dfa5e46fd5
Originally, there has been a dedicated pass for renumbering AST NodeIds to have actual values. This pass had been added by commit a5ad4c379466519a0bf977864a5cdc50a7ade385. Then, later, this step was moved to where it resides now, macro expansion. See commit c86c8d41a26b2037e80c9fd028a59313a78b3a66 or PR #36438. The comment snippet, added by the original commit, has survived the times without any change, becoming outdated at removal of the dedicated pass. Nowadays, grepping for the next_node_id function will show up multiple places in the compiler that call it, but the main rewriting that the comment talks about is still done in the expansion step, inside an innocious looking visit_id function that's called during macro invocation collection.
The rustc_ast crate contains those things concerned purely with syntax
– that is, the AST ("abstract syntax tree"), parser, pretty-printer,
lexer, macro expander, and utilities for traversing ASTs.
For more information about how these things work in rustc, see the rustc dev guide: