mgca: Lower all const paths as ConstArgKind::Path

[camelid]

When #![feature(min_generic_const_args)] is enabled, we now lower all
const paths in generic arg position to hir::ConstArgKind::Path. We
then lower assoc const paths to ty::ConstKind::Unevaluated since we
can no longer use the anon const expression lowering machinery. In the
process of implementing this, I factored out hir_ty_lowering code that
is now shared between lowering assoc types and assoc consts.

This PR also introduces a #[type_const] attribute for trait assoc
consts that are allowed as const args. However, we still need to
implement code to check that assoc const definitions satisfy
#[type_const] if present (basically is it a const path or a
monomorphic anon const).

r? @BoxyUwU

[rustbot]

Some changes occurred in const_evaluatable.rs

cc @BoxyUwU

HIR ty lowering was modified

cc @fmease

[BoxyUwU]

thx for working on this :3

[bors]

☔ The latest upstream changes (presumably #135896) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #136024) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #135726) made this pull request unmergeable. Please resolve the merge conflicts.

[BoxyUwU]

lgtm, once you've squashed the commits ill r+ (and probably should update the PR description/title)

[BoxyUwU]

@bors r+ rollup=never

[bors]

📌 Commit 177e7ff has been approved by BoxyUwU

It is now in the queue for this repository.

[bors]

⌛ Testing commit 177e7ff with merge 08db600...

[bors]

☀️ Test successful - checks-actions
Approved by: BoxyUwU
Pushing 08db600 to master...

[rust-timer]

Finished benchmarking commit (08db600): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -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 (secondary 0.6%)

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)	2.1%	[1.8%, 2.4%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.3%	[-2.3%, -2.3%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -2.1%)

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)	-2.1%	[-2.1%, -2.1%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 778.226s -> 775.839s (-0.31%)
Artifact size: 362.05 MiB -> 362.04 MiB (-0.00%)