Constify assert_eq! and assert_ne!

We provide an inferior type of formatting for use when panicking in
compile time, which should not come with additional costs as the
additional argument should be inlined and optimized away when in runtime

Author: fee1-dead

library/core/src/macros/mod.rs

@@ -42,7 +42,7 @@ macro_rules! assert_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::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::None);
+                    $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::None, $crate::concat!("assertion failed: `", $crate::stringify!($left), " == ", $crate::stringify!($right), "`"));
                 }
             }
         }
@@ -55,7 +55,7 @@ macro_rules! assert_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::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::Some($crate::format_args!($($arg)+)));
+                    $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::Some($crate::format_args!($($arg)+)), $crate::concat!("assertion failed: `", $crate::stringify!($left), " == ", $crate::stringify!($right), "`"));
                 }
             }
         }
@@ -92,7 +92,7 @@ macro_rules! assert_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::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::None);
+                    $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::None, $crate::concat!("assertion failed: `", $crate::stringify!($left), " != ", $crate::stringify!($right), "`"));
                 }
             }
         }
@@ -105,7 +105,7 @@ macro_rules! assert_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::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::Some($crate::format_args!($($arg)+)));
+                    $crate::panicking::assert_failed(kind, &*left_val, &*right_val, $crate::option::Option::Some($crate::format_args!($($arg)+)), $crate::concat!("assertion failed: `", $crate::stringify!($left), " != ", $crate::stringify!($right), "`"));
                 }
             }
         }

library/core/src/panicking.rs

@@ -199,17 +199,53 @@ pub enum AssertKind {
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
 #[doc(hidden)]
-pub fn assert_failed<T, U>(
+#[rustc_const_unstable(feature = "const_assert_eq", issue = "none")]
+pub const fn assert_failed<T, U>(
     kind: AssertKind,
     left: &T,
     right: &U,
     args: Option<fmt::Arguments<'_>>,
+    string_for_const_msg: &str,
 ) -> !
 where
     T: fmt::Debug + ?Sized,
     U: fmt::Debug + ?Sized,
 {
-    assert_failed_inner(kind, &left, &right, args)
+    #[track_caller]
+    const fn assert_failed_const_impl<T: ?Sized, U: ?Sized>(
+        _: AssertKind,
+        _: &T,
+        _: &U,
+        _: Option<fmt::Arguments<'_>>,
+        msg: &str,
+    ) -> ! {
+        panic_str(msg)
+    }
+
+    #[track_caller]
+    #[inline]
+    fn assert_failed_runtime_impl<T, U>(
+        kind: AssertKind,
+        left: &T,
+        right: &U,
+        args: Option<fmt::Arguments<'_>>,
+        _: &str,
+    ) -> !
+    where
+        T: fmt::Debug + ?Sized,
+        U: fmt::Debug + ?Sized,
+    {
+        assert_failed_inner(kind, &left, &right, args)
+    }
+
+    // SAFETY: we are panicking in both branches, so this is consistent.
+    unsafe {
+        crate::intrinsics::const_eval_select(
+            (kind, left, right, args, string_for_const_msg),
+            assert_failed_const_impl,
+            assert_failed_runtime_impl,
+        )
+    }
 }
 
 /// Internal function for `assert_match!`

tests/ui/consts/assertions.rs

@@ -0,0 +1,15 @@
+#![feature(const_assert_eq)]
+
+const _BAD1: () = {
+    assert_eq!(1, 2)
+}; //~^ ERROR: evaluation of constant value failed
+
+const _BAD2: () = {
+    assert_ne!(1, 1)
+}; //~^ ERROR: evaluation of constant value failed
+
+const _BAD3: () = {
+    assert!(false)
+}; //~^ ERROR: evaluation of constant value failed
+
+fn main() {}

tests/ui/consts/assertions.stderr

@@ -0,0 +1,27 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/assertions.rs:4:5
+   |
+LL |     assert_eq!(1, 2)
+   |     ^^^^^^^^^^^^^^^^ the evaluated program panicked at 'assertion failed: `1 == 2`', $DIR/assertions.rs:4:5
+   |
+   = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0080]: evaluation of constant value failed
+  --> $DIR/assertions.rs:8:5
+   |
+LL |     assert_ne!(1, 1)
+   |     ^^^^^^^^^^^^^^^^ the evaluated program panicked at 'assertion failed: `1 != 1`', $DIR/assertions.rs:8:5
+   |
+   = note: this error originates in the macro `assert_ne` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0080]: evaluation of constant value failed
+  --> $DIR/assertions.rs:12:5
+   |
+LL |     assert!(false)
+   |     ^^^^^^^^^^^^^^ the evaluated program panicked at 'assertion failed: false', $DIR/assertions.rs:12:5
+   |
+   = note: this error originates in the macro `assert` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0080`.