Enable warn(unused_crate_dependencies) on rustc_*` crates

[nnethercote]

Proposal

It's bad having unused dependencies in Cargo.toml files. There's probably some kind of compile time cost, but more importantly, it's just misleading when reading the code.[*]

In the past I have looked for unused dependencies in a brute force fashion: comment out all dependencies in a Cargo.toml file, and then add them back in until it compiles again. This works but is slow and tedious.

Then I learned the unused_crate_dependencies lint, which is much better. In rust-lang/rust#126063 I removed 19 unused dependencies that I found by changing unused_crate_dependencies from Allow to Warn. But there were a few false positives so doing this everywhere didn't seem feasible.

Today I redid this exercise and found another 15 unused dependencies that had crept in over the past nine months. (And I recently removed another two in rust-lang/rust#137776 that I found with my own eyeballs.) Based on that, I think it's worth pushing harder on enabling this lint for the compiler.

I propose adding warn(unused_crate_dependencies) to all rustc_* crates, except for those where the lint gives false positives, which would instead get allow(unused_crate_dependencies) along with an explanatory comment.

(An alternative is to pass -Wunused-crate-dependencies to all rustc_* crates. That would require touching fewer lines, and would also automatically extend the functionality to new crates.)

[*] Something I look at quite often is a graph of the crate dependencies, generated with these commands:

cargo +nightly depgraph --all-deps --dedup-transitive-deps --workspace-only > ~/graph.dot;
dot -Tpng ~/graph.dot > ~/graph.png

Here is an example one:

It's good to not have unused edges in this graph.

Mentors or Reviewers

None. Two possible PRs implementing this are at rust-lang/rust#137911 and rust-lang/rust#137930.

Process

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[rustbot]

Important

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cc @rust-lang/compiler

[oli-obk]

@rustbot second