track_caller: support on FFI imports

src/librustc_error_codes/error_codes.rs

@@ -416,7 +416,6 @@ E0734: include_str!("./error_codes/E0734.md"),
 E0735: include_str!("./error_codes/E0735.md"),
 E0736: include_str!("./error_codes/E0736.md"),
 E0737: include_str!("./error_codes/E0737.md"),
-E0738: include_str!("./error_codes/E0738.md"),
 E0739: include_str!("./error_codes/E0739.md"),
 E0740: include_str!("./error_codes/E0740.md"),
 E0741: include_str!("./error_codes/E0741.md"),
@@ -614,4 +613,5 @@ E0751: include_str!("./error_codes/E0751.md"),
     E0722, // Malformed `#[optimize]` attribute
     E0724, // `#[ffi_returns_twice]` is only allowed in foreign functions
     E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
+//  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
 }

src/librustc_passes/check_attr.rs

@@ -151,17 +151,7 @@ impl CheckAttrVisitor<'tcx> {
                 .emit();
                 false
             }
-            Target::ForeignFn => {
-                struct_span_err!(
-                    self.tcx.sess,
-                    *attr_span,
-                    E0738,
-                    "`#[track_caller]` is not supported on foreign functions",
-                )
-                .emit();
-                false
-            }
-            Target::Fn | Target::Method(..) => true,
+            Target::Fn | Target::Method(..) | Target::ForeignFn => true,
             _ => {
                 struct_span_err!(
                     self.tcx.sess,

src/test/ui/rfc-2091-track-caller/track-caller-ffi.rs

@@ -0,0 +1,50 @@
+// run-pass
+
+#![feature(track_caller)]
+
+use std::panic::Location;
+
+extern "Rust" {
+    #[track_caller]
+    fn rust_track_caller_ffi_test_tracked() -> &'static Location<'static>;
+    fn rust_track_caller_ffi_test_untracked() -> &'static Location<'static>;
+}
+
+fn rust_track_caller_ffi_test_nested_tracked() -> &'static Location<'static> {
+    unsafe { rust_track_caller_ffi_test_tracked() }
+}
+
+mod provides {
+    use std::panic::Location;
+    #[track_caller] // UB if we did not have this!
+    #[no_mangle]
+    fn rust_track_caller_ffi_test_tracked() -> &'static Location<'static> {
+        Location::caller()
+    }
+    #[no_mangle]
+    fn rust_track_caller_ffi_test_untracked() -> &'static Location<'static> {
+        Location::caller()
+    }
+}
+
+fn main() {
+    let location = Location::caller();
+    assert_eq!(location.file(), file!());
+    assert_eq!(location.line(), 31);
+    assert_eq!(location.column(), 20);
+
+    let tracked = unsafe { rust_track_caller_ffi_test_tracked() };
+    assert_eq!(tracked.file(), file!());
+    assert_eq!(tracked.line(), 36);
+    assert_eq!(tracked.column(), 28);
+
+    let untracked = unsafe { rust_track_caller_ffi_test_untracked() };
+    assert_eq!(untracked.file(), file!());
+    assert_eq!(untracked.line(), 26);
+    assert_eq!(untracked.column(), 9);
+
+    let contained = rust_track_caller_ffi_test_nested_tracked();
+    assert_eq!(contained.file(), file!());
+    assert_eq!(contained.line(), 14);
+    assert_eq!(contained.column(), 14);
+}