Attribute macro can replace item with expression.

@rustbot

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

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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

This might be an issue with the reference rather than a bug in implementation?

One example where this came in handy: https://docs.rs/cfg_eval/0.1.2/cfg_eval/index.html#example

@petrochenkov

@petrochenkov about your opinion

The macro invocation is in a statement position, so it can produce multiple statements, including expression statements.
So it's not necessarily an item.

Expression statements do not produce values though, right? If the macro expanded to an expression statement (as opposed to a non-statement expression), I would not have been able to print the value "weird".

They can when within braces:

{
    foo! {} // say this emits `42`
    bar! {} // and this, ``
} // <- evaluates to `42`

Playground

And..... the reference says that the result of an expression statement is always ignored.

This function compiles, and returns 42:

macro_rules! nothing {
    () => {}
}

fn demo() -> i32 {
    { 42 }
    nothing! {}
}

Based on the grammar in the reference, I believe that the { 42 } must be an expression statement. However, the result of it is clearly not ignored.

Either the implementation or the reference is also wrong here.

Yes, I believe the reference ought to mention the special case of the without-semicolon "block statement", whose value is not necessarily always discarded. Do you want to open an issue about it?

This function compiles, and returns 42:

IMO, this shouldn't work. It appears to violate the tree structure of the language in a way that macros usually can't (e.g. foo(a, b!(), c) always has 3 arguments), and thus surprisingly reduces the ability to read Rust without knowing everything about every involved macro expansion.

(e.g. foo(a, b!(), c) always has 3 arguments

What about foo(a, #[my_cfg(false)] b, c), how many arguments does it have?

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Attribute macro can replace item with expression. #144716

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Attribute macro can replace item with expression. #144716

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danielhenrymantilla commented on Jul 31, 2025

Noratrieb commented on Jul 31, 2025

petrochenkov commented on Jul 31, 2025

theemathas commented on Jul 31, 2025

danielhenrymantilla commented on Jul 31, 2025

theemathas commented on Jul 31, 2025

danielhenrymantilla commented on Jul 31, 2025

kpreid commented on Jul 31, 2025

petrochenkov commented on Jul 31, 2025

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