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Merged
merged 1 commit into from
Apr 4, 2023
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Never consider int and float vars for FnPtr candidates #109896

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10 changes: 8 additions & 2 deletions compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -998,8 +998,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
| ty::Alias(..)
| ty::Param(..)
| ty::Bound(..)
| ty::Error(_) => {}
ty::Infer(_) => {
| ty::Error(_)
| ty::Infer(
ty::InferTy::IntVar(_)
| ty::InferTy::FloatVar(_)
| ty::InferTy::FreshIntTy(_)
| ty::InferTy::FreshFloatTy(_),
) => {}
ty::Infer(ty::InferTy::TyVar(_) | ty::InferTy::FreshTy(_)) => {
candidates.ambiguous = true;
Comment on lines +1008 to 1009
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This is the only case I can think of that could still cause issues, I just hope this was already always ambiguous anyway 🤞.

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@Noratrieb Noratrieb Apr 3, 2023 •
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I tried to call a method on a ty infer var but that caused errors in typeck about not being able to do that

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rust/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

Line 39 in 1767585

if obligation.predicate.skip_binder().self_ty().is_ty_var() {

_: Trait is always ambiguous, and similarly treating fresh vars here is also fine here, since we'll only get those from freshening ty vars.

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@eddyb eddyb Apr 3, 2023 •
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I tried to call a method on a ty infer var but that caused errors in typeck about not being able to do that

Yeah, that's what I ran into as well, when trying to reproduce it w/o floats.

AFAIK you can't get it to pick a trait based on method name alone, without something like Default::default().method() or <_>::method but those give up too early because of how ambiguous they are, only int/float inference vars go farther (because they represent an unresolved choice in a small finite set of types etc.).

The fuzzier ideas I've had would involve guiding inference, but _: FnPtr will never succeed early enough to taint inference, and blocking inference would likely require associated types etc. - and the moment you pin down a trait, FnPtr can only affect it through a blanket impl of your choice, which would also apply to all the existing types too (before FnPtr was added).

EDIT: @compiler-errors' reply while I was playing around with more ideas for potential triggers, kind of invalidates all of those silly ideas, so yeah don't mind me, heh.

}
}
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12 changes: 6 additions & 6 deletions compiler/rustc_trait_selection/src/traits/select/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -177,14 +177,14 @@ struct TraitObligationStack<'prev, 'tcx> {
}

struct SelectionCandidateSet<'tcx> {
// A list of candidates that definitely apply to the current
// obligation (meaning: types unify).
/// A list of candidates that definitely apply to the current
/// obligation (meaning: types unify).
vec: Vec<SelectionCandidate<'tcx>>,

// If `true`, then there were candidates that might or might
// not have applied, but we couldn't tell. This occurs when some
// of the input types are type variables, in which case there are
// various "builtin" rules that might or might not trigger.
/// If `true`, then there were candidates that might or might
/// not have applied, but we couldn't tell. This occurs when some
/// of the input types are type variables, in which case there are
/// various "builtin" rules that might or might not trigger.
ambiguous: bool,
}

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10 changes: 10 additions & 0 deletions tests/ui/fn/fn-ptr-trait-int-float-infer-var.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,10 @@
// check-pass
trait MyCmp {
fn cmp(&self) {}
}
impl MyCmp for f32 {}

fn main() {
// Ensure that `impl<F: FnPtr> Ord for F` is never considered for int and float infer vars.
0.0.cmp();
}