macro_rules! thinks use is an ident. It isn't.

This code should fail to compile, giving the same error message as fn use() would give, but it compiles successfully.

macro_rules! foo {
    ($foo:ident) => {}
}

fn main() {
    foo!(use);
}

This code should compile successfully. However, it fails complaining that it can't decide whether use is an ident or not (hint: it isn't). It also shouldn't be trying to match use as an ident because of the semicolon, but that's a broader/deeper issue with macro_rules!

macro_rules! foo {
    ($(use ;)* $foo:ident) => {}
}

fn main() {
    foo!(use ; use ; bar);
}

@nrc

ident matcher accepts keywords (and also reserved keywords) by design (see very similar issue about self - #27899 (comment) cc @nrc).

From that issue:

I suppose, ident can be backward compatibly restricted to reject keywords and reserved identifiers with exception of path segment keywords super, self and Self (because you may want to write things like $seg: ident => $seg::foo::bar), but I don't see much benefit from such restriction.

The second case given in this issue is a very concrete benefit from that restriction. 😄

It looks like repeaters can pretty easily confuse macro_rules! even in more ridiculous cases. e.g.

macro_rules! my_struct {
    ($(#[$meta:meta])* $ident:ident) => {
        $(#[$meta])* struct $ident;
    }
}

my_struct!(#[derive(Debug)] Foo);

causes it to complain that it doesn't know if # is supposed to be an ident or not

@petrochenkov

The repetition thing is because macro_rules just doesn't attempt any backtracking. This has been reported before. You need to put a constant token in between (e.g. struct). It's obviously not backwards compatible to change the definition of ident now, I don't know what @petrochenkov was talking about in that quote...

One specific common macro that this would break is Serde's forward_to_deserialize_any which accepts "struct" and "enum" as idents.

Auto merge of #42913 - kennytm:fix-40569-ident-without-backtrack, r=j…

Closing as works-as-expected and cannot-be-changed.

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`macro_rules!` thinks `use` is an `ident`. It isn't. #40569

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macro_rules! thinks use is an ident. It isn't. #40569

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petrochenkov commented on Mar 16, 2017

sgrif commented on Mar 16, 2017

sgrif commented on Mar 16, 2017

durka commented on Mar 16, 2017

dtolnay commented on Mar 16, 2017

petrochenkov commented on Aug 14, 2019

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`macro_rules!` thinks `use` is an `ident`. It isn't. #40569