Port diagnosttic::on_unimplemented

[mejrs]

I mostly just want a perf run to see if this is the right way to go (thoughts welcome btw)

won't pass ci (needs #151516), also it's a huge mess of duplication rn

r? ghost

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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

fmt check
fmt: checked 6663 files
tidy check
tidy [rustdoc_json (src)]: `rustdoc-json-types` modified, checking format version
tidy [error_codes]: Error code `E0230` exists, but is not emitted by the compiler!
Please mark the code as no longer emitted by adding the following note to the top of the `EXXXX.md` file:
`#### Note: this error code is no longer emitted by the compiler`
Also, do not forget to mark doctests that no longer apply as `ignore (error is no longer emitted)`.
tidy [error_codes]: FAIL
tidy: Skipping binary file check, read-only filesystem
tidy [style (compiler)]: /checkout/compiler/rustc_attr_parsing/src/attributes/diagnostic/on_unimplemented.rs:13: TODO is used for tasks that should be done before merging a PR; If you want to leave a message in the codebase use FIXME
tidy [style (compiler)]: /checkout/compiler/rustc_lint/messages.ftl: missing trailing newline
tidy [style (compiler)]: FAIL
tidy: The following checks failed: error_codes, style (compiler)
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 yarn` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1612:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1358:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Build completed unsuccessfully in 0:00:52
  local time: Fri Jan 23 23:52:19 UTC 2026
  network time: Fri, 23 Jan 2026 23:52:20 GMT
##[error]Process completed with exit code 1.

[JonathanBrouwer]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: e5e5a65 (e5e5a6588240040eb81cfb52d8b9c3281547ef47, parent: d222ddc4d90743dfc1e53b610be8fc9d95893d2c)

[rust-timer]

Finished benchmarking commit (e5e5a65): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.3%	[0.3%, 0.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -1.7%, secondary -0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.9%	[3.9%, 3.9%]	1
Improvements ✅ (primary)	-1.7%	[-1.7%, -1.7%]	1
Improvements ✅ (secondary)	-4.2%	[-4.2%, -4.2%]	1
All ❌✅ (primary)	-1.7%	[-1.7%, -1.7%]	1

Cycles

Results (secondary -4.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.3%	[-4.9%, -3.0%]	3
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 470.684s -> 471.14s (0.10%)
Artifact size: 383.48 MiB -> 383.64 MiB (0.04%)