(Re-)Implement impl_trait_in_bindings

[compiler-errors]

This reimplements the impl_trait_in_bindings feature for local bindings.

"impl Trait in bindings" serve as a form of trait ascription, where the type basically functions as an infer var but additionally registering the impl Trait's trait bounds for the infer type. These trait bounds can be used to enforce that predicates hold, and can guide inference (e.g. for closure signature inference):

let _: impl Fn(&u8) -> &u8 = |x| x;

They are implemented as an additional set of bounds that are registered when the type is lowered during typeck, and then these bounds are tied to a given CanonicalUserTypeAscription for borrowck. We enforce these CanonicalUserTypeAscription bounds during borrowck to make sure that the impl Trait types are sensitive to lifetimes:

trait Static: 'static {}
impl<T> Static for T where T: 'static {}

let local = 1;
let x: impl Static = &local;
//~^ ERROR `local` does not live long enough

r? oli-obk

cc #63065

---

Why can't we just use TAIT inference or something? Well, TAITs in bodies have the problem that they cannot reference lifetimes local to a body. For example:

type TAIT = impl Display;
let local = 0;
let x: TAIT = &local;
//~^ ERROR `local` does not live long enough

That's because TAITs requires us to do opaque type inference which is pretty strict, since we need to remap all of the lifetimes of the hidden type to universal regions. This is simply not possible here.

---

I consider this part of the "impl trait everywhere" experiment. I'm not certain if this needs yet another lang team experiment.

[compiler-errors]

@bors try @rust-timer queue

[rustbot]

Some changes occurred in match lowering

cc @Nadrieril

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

HIR ty lowering was modified

cc @fmease

[bors]

⌛ Trying commit f84b2f6 with merge 917c884c739f8b62081dbc8e34bb112bab0f248f...

[compiler-errors]

@rust-timer build 917c884

[rust-timer]

Finished benchmarking commit (917c884): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (primary 0.4%, secondary -4.6%)

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)	2.1%	[2.1%, 2.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.3%	[-1.3%, -1.3%]	1
Improvements ✅ (secondary)	-4.6%	[-5.1%, -4.2%]	2
All ❌✅ (primary)	0.4%	[-1.3%, 2.1%]	2

Cycles

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

Binary size

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

Bootstrap: 771.021s -> 769.24s (-0.23%)
Artifact size: 330.96 MiB -> 331.03 MiB (0.02%)

[oli-obk]

A difference to APIT is that impl Trait<impl Debug> is allowed. r=me with a test for that added

[compiler-errors]

We should definitely not allow that, ill check and add test and maybe fix it

[compiler-errors]

Added a test which confirms that we deny impl Trait<impl Trait>.

[compiler-errors]

@bors r=oli-obk

[bors]

📌 Commit 3bbdc9e has been approved by oli-obk

It is now in the queue for this repository.

[bors]

☔ The latest upstream changes (presumably #134269) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

@bors r=oli-obk

[bors]

📌 Commit d714a22 has been approved by oli-obk

It is now in the queue for this repository.

[bors]

⌛ Testing commit d714a22 with merge f5079d0...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing f5079d0 to master...

[rust-timer]

Finished benchmarking commit (f5079d0): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (primary -1.8%, 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)	-1.8%	[-2.0%, -1.7%]	2
Improvements ✅ (secondary)	-2.1%	[-2.1%, -2.1%]	1
All ❌✅ (primary)	-1.8%	[-2.0%, -1.7%]	2

Cycles

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

Binary size

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

Bootstrap: 768.74s -> 769.535s (0.10%)
Artifact size: 333.05 MiB -> 333.08 MiB (0.01%)