`#![macro_use]` is accepted at crate root

[theemathas]

The following code compiles in lib.rs.

#![macro_use]

I don't know what it does (hopefully nothing), but it probably shouldn't compile.

cc @JonathanBrouwer @jdonszelmann

Meta

Reproducible on the playground with version 1.98.0-nightly (2026-05-31 14210df0e27ccd7d9e6a)

[JonathanBrouwer]

Huh it is explicitly marked as allowed, I wonder why, I'll look into this

rust/compiler/rustc_attr_parsing/src/attributes/macro_attrs.rs

Line 36 in 4804ad7

Allow(Target::Crate),

[JonathanBrouwer]

I looked into it, when writing the attribute parser we preserved the previous behavior, this has always been legal and there are tests covering this. As far as I can see, this has no behaviour.

At the same time, the rust reference seems to suggest this is not allowed:

The macro_use attribute may be applied to modules or extern crate.

Imo the currently implemented behaviour is sorta reasonable, the attribute takes all the macros in scope and exposes them to the global scope. When the attribute is applied to the global scope, it's behaviour is a no-op. Maybe emitting an unused attribute warning makes sense?

[ehuss]

seems to suggest this is not allowed [..] applied to modules

Sorry if it's not clear, but the crate root is considered a module (the root module).

Since there is nothing "past" the end of the root, I would expect it to do nothing.

the attribute takes all the macros in scope and exposes them to the global scope

I wouldn't say that's what it does exactly. On a module, it extends the textual scope past the point of the module, but that exposure does not propagate upwards at all (or to the prelude or the crate root). There is a significant difference between textual scope and and path scope. As in:

mod a {
    m!(); // NOT OK! `m` is not placed into the module scope.

    #[macro_use]
    mod b {
        macro_rules! m {
            () => {};
        }
    }
    m!(); // OK, `m` was extended textually from `b`.
    // textual scope ends here.
}
m!(); // NOT OK!