Improve assert_matches! documentation

This new documentation tries to avoid to limit the impact of the
conceptual pitfall, that the if guard relaxes the constraint, when
really it tightens it. This is achieved by changing the text and
examples. The previous documentation also chose a rather weird and
non-representative example for the if guard, that made it needlessly
complicated to understand.

library/core/src/macros/mod.rs

@@ -112,16 +112,19 @@ macro_rules! assert_ne {
     };
 }
 
-/// Asserts that an expression matches any of the given patterns.
+/// Asserts that an expression matches the provided pattern.
 ///
-/// Like in a `match` expression, the pattern can be optionally followed by `if`
-/// and a guard expression that has access to names bound by the pattern.
+/// This macro is generally preferable to `assert!(matches!(value, pattern))`, because it can print
+/// the debug representation, of the actual value shape that did not meet expectation. In contrast
+/// using [`assert!`] will only print that the expectation was not met, but not why.
 ///
-/// On panic, this macro will print the value of the expression with its
-/// debug representation.
+/// The pattern syntax is exactly the same as found in a match arm and the [`matches!`] macro. The
+/// optional if guard can be used to add additional checks that must be true for the matched value,
+/// otherwise this macro will panic.
 ///
-/// Like [`assert!`], this macro has a second form, where a custom
-/// panic message can be provided.
+/// On panic, this macro will print the value of the expression with its debug representation.
+///
+/// Like [`assert!`], this macro has a second form, where a custom panic message can be provided.
 ///
 /// # Examples
 ///
@@ -130,13 +133,20 @@ macro_rules! assert_ne {
 ///
 /// use std::assert_matches::assert_matches;
 ///
-/// let a = 1u32.checked_add(2);
-/// let b = 1u32.checked_sub(2);
+/// let a = Some(345);
+/// let b = Some(56);
 /// assert_matches!(a, Some(_));
-/// assert_matches!(b, None);
+/// assert_matches!(b, Some(_));
 ///
-/// let c = Ok("abc".to_string());
-/// assert_matches!(c, Ok(x) | Err(x) if x.len() < 100);
+/// assert_matches!(a, Some(345));
+/// assert_matches!(a, Some(345) | None);
+///
+/// // assert_matches!(a, None); // panics
+/// // assert_matches!(b, Some(345)); // panics
+/// // assert_matches!(b, Some(345) | None); // panics
+///
+/// assert_matches!(a, Some(x) if x > 100);
+/// // assert_matches!(a, Some(x) if x < 100); // panics
 /// ```
 #[unstable(feature = "assert_matches", issue = "82775")]
 #[allow_internal_unstable(panic_internals)]
@@ -369,21 +379,25 @@ macro_rules! debug_assert_ne {
     };
 }
 
-/// Asserts that an expression matches any of the given patterns.
+/// Asserts that an expression matches the provided pattern.
 ///
-/// Like in a `match` expression, the pattern can be optionally followed by `if`
-/// and a guard expression that has access to names bound by the pattern.
+/// This macro is generally preferable to `debug_assert!(matches!(value, pattern))`, because it can
+/// print the debug representation, of the actual value shape that did not meet expectation. In
+/// contrast using [`debug_assert!`] will only print that the expectation was not met, but not why.
 ///
-/// On panic, this macro will print the value of the expression with its
-/// debug representation.
+/// The pattern syntax is exactly the same as found in a match arm and the [`matches!`] macro. The
+/// optional if guard can be used to add additional checks that must be true for the matched value,
+/// otherwise this macro will panic.
 ///
-/// Unlike [`assert_matches!`], `debug_assert_matches!` statements are only
-/// enabled in non optimized builds by default. An optimized build will not
-/// execute `debug_assert_matches!` statements unless `-C debug-assertions` is
-/// passed to the compiler. This makes `debug_assert_matches!` useful for
-/// checks that are too expensive to be present in a release build but may be
-/// helpful during development. The result of expanding `debug_assert_matches!`
-/// is always type checked.
+/// On panic, this macro will print the value of the expression with its debug representation.
+///
+/// Like [`assert!`], this macro has a second form, where a custom panic message can be provided.
+///
+/// Unlike [`assert_matches!`], `debug_assert_matches!` statements are only enabled in non optimized
+/// builds by default. An optimized build will not execute `debug_assert_matches!` statements unless
+/// `-C debug-assertions` is passed to the compiler. This makes `debug_assert_matches!` useful for
+/// checks that are too expensive to be present in a release build but may be helpful during
+/// development. The result of expanding `debug_assert_matches!` is always type checked.
 ///
 /// # Examples
 ///
@@ -392,13 +406,20 @@ macro_rules! debug_assert_ne {
 ///
 /// use std::assert_matches::debug_assert_matches;
 ///
-/// let a = 1u32.checked_add(2);
-/// let b = 1u32.checked_sub(2);
+/// let a = Some(345);
+/// let b = Some(56);
 /// debug_assert_matches!(a, Some(_));
-/// debug_assert_matches!(b, None);
+/// debug_assert_matches!(b, Some(_));
 ///
-/// let c = Ok("abc".to_string());
-/// debug_assert_matches!(c, Ok(x) | Err(x) if x.len() < 100);
+/// debug_assert_matches!(a, Some(345));
+/// debug_assert_matches!(a, Some(345) | None);
+///
+/// // debug_assert_matches!(a, None); // panics
+/// // debug_assert_matches!(b, Some(345)); // panics
+/// // debug_assert_matches!(b, Some(345) | None); // panics
+///
+/// debug_assert_matches!(a, Some(x) if x > 100);
+/// // debug_assert_matches!(a, Some(x) if x < 100); // panics
 /// ```
 #[unstable(feature = "assert_matches", issue = "82775")]
 #[allow_internal_unstable(assert_matches)]