Don't emit no-op &T → &T reborrows

[Jules-Bertholet]

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Except inside closures, where we need to keep capture inference stable. Gets rid of MIR bloat, and improves diagnostics.

See also #163717 (would allow doing this even inside closures), #163494 (would allow doing this for mutable references outside closures).

r? compiler

@rustbot label A-coercions

[rustbot]

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

cc @rust-lang/clippy

[Jules-Bertholet]

To make tests/codegen-llvm/lib-optimizations/append-elements.rs happy. That test is really fragile…

View changes since the review

[jieyouxu]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: d77c4b4 (d77c4b4f551a87fc763f35c94b4177493a25ab83)
Base parent: 0f6e5bf (0f6e5bf4c63a0ab336cc62de446b77123effaffc)

[rust-timer]

Finished benchmarking commit (d77c4b4): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

@bors rollup=never rustc-perf
@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)	0.2%	[0.2%, 0.2%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.7%	[-3.9%, -0.1%]	175
Improvements ✅ (secondary)	-1.2%	[-7.0%, -0.1%]	151
All ❌✅ (primary)	-0.7%	[-3.9%, 0.2%]	176

Max RSS (memory usage)

Results (primary -2.1%, secondary -2.9%)

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)	3.1%	[2.1%, 4.2%]	2
Improvements ✅ (primary)	-2.1%	[-6.2%, -0.8%]	16
Improvements ✅ (secondary)	-3.7%	[-6.7%, -0.7%]	16
All ❌✅ (primary)	-2.1%	[-6.2%, -0.8%]	16

Cycles

Results (primary -2.7%, secondary -4.6%)

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)	7.1%	[7.1%, 7.1%]	1
Improvements ✅ (primary)	-2.7%	[-3.1%, -2.2%]	5
Improvements ✅ (secondary)	-5.4%	[-8.9%, -1.8%]	15
All ❌✅ (primary)	-2.7%	[-3.1%, -2.2%]	5

Binary size

Results (primary 0.1%, 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)	0.3%	[0.1%, 1.1%]	27
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	1
Improvements ✅ (primary)	-0.3%	[-1.0%, -0.0%]	22
Improvements ✅ (secondary)	-0.2%	[-0.5%, -0.1%]	12
All ❌✅ (primary)	0.1%	[-1.0%, 1.1%]	49

Bootstrap: 486.533s -> 486.402s (-0.03%)
Artifact size: 406.44 MiB -> 406.63 MiB (0.05%)

[theemathas]

Does this have implications on what Miri can catch?

[Jules-Bertholet]

Does this have implications on what Miri can catch?

It shouldn't. Miri does retags on every typed copy.

[rustbot]

This PR was rebased onto a different main 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.

[adwinwhite]

I think the intention of this PR is to skip more no-op reborrows?
Some explanation on what the "more" part is would be awesome.
As someone who's not super familiar with coercions, this isn't too obvious :>

View changes since this review

[adwinwhite]

style: the large if expr is kinda difficult to read as a conjunction component?

[Jules-Bertholet]

I've rewritten this if, hopefully it is more clear now.

[adwinwhite]

A little bit confused here and also about other similar comments.
Immutable reborrow doesn't affect capture inference since they're copy anyway?

[Jules-Bertholet]

The relevant test is https://github.com/rust-lang/rust/blob/701ce39ddc3b02efdf5146fd76c861b06c52aeb3/tests/ui/coercion/structural_identity_dependent_reborrows.rs.

Consider the following example:

let x: &u8 = &42;
let closure = || some_function(/* Implicit `&*` reborrow here? */ x);

If there is no reborrow, we capture x by reference. But if there is one, with edition 2021 precise capturing, we capture *x by reference, which is equivalent to capturing x by value.