Add rigid alias marker

[adwinwhite]

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This PR adds a rigidness marker to TyKind::Alias and ConstKind::Unevaluated. It's used to skip renormalization of rigid aliases. The tracking issue for this is #155345.

The difficulty is that rigid aliases are only valid in their own TypingEnv so we need to force them back to non-rigid when entering another TypingEnv, either because a change to the ParamEnv or because we enter another TypingMode.

Changes to the ParamEnv currently only happen by moving into a new context. We now make sure that EarlyBinder new contains any rigid aliases, this way we have to normalize all aliases contained in it after instantiating.

Changes to the TypingMode are rare, and have to be manually handled.

---

The main changes in this PR are as follows:

• we add enum IsRigid { Yes, No } as a field to TyKind::Alias and ConstKind::Unevaluated. It is always No with the old solver, this makes some of the code less nice than it will be with the old solver removed.
• if we keep an alias as rigid when proving Projection goals we equate the expected term with that alias with IsRigid::Yes
• EarlyBinder::bind now takes the tcx and eagerly sets all IsRigid to No, this is necessary as moving aliases into a different TypingEnv may cause them to no longer be rigid
• EarlyBinder::bind_iter asserts that there are no rigid aliases instead of replacing them
• type relations and generalization always structurally recurse into rigid aliases
• a lot of places in the new solver can ICE when matching on TyKind::Alias with IsRigid::No

---

There's a lot of future work here:

• coherence don't actually rely on lazy norm :<
• remove eq_structurally_relating_aliases, have a special type folder for canonical responses
• yeet AliasRelate, lazily replace non-rigid aliases with infer var + projection goal
• ParamEnv normalization hack to incorrectly mark things as rigid
• eagerly normalize when adding stuff to the fulfillment ctxt :>
• change TypeOutlives goal handling to only expect rigid aliases
• properly expect IsRigid::Yesin region handling (or yes_if_next_solver)
• probably change EarlyBinder::bind to also assert that there are no rigid aliases and manually replacing rigid aliases before then where necessary

---

This PR also adds a flag -Zrenormalize-rigid-aliases to test whether we properly handle typing mode change.

Currently only tests/ui/traits/next-solver/assembly/ambiguity-due-to-uniquification-4.rs fails this check.

r? lcnr

[adwinwhite]

Unfinished. Let's have a look at the perf impact nonetheless.

@bors try @rust-timer queue

[rust-bors]

💔 Test for 73eccb5 failed: CI. Failed job:

• try - dist-x86_64-linux-quick (web logs, enhanced plaintext logs)

[adwinwhite]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 42e2939 (42e2939c892f3c5e872d5751bb653ed74736a040, parent: 4b9792692fbb675174d4d2082e7c37b2bc930e71)

[rust-timer]

Finished benchmarking commit (42e2939): comparison URL.

Overall result: ❌ regressions - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never
@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)	0.8%	[0.2%, 2.2%]	15
Regressions ❌ (secondary)	11.3%	[0.1%, 95.1%]	29
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	2
All ❌✅ (primary)	0.8%	[0.2%, 2.2%]	15

Max RSS (memory usage)

Results (primary 1.6%, secondary -1.7%)

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

	mean	range	count
Regressions ❌ (primary)	2.1%	[1.7%, 2.7%]	7
Regressions ❌ (secondary)	2.4%	[0.8%, 3.6%]	6
Improvements ✅ (primary)	-2.4%	[-2.4%, -2.4%]	1
Improvements ✅ (secondary)	-4.1%	[-13.8%, -1.1%]	10
All ❌✅ (primary)	1.6%	[-2.4%, 2.7%]	8

Cycles

Results (primary 2.5%, secondary 17.8%)

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

	mean	range	count
Regressions ❌ (primary)	2.5%	[2.4%, 2.6%]	2
Regressions ❌ (secondary)	19.3%	[3.5%, 91.9%]	15
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.7%	[-4.7%, -4.7%]	1
All ❌✅ (primary)	2.5%	[2.4%, 2.6%]	2

Binary size

Results (primary 0.0%, secondary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	28
Regressions ❌ (secondary)	0.1%	[0.0%, 0.4%]	27
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	28

Bootstrap: 510.83s -> 514.725s (0.76%)
Artifact size: 400.58 MiB -> 401.07 MiB (0.12%)

[adwinwhite]

@bors try @rust-timer queue

[lcnr]

@bors r+ rollup=never p=1

[rust-bors]

📌 Commit 1af8ca5 has been approved by lcnr

It is now in the queue for this repository.

[lcnr]

@bors r- will conflict with the currently happening rollup

[rust-bors]

This pull request was unapproved.

View changes since this unapproval

[lcnr]

@bors delegate+ p=1

This is both important and very prone to merge conflicts. @adwinwhite please rebase and then feel free to merge this after looking over it yourself again. I'll do that first thing in the morning otherwise :>

[rust-bors]

Invalid command: Key-value argument p is not supported. Run @bors help or go to https://bors.rust-lang.org/help to see available commands.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[adwinwhite]

@bors r=lcnr

[rust-bors]

📌 Commit 2b8fa4f has been approved by lcnr

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 5. This pull request will be tested once the tree is reopened.

[rust-bors]

⚠️ A new commit 2368b4b70b41e8f35a73bfb329f7847699b39e16 was pushed.

This pull request was unapproved.

[adwinwhite]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[rust-bors]

⌛ Trying commit 2368b4b with merge cc59b8e…

To cancel the try build, run the command @bors try cancel.

Workflow: https://github.com/rust-lang/rust/actions/runs/28080269494