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introduce the idea of an "approximate match"

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In fact, however, we currently always give back the same exact answers
as ever. But we don't rely on them being exact anymore.
This commit is contained in:
Niko Matsakis 2018-09-17 13:28:19 -04:00
parent db0e62692e
commit 13d579bd62
2 changed files with 56 additions and 39 deletions
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69
src/librustc/infer/outlives/obligations.rs
View file

@ -388,20 +388,24 @@ where
// rule might not apply (but another rule might). For now, we err
// on the side of adding too few edges into the graph.
// Compute the bounds we can derive from the environment or trait
// definition. We know that the projection outlives all the
// regions in this list.
let mut declared_bounds = self.verify_bound
.projection_declared_bounds_from_env(projection_ty);
declared_bounds.extend(
self.verify_bound
.projection_declared_bounds_from_trait(projection_ty),
// Compute the bounds we can derive from the environment. This
// is an "approximate" match -- in some cases, these bounds
// may not apply.
let approx_env_bounds = self.verify_bound
.projection_approx_declared_bounds_from_env(projection_ty);
debug!(
"projection_must_outlive: approx_env_bounds={:?}",
approx_env_bounds
);
// Compute the bounds we can derive from the trait definition.
// These are guaranteed to apply, no matter the inference
// results.
let trait_bounds = self.verify_bound
.projection_declared_bounds_from_trait(projection_ty);
debug!(
"projection_must_outlive: declared_bounds={:?}",
declared_bounds
"projection_must_outlive: trait_bounds={:?}",
trait_bounds
);
// If declared bounds list is empty, the only applicable rule is
@ -419,7 +423,7 @@ where
// inference variables, we use a verify constraint instead of adding
// edges, which winds up enforcing the same condition.
let needs_infer = projection_ty.needs_infer();
if declared_bounds.is_empty() && needs_infer {
if approx_env_bounds.is_empty() && trait_bounds.is_empty() && needs_infer {
debug!("projection_must_outlive: no declared bounds");
for component_ty in projection_ty.substs.types() {
@ -434,34 +438,31 @@ where
return;
}
// If we find that there is a unique declared bound `'b`, and this bound
// appears in the trait reference, then the best action is to require that `'b:'r`,
// so do that. This is best no matter what rule we use:
// If we found a unique bound `'b` from the trait, and we
// found nothing else from the environment, then the best
// action is to require that `'b: 'r`, so do that.
//
// - OutlivesProjectionEnv or OutlivesProjectionTraitDef: these would translate to
// the requirement that `'b:'r`
// - OutlivesProjectionComponent: this would require `'b:'r` in addition to
// other conditions
if !declared_bounds.is_empty()
&& declared_bounds[1..]
// This is best no matter what rule we use:
//
// - OutlivesProjectionEnv: these would translate to the requirement that `'b:'r`
// - OutlivesProjectionTraitDef: these would translate to the requirement that `'b:'r`
// - OutlivesProjectionComponent: this would require `'b:'r`
// in addition to other conditions
if !trait_bounds.is_empty()
&& trait_bounds[1..]
.iter()
.all(|b| *b == declared_bounds[0])
.chain(&approx_env_bounds)
.all(|b| *b == trait_bounds[0])
{
let unique_bound = declared_bounds[0];
let unique_bound = trait_bounds[0];
debug!(
"projection_must_outlive: unique declared bound = {:?}",
"projection_must_outlive: unique trait bound = {:?}",
unique_bound
);
if projection_ty
.substs
.regions()
.any(|r| declared_bounds.contains(&r))
{
debug!("projection_must_outlive: unique declared bound appears in trait ref");
self.delegate
.push_sub_region_constraint(origin.clone(), region, unique_bound);
return;
}
debug!("projection_must_outlive: unique declared bound appears in trait ref");
self.delegate
.push_sub_region_constraint(origin.clone(), region, unique_bound);
return;
}
// Fallback to verifying after the fact that there exists a

26
src/librustc/infer/outlives/verify.rs
View file

@ -74,10 +74,18 @@ impl<'cx, 'gcx, 'tcx> VerifyBoundCx<'cx, 'gcx, 'tcx> {
/// Given a projection like `T::Item`, searches the environment
/// for where-clauses like `T::Item: 'a`. Returns the set of
/// regions `'a` that it finds. This is a "conservative" check --
/// it may not find all applicable bounds, but all the bounds it
/// returns can be relied upon.
pub fn projection_declared_bounds_from_env(
/// regions `'a` that it finds.
///
/// This is an "approximate" check -- it may not find all
/// applicable bounds, and not all the bounds it returns can be
/// relied upon. In particular, this check ignores region
/// identity. So, for example, if we have `<T as
/// Trait<'0>>::Item` where `'0` is a region variable, and the
/// user has `<T as Trait<'a>>::Item: 'b` in the environment, then
/// the clause from the environment only applies if `'0 = 'a`,
/// which we don't know yet. But we would still include `'b` in
/// this list.
pub fn projection_approx_declared_bounds_from_env(
&self,
projection_ty: ty::ProjectionTy<'tcx>,
) -> Vec<ty::Region<'tcx>> {
@ -103,9 +111,11 @@ impl<'cx, 'gcx, 'tcx> VerifyBoundCx<'cx, 'gcx, 'tcx> {
projection_ty
);
// Search the env for where clauses like `P: 'a`.
let mut declared_bounds =
self.projection_declared_bounds_from_env(projection_ty);
self.declared_generic_bounds_from_env(GenericKind::Projection(projection_ty));
// Extend with bounds that we can find from the trait.
declared_bounds.extend(
self.projection_declared_bounds_from_trait(projection_ty)
);
@ -139,6 +149,12 @@ impl<'cx, 'gcx, 'tcx> VerifyBoundCx<'cx, 'gcx, 'tcx> {
}
}
/// Searches the environment for where-clauses like `G: 'a` where
/// `G` is either some type parameter `T` or a projection like
/// `T::Item`. Returns a vector of the `'a` bounds it can find.
///
/// This is a conservative check -- it may not find all applicable
/// bounds, but all the bounds it returns can be relied upon.
fn declared_generic_bounds_from_env(
&self,
generic: GenericKind<'tcx>,