--- /dev/null
+// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// The outlines relation `T: 'a` or `'a: 'b`.
+
+use middle::infer::InferCtxt;
+use middle::ty::{self, RegionEscape, Ty};
+
+#[derive(Debug)]
+pub enum Component<'tcx> {
+ Region(ty::Region),
+ 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>>),
+
+ RFC1214(Vec<Component<'tcx>>),
+}
+
+/// Returns all the things that must outlive `'a` for the condition
+/// `ty0: 'a` to hold.
+pub fn components<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
+ ty0: Ty<'tcx>)
+ -> Vec<Component<'tcx>> {
+ let mut components = vec![];
+ compute_components(infcx, ty0, &mut components);
+ debug!("outlives({:?}) = {:?}", ty0, components);
+ components
+}
+
+fn compute_components<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
+ ty0: Ty<'tcx>,
+ out: &mut Vec<Component<'tcx>>) {
+ // 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).
+ let mut subtys = ty0.walk();
+ while let Some(ty) = subtys.next() {
+ match ty.sty {
+ ty::TyClosure(_, ref substs) => {
+ // FIXME(#27086). We do not accumulate from substs, since they
+ // don't represent reachable data. This means that, in
+ // practice, some of the lifetime parameters might not
+ // be in scope when the body runs, so long as there is
+ // no reachable data with that lifetime. For better or
+ // worse, this is consistent with fn types, however,
+ // which can also encapsulate data in this fashion
+ // (though it's somewhat harder, and typically
+ // requires virtual dispatch).
+ //
+ // Note that changing this (in a naive way, at least)
+ // causes regressions for what appears to be perfectly
+ // reasonable code like this:
+ //
+ // ```
+ // fn foo<'a>(p: &Data<'a>) {
+ // bar(|q: &mut Parser| q.read_addr())
+ // }
+ // fn bar(p: Box<FnMut(&mut Parser)+'static>) {
+ // }
+ // ```
+ //
+ // Note that `p` (and `'a`) are not used in the
+ // closure at all, but to meet the requirement that
+ // the closure type `C: 'static` (so it can be coerced
+ // to the object type), we get the requirement that
+ // `'a: 'static` since `'a` appears in the closure
+ // type `C`.
+ //
+ // A smarter fix might "prune" unused `func_substs` --
+ // this would avoid breaking simple examples like
+ // this, but would still break others (which might
+ // indeed be invalid, depending on your POV). Pruning
+ // would be a subtle process, since we have to see
+ // what func/type parameters are used and unused,
+ // taking into consideration UFCS and so forth.
+
+ for &upvar_ty in &substs.upvar_tys {
+ compute_components(infcx, upvar_ty, out);
+ }
+ subtys.skip_current_subtree();
+ }
+ ty::TyBareFn(..) | ty::TyTrait(..) => {
+ subtys.skip_current_subtree();
+ let temp = capture_components(infcx, ty);
+ out.push(Component::RFC1214(temp));
+ }
+ ty::TyParam(p) => {
+ out.push(Component::Param(p));
+ subtys.skip_current_subtree();
+ }
+ ty::TyProjection(ref data) => {
+ // 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`.
+
+ if !data.has_escaping_regions() {
+ // best case: no escaping reions, so push the
+ // projection and skip the subtree (thus
+ // generating no constraints for Pi).
+ out.push(Component::Projection(*data));
+ } else {
+ // fallback case: continue walking through and
+ // constrain Pi.
+ let temp = capture_components(infcx, ty);
+ out.push(Component::EscapingProjection(temp));
+ }
+ subtys.skip_current_subtree();
+ }
+ ty::TyInfer(_) => {
+ let ty = infcx.resolve_type_vars_if_possible(&ty);
+ if let ty::TyInfer(infer_ty) = ty.sty {
+ out.push(Component::UnresolvedInferenceVariable(infer_ty));
+ } else {
+ compute_components(infcx, ty, out);
+ }
+ }
+ _ => {
+ // for all other types, just constrain the regions and
+ // keep walking to find any other types.
+ push_region_constraints(out, ty.regions());
+ }
+ }
+ }
+}
+
+fn capture_components<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
+ ty: Ty<'tcx>)
+ -> Vec<Component<'tcx>> {
+ let mut temp = vec![];
+ push_region_constraints(&mut temp, ty.regions());
+ for subty in ty.walk_shallow() {
+ compute_components(infcx, subty, &mut temp);
+ }
+ temp
+}
+
+fn push_region_constraints<'tcx>(out: &mut Vec<Component<'tcx>>, regions: Vec<ty::Region>) {
+ for r in regions {
+ if !r.is_bound() {
+ out.push(Component::Region(r));
+ }
+ }
+}
+
--- /dev/null
+// Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use middle::infer::InferCtxt;
+use middle::outlives::{self, Component};
+use middle::subst::Substs;
+use middle::traits;
+use middle::ty::{self, RegionEscape, ToPredicate, Ty};
+use std::iter::once;
+use std::mem;
+use std::rc::Rc;
+use syntax::ast;
+use syntax::codemap::Span;
+use util::common::ErrorReported;
+
+/// Returns the set of obligations needed to make `ty` well-formed.
+/// If `ty` contains unresolved inference variables, this may include
+/// further WF obligations. However, if `ty` IS an unresolved
+/// inference variable, returns `None`, because we are not able to
+/// make any progress at all. This is to prevent "livelock" where we
+/// say "$0 is WF if $0 is WF".
+pub fn obligations<'a,'tcx>(infcx: &InferCtxt<'a, 'tcx>,
+ body_id: ast::NodeId,
+ ty: Ty<'tcx>,
+ span: Span,
+ rfc1214: bool)
+ -> Option<Vec<traits::PredicateObligation<'tcx>>>
+{
+ let mut wf = WfPredicates { infcx: infcx,
+ body_id: body_id,
+ span: span,
+ out: vec![],
+ rfc1214: rfc1214 };
+ if wf.compute(ty) {
+ debug!("wf::obligations({:?}, body_id={:?}) = {:?}", ty, body_id, wf.out);
+ let result = wf.normalize();
+ debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", ty, body_id, result);
+ Some(result)
+ } else {
+ None // no progress made, return None
+ }
+}
+
+/// Returns the obligations that make this trait reference
+/// well-formed. For example, if there is a trait `Set` defined like
+/// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
+/// if `Bar: Eq`.
+pub fn trait_obligations<'a,'tcx>(infcx: &InferCtxt<'a, 'tcx>,
+ body_id: ast::NodeId,
+ trait_ref: &ty::TraitRef<'tcx>,
+ span: Span,
+ rfc1214: bool)
+ -> Vec<traits::PredicateObligation<'tcx>>
+{
+ let mut wf = WfPredicates { infcx: infcx, body_id: body_id, span: span,
+ out: vec![], rfc1214: rfc1214 };
+ wf.compute_trait_ref(trait_ref);
+ wf.normalize()
+}
+
+pub fn predicate_obligations<'a,'tcx>(infcx: &InferCtxt<'a, 'tcx>,
+ body_id: ast::NodeId,
+ predicate: &ty::Predicate<'tcx>,
+ span: Span,
+ rfc1214: bool)
+ -> Vec<traits::PredicateObligation<'tcx>>
+{
+ let mut wf = WfPredicates { infcx: infcx, body_id: body_id, span: span,
+ out: vec![], rfc1214: rfc1214 };
+
+ // (*) ok to skip binders, because wf code is prepared for it
+ match *predicate {
+ ty::Predicate::Trait(ref t) => {
+ wf.compute_trait_ref(&t.skip_binder().trait_ref); // (*)
+ }
+ ty::Predicate::Equate(ref t) => {
+ wf.compute(t.skip_binder().0);
+ wf.compute(t.skip_binder().1);
+ }
+ ty::Predicate::RegionOutlives(..) => {
+ }
+ ty::Predicate::TypeOutlives(ref t) => {
+ wf.compute(t.skip_binder().0);
+ }
+ ty::Predicate::Projection(ref t) => {
+ let t = t.skip_binder(); // (*)
+ wf.compute_projection(t.projection_ty);
+ wf.compute(t.ty);
+ }
+ ty::Predicate::WellFormed(t) => {
+ wf.compute(t);
+ }
+ ty::Predicate::ObjectSafe(_) => {
+ }
+ }
+
+ wf.normalize()
+}
+
+/// Implied bounds are region relationships that we deduce
+/// automatically. The idea is that (e.g.) a caller must check that a
+/// function's argument types are well-formed immediately before
+/// calling that fn, and hence the *callee* can assume that its
+/// argument types are well-formed. This may imply certain relationships
+/// between generic parameters. For example:
+///
+/// fn foo<'a,T>(x: &'a T)
+///
+/// can only be called with a `'a` and `T` such that `&'a T` is WF.
+/// For `&'a T` to be WF, `T: 'a` must hold. So we can assume `T: 'a`.
+#[derive(Debug)]
+pub enum ImpliedBound<'tcx> {
+ RegionSubRegion(ty::Region, ty::Region),
+ RegionSubParam(ty::Region, ty::ParamTy),
+ RegionSubProjection(ty::Region, ty::ProjectionTy<'tcx>),
+}
+
+/// This routine computes the full set of well-formedness constraints
+/// that must hold for the type `ty` to appear in a context with
+/// lifetime `outer_region`.
+pub fn implied_bounds<'a,'tcx>(
+ infcx: &'a InferCtxt<'a,'tcx>,
+ body_id: ast::NodeId,
+ ty: Ty<'tcx>,
+ span: Span)
+ -> Vec<ImpliedBound<'tcx>>
+{
+ // Sometimes when we ask what it takes for T: WF, we get back that
+ // U: WF is required; in that case, we push U onto this stack and
+ // process it next. Currently (at least) these resulting
+ // predicates are always guaranteed to be a subset of the original
+ // type, so we need not fear non-termination.
+ let mut wf_types = vec![ty];
+
+ let mut implied_bounds = vec![];
+
+ while let Some(ty) = wf_types.pop() {
+ // Compute the obligations for `ty` to be well-formed. If `ty` is
+ // an unresolved inference variable, just substituted an empty set
+ // -- because the return type here is going to be things we *add*
+ // to the environment, it's always ok for this set to be smaller
+ // than the ultimate set. (Note: normally there won't be
+ // unresolved inference variables here anyway, but there might be
+ // during typeck under some circumstances.)
+ let obligations = obligations(infcx, body_id, ty, span, false).unwrap_or(vec![]);
+
+ // From the full set of obligations, just filter down to the
+ // region relationships.
+ implied_bounds.extend(
+ obligations
+ .into_iter()
+ .flat_map(|obligation| {
+ assert!(!obligation.has_escaping_regions());
+ match obligation.predicate {
+ ty::Predicate::Trait(..) |
+ ty::Predicate::Equate(..) |
+ ty::Predicate::Projection(..) |
+ ty::Predicate::ObjectSafe(..) =>
+ vec![],
+
+ ty::Predicate::WellFormed(subty) => {
+ wf_types.push(subty);
+ vec![]
+ }
+
+ ty::Predicate::RegionOutlives(ref data) =>
+ match infcx.tcx.no_late_bound_regions(data) {
+ None =>
+ vec![],
+ Some(ty::OutlivesPredicate(r_a, r_b)) =>
+ vec![ImpliedBound::RegionSubRegion(r_b, r_a)],
+ },
+
+ ty::Predicate::TypeOutlives(ref data) =>
+ match infcx.tcx.no_late_bound_regions(data) {
+ None => vec![],
+ Some(ty::OutlivesPredicate(ty_a, r_b)) => {
+ let components = outlives::components(infcx, ty_a);
+ implied_bounds_from_components(r_b, components)
+ }
+ },
+ }}));
+ }
+
+ implied_bounds
+}
+
+/// When we have an implied bound that `T: 'a`, we can further break
+/// this down to determine what relationships would have to hold for
+/// `T: 'a` to hold. We get to assume that the caller has validated
+/// those relationships.
+fn implied_bounds_from_components<'tcx>(sub_region: ty::Region,
+ sup_components: Vec<Component<'tcx>>)
+ -> Vec<ImpliedBound<'tcx>>
+{
+ sup_components
+ .into_iter()
+ .flat_map(|component| {
+ match component {
+ Component::Region(r) =>
+ vec!(ImpliedBound::RegionSubRegion(sub_region, r)),
+ Component::Param(p) =>
+ vec!(ImpliedBound::RegionSubParam(sub_region, p)),
+ Component::Projection(p) =>
+ vec!(ImpliedBound::RegionSubProjection(sub_region, p)),
+ Component::EscapingProjection(_) =>
+ // If the projection has escaping regions, don't
+ // try to infer any implied bounds even for its
+ // free components. This is conservative, because
+ // the caller will still have to prove that those
+ // free components outlive `sub_region`. But the
+ // idea is that the WAY that the caller proves
+ // that may change in the future and we want to
+ // give ourselves room to get smarter here.
+ vec!(),
+ Component::UnresolvedInferenceVariable(..) =>
+ vec!(),
+ Component::RFC1214(components) =>
+ implied_bounds_from_components(sub_region, components),
+ }
+ })
+ .collect()
+}
+
+struct WfPredicates<'a,'tcx:'a> {
+ infcx: &'a InferCtxt<'a, 'tcx>,
+ body_id: ast::NodeId,
+ span: Span,
+ out: Vec<traits::PredicateObligation<'tcx>>,
+ rfc1214: bool
+}
+
+impl<'a,'tcx> WfPredicates<'a,'tcx> {
+ fn rfc1214<R,F:FnOnce(&mut WfPredicates<'a,'tcx>) -> R>(&mut self, f: F) -> R {
+ let b = mem::replace(&mut self.rfc1214, true);
+ let r = f(self);
+ self.rfc1214 = b;
+ r
+ }
+
+ fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
+ if !self.rfc1214 {
+ traits::ObligationCause::new(self.span, self.body_id, code)
+ } else {
+ let code = traits::ObligationCauseCode::RFC1214(Rc::new(code));
+ traits::ObligationCause::new(self.span, self.body_id, code)
+ }
+ }
+
+ fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
+ let cause = self.cause(traits::MiscObligation);
+ let infcx = &mut self.infcx;
+ self.out.iter()
+ .inspect(|pred| assert!(!pred.has_escaping_regions()))
+ .flat_map(|pred| {
+ let mut selcx = traits::SelectionContext::new(infcx);
+ let pred = traits::normalize(&mut selcx, cause.clone(), pred);
+ once(pred.value).chain(pred.obligations)
+ })
+ .collect()
+ }
+
+ fn compute_rfc1214(&mut self, ty: Ty<'tcx>) {
+ let b = mem::replace(&mut self.rfc1214, true);
+ for subty in ty.walk().skip(1) {
+ self.compute(subty);
+ }
+ self.rfc1214 = b;
+ }
+
+ /// Pushes the obligations required for `trait_ref` to be WF into
+ /// `self.out`.
+ fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>) {
+ let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
+ self.out.extend(obligations);
+
+ let cause = self.cause(traits::MiscObligation);
+ self.out.extend(
+ trait_ref.substs.types
+ .as_slice()
+ .iter()
+ .filter(|ty| !ty.has_escaping_regions())
+ .map(|ty| traits::Obligation::new(cause.clone(),
+ ty::Predicate::WellFormed(ty))));
+ }
+
+ /// Pushes the obligations required for `trait_ref::Item` to be WF
+ /// into `self.out`.
+ fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
+ // A projection is well-formed if (a) the trait ref itself is
+ // WF WF and (b) the trait-ref holds. (It may also be
+ // normalizable and be WF that way.)
+
+ self.compute_trait_ref(&data.trait_ref);
+
+ if !data.has_escaping_regions() {
+ let predicate = data.trait_ref.to_predicate();
+ let cause = self.cause(traits::ProjectionWf(data));
+ self.out.push(traits::Obligation::new(cause, predicate));
+ }
+ }
+
+ /// Push new obligations into `out`. Returns true if it was able
+ /// to generate all the predicates needed to validate that `ty0`
+ /// is WF. Returns false if `ty0` is an unresolved type variable,
+ /// in which case we are not able to simplify at all.
+ fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
+ let mut subtys = ty0.walk();
+ while let Some(ty) = subtys.next() {
+ match ty.sty {
+ ty::TyBool |
+ ty::TyChar |
+ ty::TyInt(..) |
+ ty::TyUint(..) |
+ ty::TyFloat(..) |
+ ty::TyError |
+ ty::TyStr |
+ ty::TyParam(_) => {
+ // WfScalar, WfParameter, etc
+ }
+
+ ty::TySlice(subty) |
+ ty::TyArray(subty, _) => {
+ self.rfc1214(|this| {
+ if !subty.has_escaping_regions() {
+ let cause = this.cause(traits::SliceOrArrayElem);
+ match traits::trait_ref_for_builtin_bound(this.infcx.tcx,
+ ty::BoundSized,
+ subty) {
+ Ok(trait_ref) => {
+ this.out.push(
+ traits::Obligation::new(cause,
+ trait_ref.to_predicate()));
+ }
+ Err(ErrorReported) => { }
+ }
+ }
+ })
+ }
+
+ ty::TyBox(_) |
+ ty::TyTuple(_) |
+ ty::TyRawPtr(_) => {
+ // simple cases that are WF if their type args are WF
+ }
+
+ ty::TyProjection(data) => {
+ subtys.skip_current_subtree(); // subtree handled by compute_projection
+ self.compute_projection(data);
+ }
+
+ ty::TyEnum(def, substs) |
+ ty::TyStruct(def, substs) => {
+ // WfNominalType
+ let obligations = self.nominal_obligations(def.did, substs);
+ self.out.extend(obligations);
+ }
+
+ ty::TyRef(r, mt) => {
+ // WfReference
+ if !r.has_escaping_regions() && !mt.ty.has_escaping_regions() {
+ let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
+ self.out.push(
+ traits::Obligation::new(
+ cause,
+ ty::Predicate::TypeOutlives(
+ ty::Binder(
+ ty::OutlivesPredicate(mt.ty, *r)))));
+ }
+ }
+
+ ty::TyClosure(..) => {
+ // the types in a closure are always the types of
+ // local variables (or possibly references to local
+ // variables), which are separately checked w/r/t
+ // WFedness.
+ }
+
+ ty::TyBareFn(..) => {
+ // process the bound types; because the old implicator
+ // did not do this, go into RFC1214 mode.
+ subtys.skip_current_subtree();
+ self.compute_rfc1214(ty);
+ }
+
+ ty::TyTrait(ref data) => {
+ // WfObject
+ //
+ // Here, we defer WF checking due to higher-ranked
+ // regions. This is perhaps not ideal.
+ self.from_object_ty(ty, data);
+
+ // FIXME(#27579) RFC also considers adding trait
+ // obligations that don't refer to Self and
+ // checking those
+
+ let cause = self.cause(traits::MiscObligation);
+ self.out.push(
+ traits::Obligation::new(
+ cause,
+ ty::Predicate::ObjectSafe(data.principal_def_id())));
+
+ // process the bound types; because the old implicator
+ // did not do this, go into RFC1214 mode.
+ subtys.skip_current_subtree();
+ self.compute_rfc1214(ty);
+ }
+
+ // Inference variables are the complicated case, since we don't
+ // know what type they are. We do two things:
+ //
+ // 1. Check if they have been resolved, and if so proceed with
+ // THAT type.
+ // 2. If not, check whether this is the type that we
+ // started with (ty0). In that case, we've made no
+ // progress at all, so return false. Otherwise,
+ // we've at least simplified things (i.e., we went
+ // from `Vec<$0>: WF` to `$0: WF`, so we can
+ // register a pending obligation and keep
+ // moving. (Goal is that an "inductive hypothesis"
+ // is satisfied to ensure termination.)
+ ty::TyInfer(_) => {
+ let ty = self.infcx.shallow_resolve(ty);
+ if let ty::TyInfer(_) = ty.sty { // not yet resolved...
+ if ty == ty0 { // ...this is the type we started from! no progress.
+ return false;
+ }
+
+ let cause = self.cause(traits::MiscObligation);
+ self.out.push( // ...not the type we started from, so we made progress.
+ traits::Obligation::new(cause, ty::Predicate::WellFormed(ty)));
+ } else {
+ // Yes, resolved, proceed with the
+ // result. Should never return false because
+ // `ty` is not a TyInfer.
+ assert!(self.compute(ty));
+ }
+ }
+ }
+ }
+
+ // if we made it through that loop above, we made progress!
+ return true;
+ }
+
+ fn nominal_obligations(&mut self,
+ def_id: ast::DefId,
+ substs: &Substs<'tcx>)
+ -> Vec<traits::PredicateObligation<'tcx>>
+ {
+ let predicates =
+ self.infcx.tcx.lookup_predicates(def_id)
+ .instantiate(self.infcx.tcx, substs);
+ let cause = self.cause(traits::ItemObligation(def_id));
+ predicates.predicates
+ .into_iter()
+ .map(|pred| traits::Obligation::new(cause.clone(), pred))
+ .filter(|pred| !pred.has_escaping_regions())
+ .collect()
+ }
+
+ fn from_object_ty(&mut self, ty: Ty<'tcx>, data: &ty::TraitTy<'tcx>) {
+ // Imagine a type like this:
+ //
+ // trait Foo { }
+ // trait Bar<'c> : 'c { }
+ //
+ // &'b (Foo+'c+Bar<'d>)
+ // ^
+ //
+ // In this case, the following relationships must hold:
+ //
+ // 'b <= 'c
+ // 'd <= 'c
+ //
+ // The first conditions is due to the normal region pointer
+ // rules, which say that a reference cannot outlive its
+ // referent.
+ //
+ // The final condition may be a bit surprising. In particular,
+ // you may expect that it would have been `'c <= 'd`, since
+ // usually lifetimes of outer things are conservative
+ // approximations for inner things. However, it works somewhat
+ // differently with trait objects: here the idea is that if the
+ // user specifies a region bound (`'c`, in this case) it is the
+ // "master bound" that *implies* that bounds from other traits are
+ // all met. (Remember that *all bounds* in a type like
+ // `Foo+Bar+Zed` must be met, not just one, hence if we write
+ // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
+ // 'y.)
+ //
+ // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
+ // am looking forward to the future here.
+
+ if !data.has_escaping_regions() {
+ let implicit_bounds =
+ object_region_bounds(self.infcx.tcx,
+ &data.principal,
+ data.bounds.builtin_bounds);
+
+ let explicit_bound = data.bounds.region_bound;
+
+ for implicit_bound in implicit_bounds {
+ let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
+ let outlives = ty::Binder(ty::OutlivesPredicate(explicit_bound, implicit_bound));
+ self.out.push(traits::Obligation::new(cause, outlives.to_predicate()));
+ }
+ }
+ }
+}
+
+/// Given an object type like `SomeTrait+Send`, computes the lifetime
+/// bounds that must hold on the elided self type. These are derived
+/// from the declarations of `SomeTrait`, `Send`, and friends -- if
+/// they declare `trait SomeTrait : 'static`, for example, then
+/// `'static` would appear in the list. The hard work is done by
+/// `ty::required_region_bounds`, see that for more information.
+pub fn object_region_bounds<'tcx>(
+ tcx: &ty::ctxt<'tcx>,
+ principal: &ty::PolyTraitRef<'tcx>,
+ others: ty::BuiltinBounds)
+ -> Vec<ty::Region>
+{
+ // Since we don't actually *know* the self type for an object,
+ // this "open(err)" serves as a kind of dummy standin -- basically
+ // a skolemized type.
+ let open_ty = tcx.mk_infer(ty::FreshTy(0));
+
+ // Note that we preserve the overall binding levels here.
+ assert!(!open_ty.has_escaping_regions());
+ let substs = tcx.mk_substs(principal.0.substs.with_self_ty(open_ty));
+ let trait_refs = vec!(ty::Binder(ty::TraitRef::new(principal.0.def_id, substs)));
+
+ let mut predicates = others.to_predicates(tcx, open_ty);
+ predicates.extend(trait_refs.iter().map(|t| t.to_predicate()));
+
+ tcx.required_region_bounds(open_ty, predicates)
+}
+
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