fix(len_zero): don't eagerly call GenericArgs::type_at

[ada4a]

Fixes #15657

changelog: [len_zero]: fix ICE when fn len has a return type without generic type params

r? @y21

[y21]

Thanks for the quick fix! :)

View changes since this review

[y21]

I believe this fix wouldn't make it into the same release cycle as the commit that introduced it, and the repro seems not too uncommon for people to hit, so let's also: @rustbot label +beta-nominated

[samueltardieu]

It is for 0.90? In this case, this might not make it, as the tarball must be ready already. It would be a stable merge in case there is a point release.

Ping @flip1995

Edit: the bug report was for 0.91, so on the contrary we may have time.
Edit 2: yes, the bug was introduced one month ago, so it will be in 0.91 only