Make inductive cycles always ambiguous

[compiler-errors]

This makes inductive cycles always result in ambiguity rather than be treated like a stack-dependent error.

This has some interactions with specialization, and so breaks a few UI tests that I don't agree should've ever worked in the first place, and also breaks a handful of crates in a way that I don't believe is a problem.

On the bright side, it puts us in a better spot when it comes to eventually enabling coinduction everywhere.

Results

This was cratered in #116494 (comment), which boils down to two regressions:

• lu_packets - This code should have never compiled in the first place. More below.
• ALL other regressions are due to commit_verify@0.11.0-beta.1 (edit: and commit_verify@0.10.x) - This actually seems to be fixed in version 0.11.0-beta.5, which is the most up to date version, but it's still prerelease on crates.io so I don't think cargo ends up picking beta.5 when building dependent crates.

lu_packets

Firstly, this crate uses specialization, so I think it's automatically worth breaking. However, I've minimized the regression to:

// Upstream crate
pub trait Serialize {}
impl Serialize for &() {}
impl<S> Serialize for &[S] where for<'a> &'a S: Serialize {}

// ----------------------------------------------------------------------- //

// Downstream crate
#![feature(specialization)]
#![allow(incomplete_features, unused)]

use upstream::Serialize;

trait Replica {
    fn serialize();
}

impl<T> Replica for T {
    default fn serialize() {}
}

impl<T> Replica for Option<T>
where
    for<'a> &'a T: Serialize,
{
    fn serialize() {}
}

Specifically this fails when computing the specialization graph for the downstream crate.

The code ends up cycling on &[?0]: Serialize when we equate &?0 = &[?1] during impl matching, which ends up needing to prove &[?1]: Serialize, which since cycles are treated like ambiguity, ends up in a fatal overflow. For some reason this requires two crates, squashing them into one crate doesn't work.

Side-note: This code is subtly order dependent. When minimizing, I ended up having the code start failing on nightly very easily after removing and reordering impls. This seems to me all the more reason to remove this behavior altogether.

Side-note: Item Bounds (edit: this was fixed independently in #121123)

Due to the changes in #120584 where we now consider an alias's item bounds and all the item bounds of the alias's nested self type aliases, I've had to add e6b64c6 which is a hack to make sure we're not eagerly normalizing bounds that have nothing to do with the predicate we're trying to solve, and which result in.

This is fixed in a more principled way in #121123.

---

r? lcnr for an initial review

[workingjubilee]

ALL other regressions are due to commit_verify@0.11.0-beta.1 - This actually seems to be fixed in version 0.11.0-beta.5, which is the most up to date version, but it's still prerelease on crates.io so I don't think cargo ends up picking beta.5 when building dependent crates.

-beta.5 should be a higher SemVer than -beta.1, and cargo should resolve these appropriately.

[compiler-errors]

Apologies. I misread the crater report. The regressions are a combination of 0.10.x and 0.11.0-beta.1 (I guess they're hard-coding that in their cargo toml? or have their cargo locks commited? or something).

For the record, the regression was removed when a bunch of impls were reworked in: LNP-BP/client_side_validation#152.

[compiler-errors]

Regarding commit_verify, the regression is because there's a fatal overflow when trying to prove: for<'a> Holder<&'a &T, strategies::AsRef>: encode::CommitEncode:

/// Used only internally for blank implementation on reference types.
#[doc(hidden)]
pub enum AsRef {}

impl<'a, T> CommitEncode for Holder<&'a T, AsRef>
where
    T: CommitEncode,
{
    fn commit_encode(&self, e: &mut impl io::Write) {
        self.as_type().commit_encode(e);
    }
}

impl<T> CommitStrategy for &T
where
    T: CommitEncode,
{
    type Strategy = AsRef;
}

impl<T> CommitEncode for T
where
    T: CommitStrategy,
    for<'a> Holder<&'a T, T::Strategy>: CommitEncode,
{
    fn commit_encode(&self, e: &mut impl io::Write) 
        Holder::new(self).commit_encode(e)
    }
}

... which I guess because we don't treat ambiguity as error, we end up actually evaluating to overflow.

Removing AsRef (which is #[doc(hidden)] in the code anyways, and "[u]sed only internally for blank implementation on reference types") fixes the problem, for the record.

[lcnr]

which is a hack to make sure we're not eagerly normalizing bounds that have nothing to do with the predicate we're trying to solve, and which result in.

"fatal overflow" i guess?

What is a test which is affected here. How does this handle <AssocTy as Id>::Assoc: Bound? writing a test rn

[lcnr]

unrelated: isn't this just utterly broken if you have stack dependent ambiguity resulting in difference inference constraints, causing a later goal to error without stack dependence?

anyways, the old solver cannot be salvaged. This is an existing issue 😁

[lcnr]

This already fails on nightly, so this PR seems good to me. I am a bit confused why though, we somehow keep the <Self::Assoc as Id>::This: Copy as a trait where-bound instead of an alias item bound?

trait Id {
    type This: ?Sized;
}
impl<T: ?Sized> Id for T {
    type This = T;
}

trait Trait
where
    <Self::Assoc as Id>::This: Copy,
{
    type Assoc;
}

fn foo<T: Trait>(x: T::Assoc) -> (T::Assoc, T::Assoc) {
    (x, x)
}

edit: it's because that hasn't landed yet '^^ that is from a separate PR #120752

The following also already fails, which makes sense, but seems like a shortcoming of our approach to normalization

#![feature(associated_type_bounds)]
trait Id {
    type This: ?Sized;
}
impl<T: ?Sized> Id for T {
    type This = T;
}

trait Trait {
    type Assoc: Id<This: Copy>;
}

fn foo<T: Trait>(x: T::Assoc) -> (T::Assoc, T::Assoc) {
    (x, x)
}

however, why does the following compile?

#![feature(associated_type_bounds)]
trait Id {
    type This: ?Sized;
}

trait Trait {
    type Assoc: Id<This: Copy>;
}

fn foo<T: Trait>(x: T::Assoc) -> (T::Assoc, T::Assoc)
where
    T::Assoc: Id<This = T::Assoc>,
{
    (x, x)
}

please add these all as tests 😁

[lcnr]

Ignoring the question about nested item_bounds, this PR changes inductive cycles to always result in ambiguity. This matches the behavior of the new trait solver and is an imo necessary step towards changing more cycles to be coinductive, at which point the cycles change to success. This is already the behavior during coherence since #118649.

For more details, see the PR description by @compiler-errors.

@rfcbot fcp merge

[rfcbot]

Team member @lcnr has proposed to merge this. The next step is review by the rest of the tagged team members:

• @BoxyUwU
• @aliemjay
• @compiler-errors
• @jackh726
• @lcnr
• @nikomatsakis
• @oli-obk
• @spastorino

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[bors]

☔ The latest upstream changes (presumably #121123) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

Actually, yeah, it's due to candidate shadowing, just for associated item bounds shadowing the global impl.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rust-log-analyzer]

The job aarch64-gnu failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] rustc_monomorphize test:false 25.753
   Compiling rustc_trait_selection v0.0.0 (/checkout/compiler/rustc_trait_selection)
[RUSTC-TIMING] rustc_pattern_analysis test:false 26.309
[RUSTC-TIMING] rustc_smir test:false 29.654
##[error]The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

[bors]

💔 Test failed - checks-actions

[compiler-errors]

@bors retry

[bors]

⌛ Testing commit 56dbeeb with merge 40f743d...

[bors]

☀️ Test successful - checks-actions
Approved by: lcnr
Pushing 40f743d to master...

[rust-timer]

Finished benchmarking commit (40f743d): 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)	2.2%	[0.8%, 4.2%]	5
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.2%	[0.8%, 4.2%]	5

Cycles

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)	1.1%	[1.1%, 1.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.0%	[-1.0%, -1.0%]	1
All ❌✅ (primary)	1.1%	[1.1%, 1.1%]	1

Binary size

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)	-0.2%	[-0.2%, -0.2%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.2%	[-0.2%, -0.2%]	1

Bootstrap: 668.585s -> 668.957s (0.06%)
Artifact size: 315.61 MiB -> 315.62 MiB (0.00%)