77e1069a5d
The change was "Show invisible delimiters (within comments) when pretty printing". It's useful to show these delimiters, but is a breaking change for some proc macros. Fixes #97608.
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: