Rollup merge of rust-lang#145724 - folkertdev:track-caller-drop-no-mangle, r=fee1-dead

the `#[track_caller]` shim should not inherit `#[no_mangle]`

fixes rust-lang#143162

builds on rust-lang#143293 which introduced a mechanism to strip attributes from shims.

cc `@Jules-Bertholet` `@workingjubilee` `@bjorn3`

---

Summary:

This PR fixes an interaction between `#[track_caller]`, `#[no_mangle]`, and casting to a function pointer.

A function annotated with `#[track_caller]` internally has a hidden extra argument for the panic location. The `#[track_caller]` attribute is only allowed on `extern "Rust"` functions. When a function is annotated with both `#[no_mangle]` and `#[track_caller]`, the exported symbol has the signature that includes the extra panic location argument. This works on stable rust today:

```rust
extern "Rust" {
    #[track_caller]
    fn rust_track_caller_ffi_test_tracked() -> &'static Location<'static>;
}

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()
    }
}
```

When a `#[track_caller]` function is converted to a function pointer, a shim is added to drop the additional argument. So this is a valid program:

```rust
#[track_caller]
fn foo() {}

fn main() {
    let f = foo as fn();
    f();
}
```

The issue arises when `foo` is additionally annotated with `#[no_mangle]`, the generated shim currently inherits this attribute, also exporting a symbol named `foo`, but one without the hidden panic location argument. The linker rightfully complains about a duplicate symbol.

The solution of this PR is to have the generated shim drop the `#[no_mangle]` attribute.

Author: matthiaskrgr

src/abi/mod.rs

@@ -467,7 +467,7 @@ pub(crate) fn codegen_terminator_call<'tcx>(
         true
     } else {
         instance.is_some_and(|inst| {
-            fx.tcx.codegen_fn_attrs(inst.def_id()).flags.contains(CodegenFnAttrFlags::COLD)
+            fx.tcx.codegen_instance_attrs(inst.def).flags.contains(CodegenFnAttrFlags::COLD)
         })
     };
     if is_cold {