Auto merge of #4123 - Centril:rustup-let-chains-ast, r=Manishearth
Fix fallout from rust-lang/rust PR 60861
Fixes incoming breakage for unlanded https://github.com/rust-lang/rust/pull/60861.
Tests are passing locally; the Rust PR now needs to land first.
@Manishearth also says we'll want to split out to a `collapsible_if_let` once we have let-chains working in Rust nightly or something.