Defining generics in a macro inside macro causes infinite indentation

[maxcabrajac]

macro_rules! example {
    (good) => {
        macro_rules! good {
            () => {
                A
            };
        }
    };
    (bad) => {
        macro_rules! bad {
            () => {
                A<'a>
            };
        }
    };
    (alsobad) => {
        macro_rules! bad {
            () => {
                A<B>
            };
        }
    };
}

Running multiple rounds of fmt in this causes both versions of bad's body to be indented indefinitely, but not good's.
Might be related to #4609, #5044 and other issues involving infinite indentation, but none of them mentioned generics.

[Jhynjhiruu]

It seems like it's related to parsing of things that aren't valid expressions going haywire; the same bug occurs with this example:

macro_rules! bug {
    () => {
        macro_rules! whee {
            () => {
                mut
            };
        }
    };
}

but also with ref, pub, &, @, #, =, | and various other punctuation characters in place of the mut. Essentially, if the body is a valid expression, no bug occurs, otherwise it will.

(Looks like almost this has been noticed before: #4609 (comment))