ba928ba041
Teach parser to understand fake anonymous enum syntax Parse `Ty | OtherTy` in function argument and return types. Parse type ascription in top level patterns. Minimally address #100741.
The rustc_ast crate contains those things concerned purely with syntax
– that is, the AST ("abstract syntax tree"), along with some definitions for tokens and token streams, data structures/traits for mutating ASTs, and shared definitions for other AST-related parts of the compiler (like the lexer and macro-expansion).
For more information about how these things work in rustc, see the rustc dev guide: