Infinite memory use with nested * repetition in macro

[ghost]

Play rust link: http://is.gd/DVjBtk

macro_rules! a {(<$($($d:expr),*$($c:ident),*)*>) => (B<$($d,)*$($c,)*>);}a! (<>);

[huonw]

Minimal example:

macro_rules! a {
    ($($($d:tt)*)*) => ();
}

a!();

[zakarumych]

So problem code is double repetition?
Is there any workaround?

[jonas-schievink]

I don't think the root problem is a double-repetition, but rather a repetition of something that can be 0 tokens long. In this case $($d:tt)* can contain 0 tokens, which then cause the memory explosion on the attempt to repeat it. Replacing the inner repetition with $($d:tt)+ prevents this bug, changing the outer repetition to use + doesn't.

See #28085, which I think is a duplicate of this (tl;dr: replacing the matcher with just $()+ or $()* also causes this).

[jonas-schievink]

Also, this looks like a duplicate of #5067