Infinite loop when using expr attributes inside macro expansion

[Swatinem]

Reproduction using lazy_static:

lazy_static::lazy_static! {
    static ref FOO: () = #[allow(missing_docs)]
    ();
}

I’m trying to further minimize the testcase.

I get the following with RA_LOG:

[WARN hir_expand::db] fail on macro_parse: (reason: BindingError("expected subtree") macro_call: __lazy_static_internal!(@MAKETY, ,(), $N);) parents: __lazy_static_internal!(@TAIL, $N: : = =);
__lazy_static_internal!(@TAIL, $N:()= $);
__lazy_static_internal!(@MAKETY, ,(), $N);
__lazy_static_internal!(@MAKETY, ,(), $N);
__lazy_static_internal!(@TAIL, $N:()= $);
__lazy_static_internal!(@MAKETY, ,(), $N);
__lazy_static_internal!(@MAKETY, ,(), $N);
__lazy_static_internal!(@TAIL, $N:()= $);
__lazy_static_internal!(@MAKETY, ,(), $N);
__lazy_static_internal!(@MAKETY, ,(), $N);
__lazy_static_internal!(@MAKETY, ,(), $N);
__lazy_static_internal!(()staticref$N: $T= $e;);
lazy_static!((););
__lazy_static_internal!(()staticrefFOO:()= # ;(););
lazy_static::lazy_static! {
    static ref FOO: () = #[allow(missing_docs)]
    ();
}

the log message changes a bit, but its just looping there.

[Swatinem]

This can be easily worked around by wrapping both the attribute and the expression in braces. We hit this problem here btw: getsentry/relay#734