Perform unused assignment and unused variables lints on MIR.

[cjgillot]

Rebase of #101500

Fixes #51003.

The first commit moves detection of uninhabited types from the current liveness pass to MIR building.

In order to keep the same level of diagnostics, I had to instrument MIR a little more:

• keep for which original local a guard local is created;
• store in the VarBindingForm the list of introducer places and whether this was a shorthand pattern.

I am not very proud of the handling of self-assignments. The proposed scheme is in two parts: first detect probable self-assignments, by pattern matching on MIR, and second treat them specially during dataflow analysis. I welcome ideas.

Please review carefully the changes in tests. There are many small changes to behaviour, and I'm not sure all of them are desirable.

[rustbot]

r? @wesleywiser

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[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

Some changes occurred in match lowering

cc @Nadrieril

Some changes occurred in match checking

cc @Nadrieril

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[cjgillot]

@bors2 try @rust-timer queue

[rust-bors]

⌛ Trying commit 1d8df21 with merge 50ba7f2…

To cancel the try build, run the command @bors2 try cancel.

[bors]

⌛ Trying commit 1d8df21 with merge b0d7157...

[rust-bors]

☀️ Try build successful (CI)
Build commit: 50ba7f2 (50ba7f2d41092ac03e7b2d705a93e2e3e56bbf24, parent: 573a01569000d395498a5f98f916d6e5305ac81a)

[bors]

☀️ Try build successful - checks-actions
Build commit: b0d7157 (b0d7157996aa3b283f7f26ec09fc81c1612b5fce)

[rust-timer]

Finished benchmarking commit (50ba7f2): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

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.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	0.6%	[0.2%, 1.4%]	65
Regressions ❌ (secondary)	0.7%	[0.2%, 2.8%]	103
Improvements ✅ (primary)	-0.5%	[-0.6%, -0.5%]	2
Improvements ✅ (secondary)	-0.4%	[-0.7%, -0.3%]	22
All ❌✅ (primary)	0.6%	[-0.6%, 1.4%]	67

Max RSS (memory usage)

Results (primary 0.8%, secondary 0.2%)

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

	mean	range	count
Regressions ❌ (primary)	1.6%	[1.0%, 2.9%]	12
Regressions ❌ (secondary)	2.5%	[0.7%, 9.0%]	14
Improvements ✅ (primary)	-4.2%	[-7.0%, -1.4%]	2
Improvements ✅ (secondary)	-6.2%	[-10.9%, -2.3%]	5
All ❌✅ (primary)	0.8%	[-7.0%, 2.9%]	14

Cycles

Results (primary 1.3%, secondary 3.2%)

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

	mean	range	count
Regressions ❌ (primary)	1.3%	[0.9%, 1.7%]	14
Regressions ❌ (secondary)	3.2%	[2.4%, 4.4%]	11
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.3%	[0.9%, 1.7%]	14

Binary size

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

Bootstrap: 754.801s -> 755.907s (0.15%)
Artifact size: 372.21 MiB -> 372.18 MiB (-0.01%)

[davidtwco]

Apologies for the delay in getting to this. I'm not the most familiar with doing this type of analysis on the MIR but it all looks right to me and is very well commented. r=me after rebase.

[cjgillot]

@davidtwco the last commit in this rebase is an actual bugfix for the CI failures.
What's your take on the perf regression and on some of the lost diagnostic suggestions?

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[davidtwco]

What's your take on the perf regression and on some of the lost diagnostic suggestions?

I think the perf regression is reasonable for the improved lint - this seems to catch many more cases and is a more natural part of the compiler for this analysis. I had another look through the diagnostic suggestions and I couldn't see many lost suggestions, but maybe I missed them again? The diagnostics seem largely comparable to the previous ones.

[bors]

☔ The latest upstream changes (presumably #145728) made this pull request unmergeable. Please resolve the merge conflicts.