Ah, and the README at https://github.com/rust-lang/rust/tree/master/src/grammar is completely misleading
In that folder, only the lexer is implemented in ANTLR4 (a Java tool). I think there was some code to compare it with rustc native lexer, but it isn’t run as part of the build.
Also that folder contains a lexer and parser definition for lex/bison (C tools). There is no code that compares syntax trees though.