expand free alias types in the auto-trait orphan check

[Dnreikronos]

fixes #157756.

auto-trait impls are only allowed on nominal types. the check for that looked at the self type as written, without expanding it, so a lazy type alias got rejected even when it pointed straight at a type that's fine on its own:

#![feature(lazy_type_alias)]
struct Local;
type Alias = Local;
unsafe impl Sync for Alias {} // used to be E0321

the locality check right above it already normalizes the self type, so the two were looking at different things. imo the cleanest fix is to expand free aliases first, with the same expand_free_alias_tys coherence already calls in a couple of other spots. anything that should still be rejected (trait objects, opaque types, bare type params) survives the expansion and errors like before.

one thing worth a second look: impl_for_weak_alias.rs used type Alias = (impl Sized, u8) and expected an error. idk if that was deliberate, but the alias expands to a tuple, and a tuple is fine for a local auto trait if you write it by hand, so it passes now and i made it check-pass. lmk if i'm missing why it should still be rejected.

[rustbot]

r? @petrochenkov

rustbot has assigned @petrochenkov.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 73 candidates
• Random selection from 19 candidates

[fmease]

r? me

[fmease]

Shouldn't affect perf but better be certain:

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 7e108df (7e108df05a7548d79f478037d58030aca9dbaefe, parent: edff07ce1a97f3eb39d438852ace6c57dd97605e)

[rust-timer]

Finished benchmarking commit (7e108df): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 4.8%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	4.8%	[4.6%, 5.0%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	4.8%	[4.6%, 5.0%]	2

Cycles

Results (primary 1.6%, secondary 22.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.6%	[1.0%, 2.2%]	2
Regressions ❌ (secondary)	22.7%	[19.4%, 24.4%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.6%	[1.0%, 2.2%]	2

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 519.155s -> 516.812s (-0.45%)
Artifact size: 400.88 MiB -> 401.47 MiB (0.15%)