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The extra span is now recorded in the new `TokenKind::NtIdent` and `TokenKind::NtLifetime`. These both consist of a single token, and so there's no operator precedence problems with inserting them directly into the token stream. The other way to do this would be to wrap the ident/lifetime in invisible delimiters, but there's a lot of code that assumes an interpolated ident/lifetime fits in a single token, and changing all that code to work with invisible delimiters would have been a pain. (Maybe it could be done in a follow-up.) This change might not seem like much of a win, but it's a first step toward the much bigger and long-desired removal of `Nonterminal` and `TokenKind::Interpolated`. That change is big and complex enough that it's worth doing this piece separately. (Indeed, this commit is based on part of a late commit in #114647, a prior attempt at that big and complex change.)
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: