Attribute macro can replace item with expression.

I tried this code:

bar/src/lib.rs:

use proc_macro::TokenStream;

#[proc_macro_attribute]
pub fn my_macro(_: TokenStream, _: TokenStream) -> TokenStream {
    "\"weird\"".parse().unwrap()
}

src/main.rs:

fn main() {
    // prints "weird"
    println!("{}", {
        #[bar::my_macro]
        struct Thing;
    });
}

I expected the code to give a compile error. Instead, it compiles and prints weird.

The proc macro is somehow replacing an item with an expression. The reference says this isn't allowed:

The returned TokenStream replaces the item with an arbitrary number of items.

Initially discovered in #144579.

@rustbot labels +A-macros +A-proc-macros +A-attributes

Meta

rustc --version --verbose:

rustc 1.88.0 (6b00bc388 2025-06-23)
binary: rustc
commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc
commit-date: 2025-06-23
host: aarch64-apple-darwin
release: 1.88.0
LLVM version: 20.1.5

Reproduce the behavior using bar/src/lib.rs and src/main.rs with the reported rustc version, then trace how attribute macro output is validated in the compiler. Add regression coverage for the item-to-expression case and verify that the example now produces the expected compile error.