`thread_local!` doesn’t like types whose definitions cannot be written more than once

I tried this (admittedly cursed) code:

thread_local! {
    pub static FOO: [(); {
        #[unsafe(no_mangle)]
        fn dont_duplicate_me() {}
        0
    }] = [];
}

I expected to see this happen: It compiles, just like the non-thread_local! version would

Instead, this happened:

error: symbol `dont_duplicate_me` is already defined
 --> src/lib.rs:4:9
  |
4 |         fn dont_duplicate_me() {}
  |         ^^^^^^^^^^^^^^^^^^^^^^

Meta

rustc --version:

1.92.0-nightly (2025-09-26 54a8a1db604e4caff93e)

@rustbot label T-libs A-macros A-thread-locals A-const-generics

Start with the src/lib.rs reproducer using thread_local! and the nested dont_duplicate_me function, then compare it with the equivalent non-thread_local! version. Trace how the thread_local! macro handles the const-generic type and confirm that the corrected behavior compiles without the duplicate-symbol error.