Rollup merge of #120590 - compiler-errors:dead, r=Nilstrieb

Remove unused args from functions

`#[instrument]` suppresses the unused arguments from a function, *and* suppresses unused methods too! This PR removes things which are only used via `#[instrument]` calls, and fixes some other errors (privacy?) that I will comment inline.

It's possible that some of these arguments were being passed in for the purposes of being instrumented, but I am unconvinced by most of them.

compiler/rustc_trait_selection/src/solve/assembly/mod.rs

@@ -826,7 +826,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
     #[instrument(level = "debug", skip(self), ret)]
     pub(super) fn merge_candidates(
         &mut self,
-        mut candidates: Vec<Candidate<'tcx>>,
+        candidates: Vec<Candidate<'tcx>>,
     ) -> QueryResult<'tcx> {
         // First try merging all candidates. This is complete and fully sound.
         let responses = candidates.iter().map(|c| c.result).collect::<Vec<_>>();