best_blame_constraint: prioritize blaming interesting-seeming constraints
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dianne
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45b2ae935d
commit
6421d4cf80
22 changed files with 206 additions and 173 deletions
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@ -2026,87 +2026,102 @@ impl<'tcx> RegionInferenceContext<'tcx> {
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| NllRegionVariableOrigin::Existential { from_forall: true } => false,
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};
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let interesting_to_blame = |constraint: &OutlivesConstraint<'tcx>| {
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!matches!(
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constraint.category,
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ConstraintCategory::OpaqueType
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| ConstraintCategory::Boring
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| ConstraintCategory::BoringNoLocation
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| ConstraintCategory::Internal
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| ConstraintCategory::Predicate(_)
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| ConstraintCategory::Assignment { has_interesting_ty: false }
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) && constraint.span.desugaring_kind().is_none_or(|kind| {
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// Try to avoid blaming constraints from desugarings, since they may not clearly
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// clearly match what users have written. As an exception, allow blaming returns
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// generated by `?` desugaring, since the correspondence is fairly clear.
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kind == DesugaringKind::QuestionMark
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&& matches!(constraint.category, ConstraintCategory::Return(_))
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})
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// To pick a constraint to blame, we organize constraints by how interesting we expect them
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// to be in diagnostics, then pick the most interesting one closest to either the source or
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// the target on our constraint path.
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let constraint_interest = |constraint: &OutlivesConstraint<'tcx>| {
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// Try to avoid blaming constraints from desugarings, since they may not clearly match
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// match what users have written. As an exception, allow blaming returns generated by
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// `?` desugaring, since the correspondence is fairly clear.
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let category = if let Some(kind) = constraint.span.desugaring_kind()
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&& (kind != DesugaringKind::QuestionMark
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|| !matches!(constraint.category, ConstraintCategory::Return(_)))
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{
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ConstraintCategory::Boring
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} else {
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constraint.category
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};
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match category {
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// Returns usually provide a type to blame and have specially written diagnostics,
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// so prioritize them.
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ConstraintCategory::Return(_) => 0,
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// Unsizing coercions are interesting, since we have a note for that:
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// `BorrowExplanation::add_object_lifetime_default_note`.
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// FIXME(dianne): That note shouldn't depend on a coercion being blamed; see issue
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// #131008 for an example of where we currently don't emit it but should.
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// Once the note is handled properly, this case should be removed. Until then, it
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// should be as limited as possible; the note is prone to false positives and this
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// constraint usually isn't best to blame.
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ConstraintCategory::Cast {
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unsize_to: Some(unsize_ty),
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is_implicit_coercion: true,
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} if target_region == self.universal_regions().fr_static
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// Mirror the note's condition, to minimize how often this diverts blame.
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&& let ty::Adt(_, args) = unsize_ty.kind()
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&& args.iter().any(|arg| arg.as_type().is_some_and(|ty| ty.is_trait()))
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// Mimic old logic for this, to minimize false positives in tests.
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&& !path
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.iter()
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.any(|c| matches!(c.category, ConstraintCategory::TypeAnnotation)) =>
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{
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1
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}
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// Between other interesting constraints, order by their position on the `path`.
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ConstraintCategory::Yield
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| ConstraintCategory::UseAsConst
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| ConstraintCategory::UseAsStatic
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| ConstraintCategory::TypeAnnotation
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| ConstraintCategory::Cast { .. }
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| ConstraintCategory::CallArgument(_)
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| ConstraintCategory::CopyBound
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| ConstraintCategory::SizedBound
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| ConstraintCategory::Assignment { has_interesting_ty: true }
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| ConstraintCategory::Usage
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| ConstraintCategory::ClosureUpvar(_) => 2,
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// Give assignments a lower priority when flagged as less likely to be interesting.
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// In particular, de-prioritize MIR assignments lowered from argument patterns.
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ConstraintCategory::Assignment { has_interesting_ty: false } => 3,
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// We handle predicates and opaque types specially; don't prioritize them here.
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ConstraintCategory::Predicate(_) | ConstraintCategory::OpaqueType => 4,
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// `Boring` constraints can correspond to user-written code and have useful spans,
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// but don't provide any other useful information for diagnostics.
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ConstraintCategory::Boring => 5,
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// `BoringNoLocation` constraints can point to user-written code, but are less
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// specific, and are not used for relations that would make sense to blame.
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ConstraintCategory::BoringNoLocation => 6,
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// Do not blame internal constraints.
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ConstraintCategory::Internal => 7,
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ConstraintCategory::IllegalUniverse => 8,
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}
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};
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let best_choice = if blame_source {
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path.iter().rposition(interesting_to_blame)
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path.iter().enumerate().rev().min_by_key(|(_, c)| constraint_interest(c)).unwrap().0
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} else {
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path.iter().position(interesting_to_blame)
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path.iter().enumerate().min_by_key(|(_, c)| constraint_interest(c)).unwrap().0
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};
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debug!(?best_choice, ?blame_source);
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let best_constraint = match best_choice {
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Some(i)
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if let Some(next) = path.get(i + 1)
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&& matches!(path[i].category, ConstraintCategory::Return(_))
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&& next.category == ConstraintCategory::OpaqueType =>
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{
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// The return expression is being influenced by the return type being
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// impl Trait, point at the return type and not the return expr.
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*next
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}
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Some(i)
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if path[i].category == ConstraintCategory::Return(ReturnConstraint::Normal)
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&& let Some(field) = path.iter().find_map(|p| {
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if let ConstraintCategory::ClosureUpvar(f) = p.category {
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Some(f)
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} else {
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None
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}
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}) =>
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{
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OutlivesConstraint {
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category: ConstraintCategory::Return(ReturnConstraint::ClosureUpvar(field)),
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..path[i]
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}
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}
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Some(_)
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if target_region == self.universal_regions().fr_static
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&& let Some(old_best) = path.iter().min_by_key(|p| p.category)
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&& matches!(old_best.category, ConstraintCategory::Cast {
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is_implicit_coercion: true,
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unsize_to: Some(_)
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}) =>
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{
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// FIXME(dianne): This is a hack in order to emit the subdiagnostic
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// `BorrowExplanation::add_object_lifetime_default_note` more often, e.g. on
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// `tests/ui/traits/trait-object-lifetime-default-note.rs`. The subdiagnostic
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// depends on a coercion being blamed, so we fall back to an earlier version of this
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// function's blaming logic to keep the test result the same. A proper fix will
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// require rewriting the subdiagnostic not to rely on a coercion being blamed.
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// For examples of where notes are missing, see #131008 and
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// `tests/ui/suggestions/impl-on-dyn-trait-with-implicit-static-bound-needing-more-suggestions.rs`.
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// As part of fixing those, this case should be removed.
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*old_best
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}
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Some(i) => path[i],
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None => {
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// If that search fails, the only constraints on the path are those that we try not
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// to blame. In that case, find what appears to be the most interesting point to
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// report to the user via an even more ad-hoc guess.
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*path.iter().min_by_key(|p| p.category).unwrap()
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let best_constraint = if let Some(next) = path.get(best_choice + 1)
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&& matches!(path[best_choice].category, ConstraintCategory::Return(_))
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&& next.category == ConstraintCategory::OpaqueType
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{
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// The return expression is being influenced by the return type being
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// impl Trait, point at the return type and not the return expr.
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*next
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} else if path[best_choice].category == ConstraintCategory::Return(ReturnConstraint::Normal)
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&& let Some(field) = path.iter().find_map(|p| {
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if let ConstraintCategory::ClosureUpvar(f) = p.category { Some(f) } else { None }
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})
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{
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OutlivesConstraint {
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category: ConstraintCategory::Return(ReturnConstraint::ClosureUpvar(field)),
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..path[best_choice]
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}
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} else {
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path[best_choice]
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};
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let blame_constraint = BlameConstraint {
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