rustc_layout_scalar_valid_range makes ctors unsafe

[compiler-errors]

We already validate this when we use the ctor in a call, e.g. Variant(1), but not if we use the ctor as a fn ptr, e.g. .map(Variant). The easiest way to fix the latter is (afaict) is by marking the ctor as unsafe itself.

Fixes #115284

[rustbot]

r? @cjgillot

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[cjgillot]

r=me with tests blessed.

[cjgillot]

@bors r+ rollup

[bors]

📌 Commit bf66723 has been approved by cjgillot

It is now in the queue for this repository.

[bors]

⌛ Testing commit bf66723 with merge 784916c...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing 784916c to master...

[rust-timer]

Finished benchmarking commit (784916c): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was 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)	-0.4%	[-0.4%, -0.4%]	1
All ❌✅ (primary)	-	-	0

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)	-1.8%	[-3.1%, -0.7%]	4
Improvements ✅ (secondary)	-2.5%	[-2.9%, -2.2%]	3
All ❌✅ (primary)	-1.8%	[-3.1%, -0.7%]	4

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: 632.412s -> 631.374s (-0.16%)
Artifact size: 316.49 MiB -> 316.55 MiB (0.02%)