only assemble alias bound candidates for rigid aliases

[lcnr]

fixes rust-lang/trait-system-refactor-initiative#77

This also causes <Wrapper<?0> as Trait>::Unwrap: Trait to always be ambig, as we now normalize the self type before checking whether it is an inference variable.

I cannot think of an approach to the underlying issues here which does not require the "may-define means must-define" restriction for opaque types. Going to go ahead with this and added this restriction to the tracking issue for the new solver to make sure we don't stabilize it without getting types + lang signoff here.

r? @compiler-errors

[compiler-errors]

This PR ensures that the code I extracted from gimli works:

trait Reader: Default {
    fn read_u8_array<A>(&self) -> Result<A, ()>
    {
        todo!()
    }

    fn read_u8(&self) -> Result<u8, ()> {
        let a: [u8; 1] = self.read_u8_array::<_>()?;
        Ok(a[0])
    }
}

fn main() {}

Plz add it as a test, whether you want to minimize it further or not is up to you.

This has to do with taking <Result<?0, ()> as Try>::Residual: Sized and eagerly unifying it against Self: Sized in the param-env incompletely -- the old solver doesn't do this since it'll normalize <Result<?0, ()> as Try>::Residual to ?0 and the sized predicate will stall before erroneous inference is done to it.

[compiler-errors]

@rustbot author until it's ready for review

[rustbot]

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

[bors]

☔ The latest upstream changes (presumably #120401) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

@bors r+

[bors]

📌 Commit ea4e5b8 has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit ea4e5b8 with merge c401f09...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing c401f09 to master...

[rust-timer]

Finished benchmarking commit (c401f09): 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

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)	-2.8%	[-4.8%, -2.0%]	15
Improvements ✅ (secondary)	-3.0%	[-6.7%, -0.8%]	66
All ❌✅ (primary)	-2.8%	[-4.8%, -2.0%]	15

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: 662.576s -> 660.86s (-0.26%)
Artifact size: 308.05 MiB -> 308.08 MiB (0.01%)