inconsistent `stringify!`

[loynoir]

I tried this code:

#[macro_export]
macro_rules! reproduce_macro {
    () => {{
        [stringify!(a::b), stringify!(&a)]
    }};
}

#[test]
pub fn reproduce() {
    let expected = [stringify!(a::b), stringify!(&a)];
    assert_eq!(expected, ["a::b", "&a"]);

    let actual = reproduce_macro!();
    assert_eq!(actual, ["a :: b", "& a"]);
}

I expected to see this happen: explanation

actual equal expected

Instead, this happened: explanation

actual not equal expected

$ cargo test -r
       Fresh reproduce v0.1.0 (/tmp/reproduce)
    Finished `release` profile [optimized] target(s) in 0.00s
     Running `/tmp/reproduce/target/release/deps/reproduce-7e181f6f7847a00f`

running 1 test
test reproduce ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

   Doc-tests reproduce
     Running `/home/vscode/.rustup/toolchains/nightly-2025-04-14-x86_64-unknown-linux-gnu/bin/rustdoc --edition=2021 --crate-type lib --color auto --crate-name reproduce --test src/lib.rs --test-run-directory /tmp/reproduce -L dependency=/tmp/reproduce/target/release/deps -L dependency=/tmp/reproduce/target/release/deps --extern reproduce=/tmp/reproduce/target/release/deps/libreproduce-c271e11eea147c9e.rlib -C embed-bitcode=no --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values())' --error-format human`

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Meta

rustc --version --verbose:

<version>

channel = "nightly-2025-04-14"

Backtrace

<backtrace>

[Kivooeo]

This isn't a bug in stringify!, but rather a reflection of how Rust's macro system and token spacing work. That said, it's definitely confusing, and better documentation or diagnostics around this behavior could help.

Briefly:
the tokens a, ::, and b are re-tokenized when the macro is expanded. During this expansion, Rust may insert Alone spacing between tokens unless they are explicitly joined. As a result, stringify! receives tokens like a, ::, b — but with Alone spacing — and renders them with spaces: "a :: b".

This doesn't happen in regular functions because there's no macro expansion involved — the tokens come from the original source without re-parsing

[loynoir]

This isn't a bug in stringify!, but rather a reflection of how Rust's macro system and token spacing work.

Given

• X not equals to Y

To me, this sounds like Circular Reasoning

• X not equals to Y, not bug because X is designed to X

I would expected explanation like

• X not equals to Y, not bug because X is designed to X, because of problem Z