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It partially expands crate attributes before the main expansion pass (without modifying the crate), and the produced preliminary crate attribute list is used for querying a few attributes that are required very early. Crate-level cfg attributes are then expanded normally during the main expansion pass, like attributes on any other nodes.
The driver crate is effectively the "main" function for the rust
compiler. It orchestrates the compilation process and "knits together"
the code from the other crates within rustc. This crate itself does
not contain any of the "main logic" of the compiler (though it does
have some code related to pretty printing or other minor compiler
options).
For more information about how the driver works, see the rustc dev guide.