must provide a way to propagate the fact
 //! that an error has occurred back up the stack.
 //!

library/core/src/fmt/fmt_trait_method_doc.md

@@ -0,0 +1,8 @@
+Formats the value using the given formatter.
+
+# Errors
+
+This function should return [`Err`] if, and only if, the provided [`Formatter`] returns [`Err`].
+String formatting is considered an infallible operation; this function only
+returns a [`Result`] because writing to the underlying stream might fail and it must
+provide a way to propagate the fact that an error has occurred back up the stack.

library/core/src/fmt/mod.rs

@@ -72,14 +72,24 @@ 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
+/// occurred. This is because, despite the existence of this error,
+/// string formatting is considered an infallible operation.
+/// `fmt()` implementors should not return this `Error` unless they received it from their
+/// [`Formatter`]. The only time your code should create a new instance of this
+/// error is when implementing `fmt::Write`, in order to cancel the formatting operation when
+/// writing to the underlying stream fails.
+///
+/// Any extra information must be arranged to be transmitted through some other means,
+/// such as storing it in a field to be consulted after the formatting operation has been
+/// cancelled. (For example, this is how [`std::io::Write::write_fmt()`] propagates IO errors
+/// during writing.)
+///
+/// This 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::io::Write::write_fmt()`]: ../../std/io/trait.Write.html#method.write_fmt
 /// [`std::error::Error`]: ../../std/error/trait.Error.html
 ///
 /// # Examples
@@ -118,8 +128,10 @@ pub trait Write {
     /// This function will return an instance of [`std::fmt::Error`][Error] on error.
     ///
     /// The purpose of that error is to abort the formatting operation when the underlying
-    /// destination encounters some error preventing it from accepting more text; it should
-    /// generally be propagated rather than handled, at least when implementing formatting traits.
+    /// destination encounters some error preventing it from accepting more text;
+    /// in particular, it does not communicate any information about *what* error occurred.
+    /// It should generally be propagated rather than handled, at least when implementing
+    /// formatting traits.
     ///
     /// # Examples
     ///
@@ -586,7 +598,7 @@ impl Display for Arguments<'_> {
 #[rustc_diagnostic_item = "Debug"]
 #[rustc_trivial_field_reads]
 pub trait Debug {
-    /// Formats the value using the given formatter.
+    #[doc = include_str!("fmt_trait_method_doc.md")]
     ///
     /// # Examples
     ///
@@ -703,7 +715,7 @@ pub use macros::Debug;
 #[rustc_diagnostic_item = "Display"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Display {
-    /// Formats the value using the given formatter.
+    #[doc = include_str!("fmt_trait_method_doc.md")]
     ///
     /// # Examples
     ///
@@ -777,7 +789,7 @@ pub trait Display {
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Octal {
-    /// Formats the value using the given formatter.
+    #[doc = include_str!("fmt_trait_method_doc.md")]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn fmt(&self, f: &mut Formatter<'_>) -> Result;
 }
@@ -836,7 +848,7 @@ pub trait Octal {
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Binary {
-    /// Formats the value using the given formatter.
+    #[doc = include_str!("fmt_trait_method_doc.md")]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn fmt(&self, f: &mut Formatter<'_>) -> Result;
 }
@@ -891,7 +903,7 @@ pub trait Binary {
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait LowerHex {
-    /// Formats the value using the given formatter.
+    #[doc = include_str!("fmt_trait_method_doc.md")]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn fmt(&self, f: &mut Formatter<'_>) -> Result;
 }
@@ -946,7 +958,7 @@ pub trait LowerHex {
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait UpperHex {
-    /// Formats the value using the given formatter.
+    #[doc = include_str!("fmt_trait_method_doc.md")]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn fmt(&self, f: &mut Formatter<'_>) -> Result;
 }
@@ -997,7 +1009,7 @@ pub trait UpperHex {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_diagnostic_item = "Pointer"]
 pub trait Pointer {
-    /// Formats the value using the given formatter.
+    #[doc = include_str!("fmt_trait_method_doc.md")]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn fmt(&self, f: &mut Formatter<'_>) -> Result;
 }
@@ -1048,7 +1060,7 @@ pub trait Pointer {
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait LowerExp {
-    /// Formats the value using the given formatter.
+    #[doc = include_str!("fmt_trait_method_doc.md")]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn fmt(&self, f: &mut Formatter<'_>) -> Result;
 }
@@ -1099,7 +1111,7 @@ pub trait LowerExp {
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait UpperExp {
-    /// Formats the value using the given formatter.
+    #[doc = include_str!("fmt_trait_method_doc.md")]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn fmt(&self, f: &mut Formatter<'_>) -> Result;
 }