bjoernager/rust
bjoernager/rust
1
Fork 0
Code Issues Pull requests Activity

Auto merge of #122791 - compiler-errors:make-coinductive-always, r=lcnr

Browse source 
Make inductive cycles always ambiguous

 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 https://github.com/rust-lang/rust/pull/116494#issuecomment-2008657494, 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](https://crater-reports.s3.amazonaws.com/pr-116494-3/try%23d614ed876e31a5f3ad1d0fbf848fcdab3a29d1d8/gh/lcdr.lu_packets/log.txt) to:

```rust
// 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 e6b64c61941120f734657106ae2479d05b463197 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
This commit is contained in:
bors 2024-04-03 00:09:44 +00:00
commit 40f743da23
12 changed files with 104 additions and 117 deletions
Download patch file Download diff file Expand all files Collapse all files

42
compiler/rustc_trait_selection/src/traits/select/mod.rs
View file

@ -126,8 +126,6 @@ pub struct SelectionContext<'cx, 'tcx> {
/// policy. In essence, canonicalized queries need their errors propagated
/// rather than immediately reported because we do not have accurate spans.
query_mode: TraitQueryMode,
treat_inductive_cycle: TreatInductiveCycleAs,
}
// A stack that walks back up the stack frame.
@ -208,27 +206,6 @@ enum BuiltinImplConditions<'tcx> {
Ambiguous,
}
#[derive(Copy, Clone)]
pub enum TreatInductiveCycleAs {
/// This is the previous behavior, where `Recur` represents an inductive
/// cycle that is known not to hold. This is not forwards-compatible with
/// coinduction, and will be deprecated. This is the default behavior
/// of the old trait solver due to back-compat reasons.
Recur,
/// This is the behavior of the new trait solver, where inductive cycles
/// are treated as ambiguous and possibly holding.
Ambig,
}
impl From<TreatInductiveCycleAs> for EvaluationResult {
fn from(treat: TreatInductiveCycleAs) -> EvaluationResult {
match treat {
TreatInductiveCycleAs::Ambig => EvaluatedToAmbigStackDependent,
TreatInductiveCycleAs::Recur => EvaluatedToErrStackDependent,
}
}
}
impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
pub fn new(infcx: &'cx InferCtxt<'tcx>) -> SelectionContext<'cx, 'tcx> {
SelectionContext {
@ -236,19 +213,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
freshener: infcx.freshener(),
intercrate_ambiguity_causes: None,
query_mode: TraitQueryMode::Standard,
treat_inductive_cycle: TreatInductiveCycleAs::Recur,
}
}
pub fn with_treat_inductive_cycle_as_ambig(
infcx: &'cx InferCtxt<'tcx>,
) -> SelectionContext<'cx, 'tcx> {
// Should be executed in a context where caching is disabled,
// otherwise the cache is poisoned with the temporary result.
assert!(infcx.intercrate);
SelectionContext {
treat_inductive_cycle: TreatInductiveCycleAs::Ambig,
..SelectionContext::new(infcx)
}
}
@ -756,7 +720,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
stack.update_reached_depth(stack_arg.1);
return Ok(EvaluatedToOk);
} else {
return Ok(self.treat_inductive_cycle.into());
return Ok(EvaluatedToAmbigStackDependent);
}
}
return Ok(EvaluatedToOk);
@ -875,7 +839,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
}
}
ProjectAndUnifyResult::FailedNormalization => Ok(EvaluatedToAmbig),
ProjectAndUnifyResult::Recursive => Ok(self.treat_inductive_cycle.into()),
ProjectAndUnifyResult::Recursive => Ok(EvaluatedToAmbigStackDependent),
ProjectAndUnifyResult::MismatchedProjectionTypes(_) => Ok(EvaluatedToErr),
}
}
@ -1180,7 +1144,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
Some(EvaluatedToOk)
} else {
debug!("evaluate_stack --> recursive, inductive");
Some(self.treat_inductive_cycle.into())
Some(EvaluatedToAmbigStackDependent)
}
} else {
None