Emit `unused_attributes` for `#[inline]` on exported functions

I saw someone post a code sample that contained these two attributes,
which immediately made me suspicious.
My suspicions were confirmed when I did a small test and checked the
compiler source code to confirm that in these cases, `#[inline]` is
indeed ignored (because you can't exactly `LocalCopy`an unmangled symbol
since that would lead to duplicate symbols, and doing a mix of an
unmangled `GloballyShared` and mangled `LocalCopy` instantiation is too
complicated for our current instatiation mode logic, which I don't want
to change right now).

So instead, emit the usual unused attribute lint with a message saying
that the attribute is ignored in this position.

I think this is not 100% true, since I expect LLVM `inlinehint` to still
be applied to such a function, but that's not why people use this
attribute, they use it for the `LocalCopy` instantiation mode, where it
doesn't work.

1 files changed, 1 insertions, 0 deletions

diff --git a/compiler/rustc_middle/src/mir/mono.rs b/compiler/rustc_middle/src/mir/mono.rs
index 58d5c94d033..2aa31761178 100644
--- a/compiler/rustc_middle/src/mir/mono.rs
+++ b/compiler/rustc_middle/src/mir/mono.rs
@@ -117,6 +117,7 @@ impl<'tcx> MonoItem<'tcx> {
 
         // If the function is #[naked] or contains any other attribute that requires exactly-once
         // instantiation:
+        // We emit an unused_attributes lint for this case, which should be kept in sync if possible.
         let codegen_fn_attrs = tcx.codegen_fn_attrs(instance.def_id());
         if codegen_fn_attrs.contains_extern_indicator()
             || codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED)