Lint on invalid usage of UnsafeCell::raw_get in reference casting

[Urgau]

This PR proposes to take into account UnsafeCell::raw_get method call for non-Freeze types for the invalid_reference_casting lint.

The goal of this is to catch those kind of invalid reference casting:

fn as_mut<T>(x: &T) -> &mut T {
    unsafe { &mut *std::cell::UnsafeCell::raw_get(x as *const _ as *const _) }
    //~^ ERROR casting `&T` to `&mut T` is undefined behavior
}

r? @est31

[Urgau]

@RalfJung can you confirm (or deny) that we can lint on those expressions ?

[RalfJung]

Those expressions are definitely bogus.

Can you confirm that the lint will not fire if the type of x is changed to &Cell<i32>?

[Urgau]

Yes I can.

I already added some tests below to make sure we don't lint on them, but added a test with &Cell<i32> anyway (just to be on the safe side).

[est31]

@bors r+

[bors]

📌 Commit 89800a2 has been approved by est31

It is now in the queue for this repository.

[bors]

⌛ Testing commit 89800a2 with merge 4e5b31c...

[bors]

☀️ Test successful - checks-actions
Approved by: est31
Pushing 4e5b31c to master...

[rust-timer]

Finished benchmarking commit (4e5b31c): 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)	-0.2%	[-0.2%, -0.2%]	1
Improvements ✅ (secondary)	-0.6%	[-0.6%, -0.6%]	2
All ❌✅ (primary)	-0.2%	[-0.2%, -0.2%]	1

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)	0.7%	[0.7%, 0.7%]	1
Regressions ❌ (secondary)	1.1%	[1.1%, 1.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.7%	[0.7%, 0.7%]	1

Cycles

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)	3.3%	[3.3%, 3.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 628.074s -> 628.991s (0.15%)
Artifact size: 317.96 MiB -> 317.97 MiB (0.00%)