Represent type-level consts with new-and-improved hir::ConstArg

[camelid]

Summary

This is a step toward min_generic_const_exprs. We now represent all const
generic arguments using an enum that differentiates between const paths
(temporarily just bare const params) and arbitrary anon consts that may perform
computations. This will enable us to cleanly implement the min_generic_const_args
plan of allowing the use of generics in paths used as const args, while
disallowing their use in arbitrary anon consts. Here is a summary of the salient
aspects of this change:

• Add current_def_id_parent to LoweringContext

  This is needed to track anon const parents properly once we implement
  ConstArgKind::Path (which requires moving anon const def-creation
  outside of DefCollector).

• Create hir::ConstArgKind enum with Path and Anon variants. Use it in the
  existing hir::ConstArg struct, replacing the previous hir::AnonConst field.

• Use ConstArg for all instances of const args. Specifically, use it instead
  of AnonConst for assoc item constraints, array lengths, and const param
  defaults.

• Some ast::AnonConsts now have their DefIds created in
  rustc_ast_lowering rather than DefCollector. This is because in some
  cases they will end up becoming a ConstArgKind::Path instead, which
  has no DefId. We have to solve this in a hacky way where we guess
  whether the AnonConst could end up as a path const since we can't
  know for sure until after name resolution (N could refer to a free
  const or a nullary struct). If it has no chance as being a const
  param, then we create a DefId in DefCollector -- otherwise we
  decide during ast_lowering. This will have to be updated once all path
  consts use ConstArgKind::Path.

• We explicitly use ConstArgHasType for array lengths, rather than
  implicitly relying on anon const type feeding -- this is due to the
  addition of ConstArgKind::Path.

• Some tests have their outputs changed, but the changes are for the
  most part minor (including removing duplicate or almost-duplicate
  errors). One test now ICEs, but it is for an incomplete, unstable
  feature and is now tracked at ICE(non_lifetime_binders): const bound params not handled correctly #127009.

Followup items post-merge

• Use ConstArgKind::Path for all const paths, not just const params.
• Fix (no github dont close this issue) ICE(non_lifetime_binders): const bound params not handled correctly #127009

r? @BoxyUwU

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

HIR ty lowering was modified

cc @fmease

[camelid]

Let's re-approve this so it enters the queue once the tree is re-opened. @bors r=BoxyUwU

[bors]

📌 Commit c8457e6 has been approved by BoxyUwU

It is now in the queue for this repository.

[bors]

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.

[bors]

⌛ Testing commit c8457e6 with merge 8c3a94a...

[bors]

☀️ Test successful - checks-actions
Approved by: BoxyUwU
Pushing 8c3a94a to master...

[rust-timer]

Finished benchmarking commit (8c3a94a): 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)	-0.6%	[-0.8%, -0.5%]	6
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.6%	[-0.8%, -0.5%]	6

Max RSS (memory usage)

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

Cycles

Results (primary 2.2%, secondary 2.2%)

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)	2.2%	[2.2%, 2.2%]	1
Regressions ❌ (secondary)	2.2%	[1.5%, 2.7%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.2%	[2.2%, 2.2%]	1

Binary size

Results (primary -0.1%)

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)	0.2%	[0.0%, 0.6%]	36
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.5%	[-1.1%, -0.0%]	23
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.1%	[-1.1%, 0.6%]	59

Bootstrap: 768.827s -> 770.782s (0.25%)
Artifact size: 328.75 MiB -> 328.81 MiB (0.02%)