> I kinda thought something like this was already being done
It is. When building a crate, Cargo doesn't need to wait for all of that crate's dependencies to be completely finished compiling, instead it only needs to wait for each dependency to produce its metadata. The prototype here builds upon that existing machinery by causing metadata to be written earlier than it would otherwise be, prior to complete typechecking. It has the potential to increase the level of effective parallelism, so whether or not this has a benefit for a particular crate graph will depend on whether or not all of your cores are already effectively occupied during compilation or whether Cargo is letting cores lie idle while waiting on metadata to be produced.
It is. When building a crate, Cargo doesn't need to wait for all of that crate's dependencies to be completely finished compiling, instead it only needs to wait for each dependency to produce its metadata. The prototype here builds upon that existing machinery by causing metadata to be written earlier than it would otherwise be, prior to complete typechecking. It has the potential to increase the level of effective parallelism, so whether or not this has a benefit for a particular crate graph will depend on whether or not all of your cores are already effectively occupied during compilation or whether Cargo is letting cores lie idle while waiting on metadata to be produced.