Work through temporarily allowed clippy lints, part 1
This is the first batch of not allowing but actually fixing the clippy lints. Each commit removes one lint from the lint table and then fixes the resulting warnings.
Follow-up to #16401
Expand lint tables && make clippy happy 🎉
This PR expands the lint tables on `./Cargo.toml` and thereby makes `cargo clippy` exit successfully! 🎉Fixes#15918
## How?
In the beginning there are some warnings for rustc.
Next, and most importantly, there is the clippy lint table. There are a few sections in there.
First there are the lint groups.
Second there are all lints which are permanently allowed with the reasoning why they are allowed.
Third there is a huge list of temporarily allowed lints. They should be removed in the mid-term, but incur a substantial amount of work, therefore they are allowed for now and can be worked on bit by bit.
Fourth there are all lints which should warn.
Additionally there are a few allow statements in the code for lints which should be permanently allowed in this specific place, but not in the whole code base.
## Follow up work
- [ ] Run clippy in CI
- [ ] Remove tidy test (at least `@Veykril` wrote this in #15017)
- [ ] Work on temporarily allowed lints
internal: Record FnAbi
This unfortunately breaks our lub coercions, so will need to look into fixing that first, though I am not sure what is going wrong where...
Stubbed some stuff out for the time being.
