Clean up some traits

[nnethercote]

I was looking at various traits and found some unnecessary trait bounds, and some unnecessary traits. Details in individual commits.

r? @Nadrieril

[rustbot]

Some changes occurred in exhaustiveness checking

cc @Nadrieril

rustc_error_messages was changed

cc @TaKO8Ki

[rustbot]

Nadrieril is currently at their maximum review capacity.
They may take a while to respond.

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

  local time: Fri May  1 03:35:52 UTC 2026
  network time: Fri, 01 May 2026 03:35:52 GMT
##[endgroup]
sccache: Starting the server...
sccache: error: Server startup failed: cache storage failed to read: ConfigInvalid (permanent) at read => S3Error { code: "NoSuchBucket", message: "The specified bucket does not exist", resource: "", request_id: "1K1AZHQE18A8M48V" }

Context:
   uri: https://s3.us-west-1.amazonaws.com/rust-lang-ci-sccache2/.sccache_check
   response: Parts { status: 404, version: HTTP/1.1, headers: {"x-amz-request-id": "1K1AZHQE18A8M48V", "x-amz-id-2": "bCg8tcYfcWq6poWKOQDT7WHmJ4kaRu3WsiccWsvK3UCtGrP//HPOCpbxsmy/eKxatiePAMC94YTfke8qZeaOO9ldvfrOI5/Z", "content-type": "application/xml", "transfer-encoding": "chunked", "date": "Fri, 01 May 2026 03:35:52 GMT", "server": "AmazonS3"} }
   service: s3
   path: .sccache_check
   range: 0-

Backtrace:
---
[TIMING:end] compile::StdLink { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target_compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu, crates: [], force_recompile: false } -- 0.001
##[group]Building stage1 compiler artifacts (stage0 -> stage1, x86_64-unknown-linux-gnu)
error: process didn't exit successfully: `sccache /checkout/obj/build/bootstrap/debug/rustc -vV` (exit status: 2)
--- stderr
sccache: error: Server startup failed: cache storage failed to read: ConfigInvalid (permanent) at read => S3Error { code: "NoSuchBucket", message: "The specified bucket does not exist", resource: "", request_id: "P9636RSR62Y0P898" }

Context:
   uri: https://s3.us-west-1.amazonaws.com/rust-lang-ci-sccache2/.sccache_check
   response: Parts { status: 404, version: HTTP/1.1, headers: {"x-amz-request-id": "P9636RSR62Y0P898", "x-amz-id-2": "as/7i9zfbHJvTZ+XTImWsnvMwJnHj/EC6mIPH9ac8mHB/PU3LjphiAHEFFrZE0tpvKwwBnxT5gI=", "content-type": "application/xml", "transfer-encoding": "chunked", "date": "Fri, 01 May 2026 03:36:54 GMT", "server": "AmazonS3"} }
   service: s3
   path: .sccache_check
   range: 0-

Backtrace:
---
  11: <unknown>
  12: <unknown>


Run with SCCACHE_LOG=debug SCCACHE_NO_DAEMON=1 to get more information

Bootstrap failed while executing `build --stage 2 compiler rustdoc`
Build completed unsuccessfully in 0:00:26
  local time: Fri May  1 03:36:55 UTC 2026
  network time: Fri, 01 May 2026 03:36:55 GMT