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]

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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

[rust-log-analyzer]

The job mingw-check-1 failed! Check out the build log: (web) (plain)

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

[RUSTC-TIMING] tracing_core test:false 0.800
error: value assigned to `modified` is never read
   --> src/tools/rust-analyzer/crates/ide-db/src/imports/merge_imports.rs:385:29
    |
385 | / ...                   modified |=
386 | | ...                       recursive_normalize(&sub_sub_tree, NormalizationStyle::Default)
387 | | ...                           .is_some();
    | |________________________________________^
    |
    = help: maybe it is overwritten before being read?
    = note: `-D unused-assignments` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(unused_assignments)]`

[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%)

[flip1995]

NIT: Isn't unused allowed by default in the ui_test framework? Why is this explicit allow necessary here? 🤔