fd7a159710
Convert all the crates that have had their diagnostic migration completed (except save_analysis because that will be deleted soon and apfloat because of the licensing problem).
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: