Make sure std::panic::Location::caller() gets optimized away.

Author: michaelwoerister

library/core/src/panic/location.rs

@@ -83,6 +83,7 @@ impl<'a> Location<'a> {
     #[stable(feature = "track_caller", since = "1.46.0")]
     #[rustc_const_unstable(feature = "const_caller_location", issue = "76156")]
     #[track_caller]
+    #[inline(always)] // Mark as inline so that the call gets optimized away.
     pub const fn caller() -> &'static Location<'static> {
         crate::intrinsics::caller_location()
     }

src/test/codegen/track_caller_inlined.rs

@@ -0,0 +1,28 @@
+// This test makes sure that calls to std::panic::Location::caller()
+// don't result in an actual function call. The caller location is
+// known at compile time so the call can always be optimized away.
+
+// compile-flags: -Copt-level=2
+
+#![crate_type = "lib"]
+#![feature(bench_black_box)]
+
+// The first check makes sure that the caller location is used at all,
+// i.e. that std::hint::black_box() works.
+// CHECK: %0 = alloca %"core::panic::location::Location"*
+
+// This check makes sure that no call to `std::panic::Location::caller()`
+// is emitted. The sequence of characters is valid for both v0 and legacy
+// mangling.
+// CHECK-NOT: call {{.*}}8Location6caller
+
+// CHECK: call void asm sideeffect {{.*}}(%"core::panic::location::Location"** nonnull %0)
+
+#[track_caller]
+fn foo() {
+    std::hint::black_box(std::panic::Location::caller());
+}
+
+pub fn bar() {
+    foo();
+}