`#[unsafe(no_mangle)]` erroneously accepted inside `thread_local!`

I tried this code:

thread_local! {
    #[unsafe(no_mangle)]
    static FOO: () = const { () };
}

I expected to see this happen:

error: const items should never be `#[no_mangle]`

(The macro expands to a const item that has the #[unsafe(no_mangle)] applied to it.)

Instead, this happened:

It compiled successfully. The attribute is ignored

Meta

rustc --version --verbose:

rustc 1.91.0-nightly (02c7b1a7a 2025-09-13)
binary: rustc
commit-hash: 02c7b1a7ac1d739663878030510508372e46f254
commit-date: 2025-09-13
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 21.1.1

@rustbot label A-attributes A-macros T-compiler

Edit: see https://github.com/rust-lang/rust/issues/146557#issuecomment-3294496645 for root cause

Start by compiling the provided thread_local! reproducer and inspect its macro expansion, especially how #[unsafe(no_mangle)] reaches the generated const item. Trace the attribute handling in the compiler's macro and attribute diagnostics, then verify that the example produces the expected error without changing valid thread_local! uses.