Macro Expansion Hangs/OOMs

[levinwinter]

macro_rules! m {
    [ $( $( ),+ )* ] => {}
}

m! {}

The above code hangs and quickly exhausts memory. This happens on nightly and dates back all the way back to version 1.0.0.

You can safely run the code on the playground here.

This bug was found using fuzzing.

@rustbot label +C-bug +A-macros +I-hang +S-has-mcve

[matthiaskrgr]

is this a duplicate of #123848 ?

[levinwinter]

I checked the backtrace in both cases with gdb, and they do both end up stuck in fully_expand_fragment. That said, I think this issue is distinct from the one you linked.

In the other issue, the problem comes from a recursive macro that doubles its input on each expansion step. A simplified version looks like this:

macro_rules! m {
    [ $($tt:tt)* ] => { m! { $($tt)* $($tt)* } }
}

m! { _ }

Maybe it helps to look at a variant of this report that does not loop.

 macro_rules! m {
-    [ $( $( ),+ )* ] => {}
+    [ $( $( ) + )* ] => {}
 }

 m! {}

This fails with error: repetition matches empty token tree.

I’m not sure whether this report should trigger that same error instead of looping, but at least the underlying issue seems different from the recursive case.