Modify implicator to take a "syntactic" approach, ensuring that fns and

object types impose minimal constraints on their arguments.  Also modify
regionck to consider `<P0 as TraitRef<P1..Pn>>::Foo: 'r` to hold if `Pi:
'r` (for all `i`). This is a fallback, because in some cases we can
impose looser requirements. Currently the inference is sound but not
precise. Fixes rust-lang#24622.

Adapted cherry-pick of 45f93290969f19d8217a2e4dff7b12dc81957eb6.

Author: nikomatsakis

src/librustc/middle/expr_use_visitor.rs

@@ -239,7 +239,7 @@ impl OverloadedCallType {
 // supplies types from the tree. After type checking is complete, you
 // can just use the tcx as the typer.
 
-pub struct ExprUseVisitor<'d,'t,'a: 't, 'tcx:'a> {
+pub struct ExprUseVisitor<'d, 't, 'a: 't, 'tcx:'a+'d> {
     typer: &'t infer::InferCtxt<'a, 'tcx>,
     mc: mc::MemCategorizationContext<'t, 'a, 'tcx>,
     delegate: &'d mut (Delegate<'tcx>+'d),

src/librustc/middle/implicator.rs

@@ -41,8 +41,9 @@ struct Implicator<'a, 'tcx: 'a> {
     visited: FnvHashSet<Ty<'tcx>>,
 }
 
-/// This routine computes the well-formedness constraints that must hold for the type `ty` to
-/// appear in a context with lifetime `outer_region`
+/// 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 implications<'a,'tcx>(
     infcx: &'a InferCtxt<'a,'tcx>,
     body_id: ast::NodeId,
@@ -90,12 +91,15 @@ impl<'a, 'tcx> Implicator<'a, 'tcx> {
             ty::TyInt(..) |
             ty::TyUint(..) |
             ty::TyFloat(..) |
-            ty::TyBareFn(..) |
             ty::TyError |
             ty::TyStr => {
                 // No borrowed content reachable here.
             }
 
+            ty::TyBareFn(..) => {
+                self.accumulate_referenced_regions_and_types(ty);
+            }
+
             ty::TyClosure(def_id, substs) => {
                 let &(r_a, opt_ty) = self.stack.last().unwrap();
                 self.out.push(Implication::RegionSubClosure(opt_ty, r_a, def_id, substs));
@@ -382,6 +386,37 @@ impl<'a, 'tcx> Implicator<'a, 'tcx> {
             // constraint above
             self.out.push(Implication::RegionSubRegion(Some(ty), r_d, r_c));
         }
+
+        self.accumulate_referenced_regions_and_types(ty);
+    }
+
+    fn accumulate_referenced_regions_and_types(&mut self,
+                                               iface_ty: Ty<'tcx>)
+    {
+        // for now, `iface_ty` represents some type that is a fn or
+        // trait object argument, and because those appear underneath
+        // forall binders, we do not enforce the full WF requirements,
+        // just the lifetime "outlives" requirements.
+        for ty in iface_ty.walk() {
+            match ty.sty {
+                ty::TyParam(p) => {
+                    self.push_param_constraint_from_top(p);
+                }
+
+                _ => { }
+            }
+
+            for r in ty.regions() {
+                match r {
+                    ty::ReLateBound(..) => {
+                        // skip late-bound regions
+                    }
+                    _ => {
+                        self.push_region_constraint_from_top(r);
+                    }
+                }
+            }
+        }
     }
 
     fn fully_normalize<T>(&self, value: &T) -> Result<T,ErrorReported>

src/librustc/middle/infer/error_reporting.rs

@@ -241,8 +241,7 @@ pub trait ErrorReporting<'tcx> {
     fn report_generic_bound_failure(&self,
                                     origin: SubregionOrigin<'tcx>,
                                     kind: GenericKind<'tcx>,
-                                    sub: Region,
-                                    sups: Vec<Region>);
+                                    sub: Region);
 
     fn report_sub_sup_conflict(&self,
                                var_origin: RegionVariableOrigin,
@@ -294,8 +293,8 @@ impl<'a, 'tcx> ErrorReporting<'tcx> for InferCtxt<'a, 'tcx> {
                     self.report_concrete_failure(origin, sub, sup);
                 }
 
-                GenericBoundFailure(kind, param_ty, sub, sups) => {
-                    self.report_generic_bound_failure(kind, param_ty, sub, sups);
+                GenericBoundFailure(kind, param_ty, sub) => {
+                    self.report_generic_bound_failure(kind, param_ty, sub);
                 }
 
                 SubSupConflict(var_origin,
@@ -529,8 +528,7 @@ impl<'a, 'tcx> ErrorReporting<'tcx> for InferCtxt<'a, 'tcx> {
     fn report_generic_bound_failure(&self,
                                     origin: SubregionOrigin<'tcx>,
                                     bound_kind: GenericKind<'tcx>,
-                                    sub: Region,
-                                    _sups: Vec<Region>)
+                                    sub: Region)
     {
         // FIXME: it would be better to report the first error message
         // with the span of the parameter itself, rather than the span

src/librustc/middle/infer/mod.rs

@@ -20,7 +20,7 @@ pub use self::ValuePairs::*;
 pub use self::fixup_err::*;
 pub use middle::ty::IntVarValue;
 pub use self::freshen::TypeFreshener;
-pub use self::region_inference::GenericKind;
+pub use self::region_inference::{GenericKind, VerifyBound};
 
 use middle::free_region::FreeRegionMap;
 use middle::mem_categorization as mc;
@@ -1284,13 +1284,13 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                                 origin: SubregionOrigin<'tcx>,
                                 kind: GenericKind<'tcx>,
                                 a: ty::Region,
-                                bs: Vec<ty::Region>) {
+                                bound: VerifyBound) {
         debug!("verify_generic_bound({:?}, {:?} <: {:?})",
                kind,
                a,
-               bs);
+               bound);
 
-        self.region_vars.verify_generic_bound(origin, kind, a, bs);
+        self.region_vars.verify_generic_bound(origin, kind, a, bound);
     }
 
     pub fn can_equate<'b,T>(&'b self, a: &T, b: &T) -> UnitResult<'tcx>

src/librustc/middle/infer/region_inference/mod.rs

@@ -65,15 +65,41 @@ pub enum Verify<'tcx> {
     // outlive `RS`. Therefore verify that `R <= RS[i]` for some
     // `i`. Inference variables may be involved (but this verification
     // step doesn't influence inference).
-    VerifyGenericBound(GenericKind<'tcx>, SubregionOrigin<'tcx>, Region, Vec<Region>),
+    VerifyGenericBound(GenericKind<'tcx>, SubregionOrigin<'tcx>, Region, VerifyBound),
 }
 
-#[derive(Clone, PartialEq, Eq)]
+#[derive(Copy, Clone, PartialEq, Eq)]
 pub enum GenericKind<'tcx> {
     Param(ty::ParamTy),
     Projection(ty::ProjectionTy<'tcx>),
 }
 
+// When we introduce a verification step, we wish to test that a
+// particular region (let's call it `'min`) meets some bound.
+// The bound is described the by the following grammar:
+#[derive(Debug)]
+pub enum VerifyBound {
+    // B = exists {R} --> some 'r in {R} must outlive 'min
+    //
+    // Put another way, the subject value is known to outlive all
+    // regions in {R}, so if any of those outlives 'min, then the
+    // bound is met.
+    AnyRegion(Vec<Region>),
+
+    // B = forall {R} --> all 'r in {R} must outlive 'min
+    //
+    // Put another way, the subject value is known to outlive some
+    // region in {R}, so if all of those outlives 'min, then the bound
+    // is met.
+    AllRegions(Vec<Region>),
+
+    // B = exists {B} --> 'min must meet some bound b in {B}
+    AnyBound(Vec<VerifyBound>),
+
+    // B = forall {B} --> 'min must meet all bounds b in {B}
+    AllBounds(Vec<VerifyBound>),
+}
+
 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
 pub struct TwoRegions {
     a: Region,
@@ -103,12 +129,11 @@ pub enum RegionResolutionError<'tcx> {
     /// `o` requires that `a <= b`, but this does not hold
     ConcreteFailure(SubregionOrigin<'tcx>, Region, Region),
 
-    /// `GenericBoundFailure(p, s, a, bs)
+    /// `GenericBoundFailure(p, s, a)
     ///
     /// The parameter/associated-type `p` must be known to outlive the lifetime
-    /// `a`, but it is only known to outlive `bs` (and none of the
-    /// regions in `bs` outlive `a`).
-    GenericBoundFailure(SubregionOrigin<'tcx>, GenericKind<'tcx>, Region, Vec<Region>),
+    /// `a` (but none of the known bounds are sufficient).
+    GenericBoundFailure(SubregionOrigin<'tcx>, GenericKind<'tcx>, Region),
 
     /// `SubSupConflict(v, sub_origin, sub_r, sup_origin, sup_r)`:
     ///
@@ -409,6 +434,14 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
         debug!("RegionVarBindings: add_verify({:?})",
                verify);
 
+        // skip no-op cases known to be satisfied
+        match verify {
+            VerifyGenericBound(_, _, _, VerifyBound::AllBounds(ref bs)) if bs.len() == 0 => {
+                return;
+            }
+            _ => { }
+        }
+
         let mut verifys = self.verifys.borrow_mut();
         let index = verifys.len();
         verifys.push(verify);
@@ -498,8 +531,8 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
                                 origin: SubregionOrigin<'tcx>,
                                 kind: GenericKind<'tcx>,
                                 sub: Region,
-                                sups: Vec<Region>) {
-        self.add_verify(VerifyGenericBound(kind, origin, sub, sups));
+                                bound: VerifyBound) {
+        self.add_verify(VerifyGenericBound(kind, origin, sub, bound));
     }
 
     pub fn lub_regions(&self,
@@ -664,12 +697,11 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
                                     &mut result_set, r,
                                     a, b);
                             }
-                            VerifyGenericBound(_, _, a, ref bs) => {
-                                for &b in bs {
-                                    consider_adding_bidirectional_edges(
-                                        &mut result_set, r,
-                                        a, b);
-                                }
+                            VerifyGenericBound(_, _, a, ref bound) => {
+                                bound.for_each_region(&mut |b| {
+                                    consider_adding_bidirectional_edges(&mut result_set, r,
+                                                                        a, b)
+                                });
                             }
                         }
                     }
@@ -1264,20 +1296,13 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
                     errors.push(ConcreteFailure((*origin).clone(), sub, sup));
                 }
 
-                VerifyGenericBound(ref kind, ref origin, sub, ref sups) => {
+                VerifyGenericBound(ref kind, ref origin, sub, ref bound) => {
                     let sub = normalize(values, sub);
-                    if sups.iter()
-                           .map(|&sup| normalize(values, sup))
-                           .any(|sup| free_regions.is_subregion_of(self.tcx, sub, sup))
-                    {
+                    if bound.is_met(self.tcx, free_regions, values, sub) {
                         continue;
                     }
 
-                    let sups = sups.iter().map(|&sup| normalize(values, sup))
-                                          .collect();
-                    errors.push(
-                        GenericBoundFailure(
-                            (*origin).clone(), kind.clone(), sub, sups));
+                    errors.push(GenericBoundFailure((*origin).clone(), kind.clone(), sub));
                 }
             }
         }
@@ -1723,3 +1748,82 @@ impl<'tcx> GenericKind<'tcx> {
         }
     }
 }
+
+impl VerifyBound {
+    fn for_each_region(&self, f: &mut FnMut(ty::Region)) {
+        match self {
+            &VerifyBound::AnyRegion(ref rs) |
+            &VerifyBound::AllRegions(ref rs) =>
+                for &r in rs { f(r); },
+
+            &VerifyBound::AnyBound(ref bs) |
+            &VerifyBound::AllBounds(ref bs) =>
+                for b in bs { b.for_each_region(f); },
+        }
+    }
+
+    pub fn must_hold(&self) -> bool {
+        match self {
+            &VerifyBound::AnyRegion(ref bs) => bs.contains(&ty::ReStatic),
+            &VerifyBound::AllRegions(ref bs) => bs.is_empty(),
+            &VerifyBound::AnyBound(ref bs) => bs.iter().any(|b| b.must_hold()),
+            &VerifyBound::AllBounds(ref bs) => bs.iter().all(|b| b.must_hold()),
+        }
+    }
+
+    pub fn cannot_hold(&self) -> bool {
+        match self {
+            &VerifyBound::AnyRegion(ref bs) => bs.is_empty(),
+            &VerifyBound::AllRegions(ref bs) => bs.contains(&ty::ReEmpty),
+            &VerifyBound::AnyBound(ref bs) => bs.iter().all(|b| b.cannot_hold()),
+            &VerifyBound::AllBounds(ref bs) => bs.iter().any(|b| b.cannot_hold()),
+        }
+    }
+
+    pub fn or(self, vb: VerifyBound) -> VerifyBound {
+        if self.must_hold() || vb.cannot_hold() {
+            self
+        } else if self.cannot_hold() || vb.must_hold() {
+            vb
+        } else {
+            VerifyBound::AnyBound(vec![self, vb])
+        }
+    }
+
+    pub fn and(self, vb: VerifyBound) -> VerifyBound {
+        if self.must_hold() && vb.must_hold() {
+            self
+        } else if self.cannot_hold() && vb.cannot_hold() {
+            self
+        } else {
+            VerifyBound::AllBounds(vec![self, vb])
+        }
+    }
+
+    fn is_met<'tcx>(&self,
+                    tcx: &ty::ctxt<'tcx>,
+                    free_regions: &FreeRegionMap,
+                    var_values: &Vec<VarValue>,
+                    min: ty::Region)
+                    -> bool {
+        match self {
+            &VerifyBound::AnyRegion(ref rs) =>
+                rs.iter()
+                  .map(|&r| normalize(var_values, r))
+                  .any(|r| free_regions.is_subregion_of(tcx, min, r)),
+
+            &VerifyBound::AllRegions(ref rs) =>
+                rs.iter()
+                  .map(|&r| normalize(var_values, r))
+                  .all(|r| free_regions.is_subregion_of(tcx, min, r)),
+
+            &VerifyBound::AnyBound(ref bs) =>
+                bs.iter()
+                  .any(|b| b.is_met(tcx, free_regions, var_values, min)),
+
+            &VerifyBound::AllBounds(ref bs) =>
+                bs.iter()
+                  .all(|b| b.is_met(tcx, free_regions, var_values, min)),
+        }
+    }
+}