Allow partially moved values in match

[cjgillot]

This PR attempts to unify the behaviour between let _ = PLACE, let _: TY = PLACE; and match PLACE { _ => {} }.
The logical conclusion is that the match version should not check for uninitialised places nor check that borrows are still live.

The match PLACE {} case is handled by keeping a FakeRead in the unreachable fallback case to verify that PLACE has a legal value.

Schematically, match PLACE { arms } in surface rust becomes in MIR:

PlaceMention(PLACE)
match PLACE {
  // Decision tree for the explicit arms
  arms,
  // An extra fallback arm
  _ => {
    FakeRead(ForMatchedPlace, PLACE);
    unreachable
  }
}

match *borrow { _ => {} } continues to check that *borrow is live, but does not read the value.
match *borrow {} both checks that *borrow is live, and fake-reads the value.

Continuation of #102256 #104844

Fixes #99180 #53114

[rustbot]

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @davidtwco, @celinval, @vakaras

[rust-highfive]

r? @eholk

(rust-highfive has picked a reviewer for you, use r? to override)

[rustbot]

Some changes occurred to the CTFE / Miri engine

cc @rust-lang/miri

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

[pnkfelix]

The lang team wants to have a design meeting about PR #102256 and PR #103208 ; @cjgillot I propose that you and I work together to make a design doc about these changes in terms of what is surfaced at the language level.

[scottmcm]

On the let _: Ty = b; example:

What happens there when it's more than just an ascription because it's a coercion site?

For example,

let b: Box<String> = …;
let _: Box<dyn Display> = b;
return b;

I guess that the coercion becomes an explicit statement in MIR, so that would still be rejected even though the AscribeUserType in MIR wouldn't be considered a use any more?

[eholk]

I'm probably not the best person to review this, so I'll re-roll.

r? @rust-lang/compiler

[bors]

☔ The latest upstream changes (presumably #103787) made this pull request unmergeable. Please resolve the merge conflicts.

[lcnr]

r? @oli-obk maybe 🤔 cc @RalfJung

[bors]

☔ The latest upstream changes (presumably #103138) made this pull request unmergeable. Please resolve the merge conflicts.

[oli-obk]

Triage: This is still waiting on #102256

[rustbot]

The Miri subtree was changed

cc @rust-lang/miri

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[RalfJung]

The Miri tests LGTM. @oli-obk wrote

The impl lgtm.

but that was in May. Oli if you could briefly skim this again to make sure it still looks good to you that would be great. :)

[oli-obk]

Did another review of the impl and mir opt test changes. Nothing in mir or mir building has changed that would affect these changes, and they look good to me (I don't remember them from the last review, so this was a review from scratch 😅)

@bors r=oli-obk,RalfJung

[bors]

📌 Commit 479fb4b has been approved by oli-obk,RalfJung

It is now in the queue for this repository.

[bors]

⌛ Testing commit 479fb4b with merge 59bb950...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk,RalfJung
Pushing 59bb950 to master...

[rust-timer]

Finished benchmarking commit (59bb950): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.9%	[1.2%, 3.5%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

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

Binary size

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

Bootstrap: 635.347s -> 636.148s (0.13%)
Artifact size: 304.54 MiB -> 304.51 MiB (-0.01%)

[RalfJung]

It took more than a year, but it's finally done. :) Thanks a lot @cjgillot for seeing this through to the end!

[WaffleLapkin]

Should we add relnotes Marks issues that should be documented in the release notes of the next release. here? Might be useful to include in the release notes that we now accept more code

[RalfJung]

Sure, why not.