ICE in E0657.rs and impl_trait_projections.rs on nightly

[nikomatsakis]

Nightly ICEs when building two tests from the ui test suite for me:

• src/test/ui/error-codes/E0657.rs
• src/test/ui/impl_trait_projections.rs

Transcript here.

Apparently this does not always occur when running the full test suite.

cc @oli-obk

[oli-obk]

Fixed by #51773