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This makes it possible to use liballoc/libstd in combination with `--emit obj` if you use `#[global_allocator]`. Making it work for the default libstd allocator would require weak functions, which are not well supported on all systems.
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: