From eb18746bc6c6c5c710ad674873438cbad5894f06 Mon Sep 17 00:00:00 2001
From: Mara Bos <m-ou.se@m-ou.se>
Date: Thu, 4 Mar 2021 17:55:23 +0100
Subject: [PATCH] Add assert_matches!(expr, pat).

---
 library/core/src/macros/mod.rs | 52 +++++++++++++++++++++++++
 library/core/src/panicking.rs  | 69 ++++++++++++++++++++++------------
 library/std/src/lib.rs         |  5 ++-
 3 files changed, 101 insertions(+), 25 deletions(-)

diff --git a/library/core/src/macros/mod.rs b/library/core/src/macros/mod.rs
index 9a54921f07b49..3e31a9e8910e0 100644
--- a/library/core/src/macros/mod.rs
+++ b/library/core/src/macros/mod.rs
@@ -110,6 +110,58 @@ macro_rules! assert_ne {
     });
 }
 
+/// Asserts that an expression matches a pattern.
+///
+/// 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
+///
+/// ```
+/// let a = 1u32.checked_add(2);
+/// let b = 1u32.checked_sub(2);
+/// assert_matches!(a, Some(_));
+/// assert_matches!(b, None);
+/// ```
+#[macro_export]
+#[unstable(feature = "assert_matches", issue = "none")]
+#[allow_internal_unstable(core_panic)]
+macro_rules! assert_matches {
+    ($left:expr, $right:pat $(,)?) => ({
+        match &$left {
+            left_val => {
+                if let $right = left_val {
+                    // OK
+                } else {
+                    $crate::panicking::assert_matches_failed(
+                        &*left_val,
+                        $crate::stringify!($right),
+                        $crate::option::Option::None
+                    );
+                }
+            }
+        }
+    });
+    ($left:expr, $right:expr, $($arg:tt)+) => ({
+        match &$left {
+            left_val => {
+                if let $right = left_val {
+                    // OK
+                } else {
+                    $crate::panicking::assert_matches_failed(
+                        &*left_val,
+                        $crate::stringify!($right),
+                        $crate::option::Option::Some($crate::format_args!($($arg)+))
+                    );
+                }
+            }
+        }
+    });
+}
+
 /// Asserts that a boolean expression is `true` at runtime.
 ///
 /// This will invoke the [`panic!`] macro if the provided expression cannot be
diff --git a/library/core/src/panicking.rs b/library/core/src/panicking.rs
index af8a6101392a4..12acf5b4329db 100644
--- a/library/core/src/panicking.rs
+++ b/library/core/src/panicking.rs
@@ -97,6 +97,7 @@ pub fn panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
 pub enum AssertKind {
     Eq,
     Ne,
+    Match,
 }
 
 /// Internal function for `assert_eq!` and `assert_ne!` macros
@@ -113,32 +114,54 @@ where
     T: fmt::Debug + ?Sized,
     U: fmt::Debug + ?Sized,
 {
-    #[track_caller]
-    fn inner(
-        kind: AssertKind,
-        left: &dyn fmt::Debug,
-        right: &dyn fmt::Debug,
-        args: Option<fmt::Arguments<'_>>,
-    ) -> ! {
-        let op = match kind {
-            AssertKind::Eq => "==",
-            AssertKind::Ne => "!=",
-        };
-
-        match args {
-            Some(args) => panic!(
-                r#"assertion failed: `(left {} right)`
+    assert_failed_inner(kind, &left, &right, args)
+}
+
+/// Internal function for `assert_match!`
+#[cold]
+#[track_caller]
+#[doc(hidden)]
+pub fn assert_matches_failed<T: fmt::Debug + ?Sized>(
+    left: &T,
+    right: &str,
+    args: Option<fmt::Arguments<'_>>,
+) -> ! {
+    // Use the Display implementation to display the pattern.
+    struct Pattern<'a>(&'a str);
+    impl fmt::Debug for Pattern<'_> {
+        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+            fmt::Display::fmt(self.0, f)
+        }
+    }
+    assert_failed_inner(AssertKind::Match, &left, &Pattern(right), args);
+}
+
+/// Non-generic version of the above functions, to avoid code bloat.
+#[track_caller]
+fn assert_failed_inner(
+    kind: AssertKind,
+    left: &dyn fmt::Debug,
+    right: &dyn fmt::Debug,
+    args: Option<fmt::Arguments<'_>>,
+) -> ! {
+    let op = match kind {
+        AssertKind::Eq => "==",
+        AssertKind::Ne => "!=",
+        AssertKind::Match => "matches",
+    };
+
+    match args {
+        Some(args) => panic!(
+            r#"assertion failed: `(left {} right)`
   left: `{:?}`,
  right: `{:?}: {}`"#,
-                op, left, right, args
-            ),
-            None => panic!(
-                r#"assertion failed: `(left {} right)`
+            op, left, right, args
+        ),
+        None => panic!(
+            r#"assertion failed: `(left {} right)`
   left: `{:?}`,
  right: `{:?}`"#,
-                op, left, right,
-            ),
-        }
+            op, left, right,
+        ),
     }
-    inner(kind, &left, &right, args)
 }
diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index ba49dee38e642..ddceb492765da 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -228,6 +228,7 @@
 #![feature(arbitrary_self_types)]
 #![feature(array_error_internals)]
 #![feature(asm)]
+#![feature(assert_matches)]
 #![feature(associated_type_bounds)]
 #![feature(atomic_mut_ptr)]
 #![feature(box_syntax)]
@@ -550,8 +551,8 @@ pub use std_detect::detect;
 #[stable(feature = "rust1", since = "1.0.0")]
 #[allow(deprecated, deprecated_in_future)]
 pub use core::{
-    assert_eq, assert_ne, debug_assert, debug_assert_eq, debug_assert_ne, matches, r#try, todo,
-    unimplemented, unreachable, write, writeln,
+    assert_eq, assert_matches, assert_ne, debug_assert, debug_assert_eq, debug_assert_ne, matches,
+    r#try, todo, unimplemented, unreachable, write, writeln,
 };
 
 // Re-export built-in macros defined through libcore.