| // The outlines relation `T: 'a` or `'a: 'b`. This code frequently |
| // refers to rules defined in RFC 1214 (`OutlivesFooBar`), so see that |
| // RFC for reference. |
| |
| use crate::ty::subst::{GenericArg, GenericArgKind}; |
| use crate::ty::{self, Ty, TyCtxt, TypeFoldable}; |
| use smallvec::SmallVec; |
| |
| #[derive(Debug)] |
| pub enum Component<'tcx> { |
| Region(ty::Region<'tcx>), |
| Param(ty::ParamTy), |
| UnresolvedInferenceVariable(ty::InferTy), |
| |
| // Projections like `T::Foo` are tricky because a constraint like |
| // `T::Foo: 'a` can be satisfied in so many ways. There may be a |
| // where-clause that says `T::Foo: 'a`, or the defining trait may |
| // include a bound like `type Foo: 'static`, or -- in the most |
| // conservative way -- we can prove that `T: 'a` (more generally, |
| // that all components in the projection outlive `'a`). This code |
| // is not in a position to judge which is the best technique, so |
| // we just product the projection as a component and leave it to |
| // the consumer to decide (but see `EscapingProjection` below). |
| Projection(ty::ProjectionTy<'tcx>), |
| |
| // In the case where a projection has escaping regions -- meaning |
| // regions bound within the type itself -- we always use |
| // the most conservative rule, which requires that all components |
| // outlive the bound. So for example if we had a type like this: |
| // |
| // for<'a> Trait1< <T as Trait2<'a,'b>>::Foo > |
| // ~~~~~~~~~~~~~~~~~~~~~~~~~ |
| // |
| // then the inner projection (underlined) has an escaping region |
| // `'a`. We consider that outer trait `'c` to meet a bound if `'b` |
| // outlives `'b: 'c`, and we don't consider whether the trait |
| // declares that `Foo: 'static` etc. Therefore, we just return the |
| // free components of such a projection (in this case, `'b`). |
| // |
| // However, in the future, we may want to get smarter, and |
| // actually return a "higher-ranked projection" here. Therefore, |
| // we mark that these components are part of an escaping |
| // projection, so that implied bounds code can avoid relying on |
| // them. This gives us room to improve the regionck reasoning in |
| // the future without breaking backwards compat. |
| EscapingProjection(Vec<Component<'tcx>>), |
| } |
| |
| impl<'tcx> TyCtxt<'tcx> { |
| /// Push onto `out` all the things that must outlive `'a` for the condition |
| /// `ty0: 'a` to hold. Note that `ty0` must be a **fully resolved type**. |
| pub fn push_outlives_components(self, ty0: Ty<'tcx>, out: &mut SmallVec<[Component<'tcx>; 4]>) { |
| compute_components(self, ty0, out); |
| debug!("components({:?}) = {:?}", ty0, out); |
| } |
| } |
| |
| fn compute_components(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, out: &mut SmallVec<[Component<'tcx>; 4]>) { |
| // Descend through the types, looking for the various "base" |
| // components and collecting them into `out`. This is not written |
| // with `collect()` because of the need to sometimes skip subtrees |
| // in the `subtys` iterator (e.g., when encountering a |
| // projection). |
| match ty.kind { |
| ty::FnDef(_, substs) => { |
| // HACK(eddyb) ignore lifetimes found shallowly in `substs`. |
| // This is inconsistent with `ty::Adt` (including all substs) |
| // and with `ty::Closure` (ignoring all substs other than |
| // upvars, of which a `ty::FnDef` doesn't have any), but |
| // consistent with previous (accidental) behavior. |
| // See https://github.com/rust-lang/rust/issues/70917 |
| // for further background and discussion. |
| for child in substs { |
| match child.unpack() { |
| GenericArgKind::Type(ty) => { |
| compute_components(tcx, ty, out); |
| } |
| GenericArgKind::Lifetime(_) => {} |
| GenericArgKind::Const(_) => { |
| compute_components_recursive(tcx, child, out); |
| } |
| } |
| } |
| } |
| |
| ty::Array(element, _) => { |
| // Don't look into the len const as it doesn't affect regions |
| compute_components(tcx, element, out); |
| } |
| |
| ty::Closure(_, ref substs) => { |
| for upvar_ty in substs.as_closure().upvar_tys() { |
| compute_components(tcx, upvar_ty, out); |
| } |
| } |
| |
| ty::Generator(_, ref substs, _) => { |
| // Same as the closure case |
| for upvar_ty in substs.as_generator().upvar_tys() { |
| compute_components(tcx, upvar_ty, out); |
| } |
| |
| // We ignore regions in the generator interior as we don't |
| // want these to affect region inference |
| } |
| |
| // All regions are bound inside a witness |
| ty::GeneratorWitness(..) => (), |
| |
| // OutlivesTypeParameterEnv -- the actual checking that `X:'a` |
| // is implied by the environment is done in regionck. |
| ty::Param(p) => { |
| out.push(Component::Param(p)); |
| } |
| |
| // For projections, we prefer to generate an obligation like |
| // `<P0 as Trait<P1...Pn>>::Foo: 'a`, because this gives the |
| // regionck more ways to prove that it holds. However, |
| // regionck is not (at least currently) prepared to deal with |
| // higher-ranked regions that may appear in the |
| // trait-ref. Therefore, if we see any higher-ranke regions, |
| // we simply fallback to the most restrictive rule, which |
| // requires that `Pi: 'a` for all `i`. |
| ty::Projection(ref data) => { |
| if !data.has_escaping_bound_vars() { |
| // best case: no escaping regions, so push the |
| // projection and skip the subtree (thus generating no |
| // constraints for Pi). This defers the choice between |
| // the rules OutlivesProjectionEnv, |
| // OutlivesProjectionTraitDef, and |
| // OutlivesProjectionComponents to regionck. |
| out.push(Component::Projection(*data)); |
| } else { |
| // fallback case: hard code |
| // OutlivesProjectionComponents. Continue walking |
| // through and constrain Pi. |
| let mut subcomponents = smallvec![]; |
| compute_components_recursive(tcx, ty.into(), &mut subcomponents); |
| out.push(Component::EscapingProjection(subcomponents.into_iter().collect())); |
| } |
| } |
| |
| // We assume that inference variables are fully resolved. |
| // So, if we encounter an inference variable, just record |
| // the unresolved variable as a component. |
| ty::Infer(infer_ty) => { |
| out.push(Component::UnresolvedInferenceVariable(infer_ty)); |
| } |
| |
| // Most types do not introduce any region binders, nor |
| // involve any other subtle cases, and so the WF relation |
| // simply constraints any regions referenced directly by |
| // the type and then visits the types that are lexically |
| // contained within. (The comments refer to relevant rules |
| // from RFC1214.) |
| ty::Bool | // OutlivesScalar |
| ty::Char | // OutlivesScalar |
| ty::Int(..) | // OutlivesScalar |
| ty::Uint(..) | // OutlivesScalar |
| ty::Float(..) | // OutlivesScalar |
| ty::Never | // ... |
| ty::Adt(..) | // OutlivesNominalType |
| ty::Opaque(..) | // OutlivesNominalType (ish) |
| ty::Foreign(..) | // OutlivesNominalType |
| ty::Str | // OutlivesScalar (ish) |
| ty::Slice(..) | // ... |
| ty::RawPtr(..) | // ... |
| ty::Ref(..) | // OutlivesReference |
| ty::Tuple(..) | // ... |
| ty::FnPtr(_) | // OutlivesFunction (*) |
| ty::Dynamic(..) | // OutlivesObject, OutlivesFragment (*) |
| ty::Placeholder(..) | |
| ty::Bound(..) | |
| ty::Error(_) => { |
| // (*) Function pointers and trait objects are both binders. |
| // In the RFC, this means we would add the bound regions to |
| // the "bound regions list". In our representation, no such |
| // list is maintained explicitly, because bound regions |
| // themselves can be readily identified. |
| compute_components_recursive(tcx, ty.into(), out); |
| } |
| } |
| } |
| |
| fn compute_components_recursive( |
| tcx: TyCtxt<'tcx>, |
| parent: GenericArg<'tcx>, |
| out: &mut SmallVec<[Component<'tcx>; 4]>, |
| ) { |
| for child in parent.walk_shallow() { |
| match child.unpack() { |
| GenericArgKind::Type(ty) => { |
| compute_components(tcx, ty, out); |
| } |
| GenericArgKind::Lifetime(lt) => { |
| // Ignore late-bound regions. |
| if !lt.is_late_bound() { |
| out.push(Component::Region(lt)); |
| } |
| } |
| GenericArgKind::Const(_) => { |
| compute_components_recursive(tcx, child, out); |
| } |
| } |
| } |
| } |
