Auto merge of #79100 - a1phyr:better_assert_eq, r=m-ou-se

Improve assert_eq! and assert_ne!

This PR improves `assert_eq!` and `assert_ne!` by moving the panicking code in an external function.

It does not change the fast path, but the move of the formatting in the cold path (the panic) may have a positive effect on in instruction cache use and with inlining.

Moreover, the use of trait objects instead of generic may improve compile times for `assert_eq!`-heavy code.

Godbolt link: ~~https://rust.godbolt.org/z/TYa9MT~~ \
Updated: https://rust.godbolt.org/z/bzE84x

Author: bors

library/core/src/macros/mod.rs

@@ -53,32 +53,30 @@ macro_rules! panic {
 /// ```
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[allow_internal_unstable(core_panic)]
 macro_rules! assert_eq {
     ($left:expr, $right:expr $(,)?) => ({
         match (&$left, &$right) {
             (left_val, right_val) => {
                 if !(*left_val == *right_val) {
+                    let kind = $crate::panicking::AssertKind::Eq;
                     // The reborrows below are intentional. Without them, the stack slot for the
                     // borrow is initialized even before the values are compared, leading to a
                     // noticeable slow down.
-                    $crate::panic!(r#"assertion failed: `(left == right)`
-  left: `{:?}`,
- right: `{:?}`"#, &*left_val, &*right_val)
+                    $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::None);
                 }
             }
         }
     });
     ($left:expr, $right:expr, $($arg:tt)+) => ({
-        match (&($left), &($right)) {
+        match (&$left, &$right) {
             (left_val, right_val) => {
                 if !(*left_val == *right_val) {
+                    let kind = $crate::panicking::AssertKind::Eq;
                     // The reborrows below are intentional. Without them, the stack slot for the
                     // borrow is initialized even before the values are compared, leading to a
                     // noticeable slow down.
-                    $crate::panic!(r#"assertion failed: `(left == right)`
-  left: `{:?}`,
- right: `{:?}`: {}"#, &*left_val, &*right_val,
-                           $crate::format_args!($($arg)+))
+                    $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::Some($crate::format_args!($($arg)+)));
                 }
             }
         }
@@ -104,17 +102,17 @@ macro_rules! assert_eq {
 /// ```
 #[macro_export]
 #[stable(feature = "assert_ne", since = "1.13.0")]
+#[allow_internal_unstable(core_panic)]
 macro_rules! assert_ne {
     ($left:expr, $right:expr $(,)?) => ({
         match (&$left, &$right) {
             (left_val, right_val) => {
                 if *left_val == *right_val {
+                    let kind = $crate::panicking::AssertKind::Ne;
                     // The reborrows below are intentional. Without them, the stack slot for the
                     // borrow is initialized even before the values are compared, leading to a
                     // noticeable slow down.
-                    $crate::panic!(r#"assertion failed: `(left != right)`
-  left: `{:?}`,
- right: `{:?}`"#, &*left_val, &*right_val)
+                    $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::None);
                 }
             }
         }
@@ -123,13 +121,11 @@ macro_rules! assert_ne {
         match (&($left), &($right)) {
             (left_val, right_val) => {
                 if *left_val == *right_val {
+                    let kind = $crate::panicking::AssertKind::Ne;
                     // The reborrows below are intentional. Without them, the stack slot for the
                     // borrow is initialized even before the values are compared, leading to a
                     // noticeable slow down.
-                    $crate::panic!(r#"assertion failed: `(left != right)`
-  left: `{:?}`,
- right: `{:?}`: {}"#, &*left_val, &*right_val,
-                           $crate::format_args!($($arg)+))
+                    $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::Some($crate::format_args!($($arg)+)));
                 }
             }
         }

library/core/src/panicking.rs

@@ -91,3 +91,54 @@ pub fn panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
     // SAFETY: `panic_impl` is defined in safe Rust code and thus is safe to call.
     unsafe { panic_impl(&pi) }
 }
+
+#[derive(Debug)]
+#[doc(hidden)]
+pub enum AssertKind {
+    Eq,
+    Ne,
+}
+
+/// Internal function for `assert_eq!` and `assert_ne!` macros
+#[cold]
+#[track_caller]
+#[doc(hidden)]
+pub fn assert_failed<T, U>(
+    kind: AssertKind,
+    left: &T,
+    right: &U,
+    args: Option<fmt::Arguments<'_>>,
+) -> !
+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)`
+  left: `{:?}`,
+ right: `{:?}: {}`"#,
+                op, left, right, args
+            ),
+            None => panic!(
+                r#"assertion failed: `(left {} right)`
+  left: `{:?}`,
+ right: `{:?}`"#,
+                op, left, right,
+            ),
+        }
+    }
+    inner(kind, &left, &right, args)
+}