Convert moves of references to copies in ReferencePropagation

[saethlin]

This is a fix for #141101.

The root cause of this miscompile is that the SsaLocals analysis that MIR transforms use is supposed to detect locals that are only written to once, in their single assignment. But that analysis is subtly wrong; it does not consider Operand::Move to be a write even though the meaning ascribed to Operand::Move (at least as a function parameter) by Miri is that the callee may have done arbitrary writes to the caller's Local that the Operand wraps (because Move is pass-by-pointer). So Miri conwiders Operand::Move to be a write but both the MIR visitor system considers it a read, and so does SsaLocals.

I have tried fixing this by changing the PlaceContext that is ascribed to an Operand::Move to a MutatingUseContext but that seems to have borrow checker implications, and changing SsaLocals seems to have wide-ranging regressions in MIR optimizations.

So instead of doing those, this PR adds a new kludge to ReferencePropagation, which follows the same line of thinking as the kludge in CopyProp that solves this same problem inside that pass:

rust/compiler/rustc_mir_transform/src/copy_prop.rs

Lines 65 to 98 in a5584a8

	/// `SsaLocals` computed equivalence classes between locals considering copy/move assignments.
	///
	/// This function also returns whether all the `move?` in the pattern are `move` and not copies.
	/// A local which is in the bitset can be replaced by `move _a`. Otherwise, it must be
	/// replaced by `copy _a`, as we cannot move multiple times from `_a`.
	///
	/// If an operand copies `_c`, it must happen before the assignment `_d = _c`, otherwise it is UB.
	/// This means that replacing it by a copy of `_a` if ok, since this copy happens before `_c` is
	/// moved, and therefore that `_d` is moved.
	#[instrument(level = "trace", skip(ssa, body))]
	fn fully_moved_locals(ssa: &SsaLocals, body: &Body<'_>) -> DenseBitSet<Local> {
	let mut fully_moved = DenseBitSet::new_filled(body.local_decls.len());
	for (_, rvalue, _) in ssa.assignments(body) {
	let (Rvalue::Use(Operand::Copy(place) | Operand::Move(place))
	| Rvalue::CopyForDeref(place)) = rvalue
	else {
	continue;
	};
	let Some(rhs) = place.as_local() else { continue };
	if !ssa.is_ssa(rhs) {
	continue;
	}
	if let Rvalue::Use(Operand::Copy(_)) | Rvalue::CopyForDeref(_) = rvalue {
	fully_moved.remove(rhs);
	}
	}
	ssa.meet_copy_equivalence(&mut fully_moved);
	fully_moved
	}

[saethlin]

@bors try @rust-timer queue

[bors]

⌛ Trying commit e41bd6c with merge 920f4b7...

[bors]

☀️ Try build successful - checks-actions
Build commit: 920f4b7 (920f4b72087ccff7231e54c1928f5f1ce0a2b98e)

[rust-timer]

Finished benchmarking commit (920f4b7): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

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

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	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 1.1%, secondary 9.7%)

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)	1.1%	[1.1%, 1.1%]	1
Regressions ❌ (secondary)	9.7%	[9.7%, 9.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.1%	[1.1%, 1.1%]	1

Cycles

Results (secondary 0.7%)

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)	4.3%	[4.3%, 4.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.8%	[-2.8%, -2.8%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 752.018s -> 753.039s (0.14%)
Artifact size: 372.42 MiB -> 372.38 MiB (-0.01%)