rc,sync: Do not create references to uninitialized values

[taiki-e]

Closes#119241

r? @RalfJung

[RalfJung]

Sorry, I can't review libs changes.
r? libs

[Noratrieb]

Even if it's decided to not be UB, I think this still makes the code nicer. Thanks!
@bors r+

[bors]

📌 Commit 4e973b0 has been approved by Nilstrieb

It is now in the queue for this repository.

[bors]

⌛ Testing commit 4e973b0 with merge dfe53af...

[bors]

☀️ Test successful - checks-actions
Approved by: Nilstrieb
Pushing dfe53af to master...

[rust-timer]

Finished benchmarking commit (dfe53af): 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)	-	-	0
Improvements ✅ (primary)	-5.5%	[-5.5%, -5.5%]	1
Improvements ✅ (secondary)	-3.1%	[-3.1%, -3.1%]	1
All ❌✅ (primary)	-5.5%	[-5.5%, -5.5%]	1

Cycles

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

Binary size

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.2%]	4
All ❌✅ (primary)	-	-	0

Bootstrap: 662.447s -> 662.823s (0.06%)
Artifact size: 308.28 MiB -> 308.32 MiB (0.01%)