The flate2 dependency is included by cargo in the following way:

This includes the on-by-default rust_backend feature, which adds the miniz_oxide dependency to flate2 and the adler32 transitive dependency. In the rustc repo you can find the following note though, saying that adler32 is not used:

rust-lang/rust@61fe2e4#diff-3ac60df36be32d72842bf5351fc2bb1dR43

It might be better to turn off the default features of flate2 so that the unused crate doesn't have to be compiled by people.