#[no_mangle] in thread_local! requires unsafe wrapping in edition 2021

[elftausend]

I tried this code to compile in Edition 2021:

#[no_mangle] // fine
static TEST_OUTSIDE: usize = 0;

thread_local! {
    #[no_mangle] // unsafe attribute used without unsafe wrap error
    static TEST: usize = 0;
}

However, compilation fails because the attribute must be wrapped in unsafe(...).
This breaks existing libraries like litequad, even though it wasn't upgraded to edition 2024.

Meta

[package]
name = "no_mangle_test"
version = "0.1.0"
edition = "2021"

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=a63572490a930b126b003b57c58c9df0

error: unsafe attribute used without unsafe
 --> src/main.rs:6:7
  |
6 |     #[no_mangle]
  |       ^^^^^^^^^ usage of unsafe attribute
  |
help: wrap the attribute in `unsafe(...)`
  |
6 |     #[unsafe(no_mangle)]
  |       +++++++         +

error: could not compile `no_mangle_test` (bin "no_mangle_test") due to 1 previous error

[workingjubilee]

this seems to be a similar class of bug as to what we fixed via #138717 because the spans get their edition based on the crate the macro is defined in, and for a more recent rustc, std is Edition 2024

[bjorn3]

I don't think #[no_mangle] inside thread_local! {} is supposed to work. What would it actually apply to? The #[thread_local] static? (there may not be any), the static containing the pthread_key_t (again there may not be any), the std::thread::LocalKey (likely to surprise people who want to interoperate with C and it may not even be stored in a static in the first place) or something else?

[workingjubilee]

...that is an interesting question, yeah.

[apiraino]

@elftausend I'm slightly confused about which edition that snippet used to compile for you (IIUC your report). Does it compile with --edition=2018 or --edition=2021? Can you share how do you exactly compile that snippet? Thanks