Make GATs less ICE-prone.

src/librustc/infer/error_reporting/mod.rs

@@ -1912,6 +1912,7 @@ impl<'tcx> ObligationCause<'tcx> {
         use crate::traits::ObligationCauseCode::*;
         match self.code {
             CompareImplMethodObligation { .. } => Error0308("method not compatible with trait"),
+            CompareImplTypeObligation { .. } => Error0308("type not compatible with trait"),
             MatchExpressionArm(box MatchExpressionArmCause { source, .. }) =>
                 Error0308(match source {
                     hir::MatchSource::IfLetDesugar { .. } =>
@@ -1948,6 +1949,7 @@ impl<'tcx> ObligationCause<'tcx> {
         use crate::traits::ObligationCauseCode::*;
         match self.code {
             CompareImplMethodObligation { .. } => "method type is compatible with trait",
+            CompareImplTypeObligation { .. } => "associated type is compatible with trait",
             ExprAssignable => "expression is assignable",
             MatchExpressionArm(box MatchExpressionArmCause { source, .. }) => match source {
                 hir::MatchSource::IfLetDesugar { .. } => "`if let` arms have compatible types",

src/librustc/traits/error_reporting.rs

@@ -702,6 +702,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             SelectionError::Unimplemented => {
                 if let ObligationCauseCode::CompareImplMethodObligation {
                     item_name, impl_item_def_id, trait_item_def_id,
+                } | ObligationCauseCode::CompareImplTypeObligation {
+                    item_name, impl_item_def_id, trait_item_def_id,
                 } = obligation.cause.code {
                     self.report_extra_impl_obligation(
                         span,
@@ -2631,6 +2633,12 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                               but not on the corresponding trait method",
                              predicate));
             }
+            ObligationCauseCode::CompareImplTypeObligation { .. } => {
+                err.note(&format!(
+                    "the requirement `{}` appears on the associated impl type\
+                     but not on the corresponding associated trait type",
+                     predicate));
+            }
             ObligationCauseCode::ReturnType |
             ObligationCauseCode::ReturnValue(_) |
             ObligationCauseCode::BlockTailExpression(_) => (),

src/librustc/traits/mod.rs

@@ -230,6 +230,13 @@ pub enum ObligationCauseCode<'tcx> {
         trait_item_def_id: DefId,
     },
 
+    /// Error derived when matching traits/impls; see ObligationCause for more details
+    CompareImplTypeObligation {
+        item_name: ast::Name,
+        impl_item_def_id: DefId,
+        trait_item_def_id: DefId,
+    },
+
     /// Checking that this expression can be assigned where it needs to be
     // FIXME(eddyb) #11161 is the original Expr required?
     ExprAssignable,

src/librustc/traits/project.rs

@@ -23,6 +23,7 @@ use crate::ty::subst::{Subst, InternalSubsts};
 use crate::ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt};
 use crate::ty::fold::{TypeFoldable, TypeFolder};
 use crate::util::common::FN_OUTPUT_NAME;
+use syntax_pos::DUMMY_SP;
 
 /// Depending on the stage of compilation, we want projection to be
 /// more or less conservative.
@@ -1437,11 +1438,14 @@ fn confirm_impl_candidate<'cx, 'tcx>(
     obligation: &ProjectionTyObligation<'tcx>,
     impl_vtable: VtableImplData<'tcx, PredicateObligation<'tcx>>,
 ) -> Progress<'tcx> {
+    let tcx = selcx.tcx();
+
     let VtableImplData { impl_def_id, substs, nested } = impl_vtable;
+    let assoc_item_id = obligation.predicate.item_def_id;
+    let trait_def_id = tcx.trait_id_of_impl(impl_def_id).unwrap();
 
-    let tcx = selcx.tcx();
     let param_env = obligation.param_env;
-    let assoc_ty = assoc_ty_def(selcx, impl_def_id, obligation.predicate.item_def_id);
+    let assoc_ty = assoc_ty_def(selcx, impl_def_id, assoc_item_id);
 
     if !assoc_ty.item.defaultness.has_value() {
         // This means that the impl is missing a definition for the
@@ -1456,16 +1460,28 @@ fn confirm_impl_candidate<'cx, 'tcx>(
             obligations: nested,
         };
     }
+    let substs = obligation.predicate.substs.rebase_onto(tcx, trait_def_id, substs);
     let substs = translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.node);
     let ty = if let ty::AssocKind::OpaqueTy = assoc_ty.item.kind {
         let item_substs = InternalSubsts::identity_for_item(tcx, assoc_ty.item.def_id);
         tcx.mk_opaque(assoc_ty.item.def_id, item_substs)
     } else {
         tcx.type_of(assoc_ty.item.def_id)
     };
-    Progress {
-        ty: ty.subst(tcx, substs),
-        obligations: nested,
+    if substs.len() != tcx.generics_of(assoc_ty.item.def_id).count() {
+        tcx.sess.delay_span_bug(
+            DUMMY_SP,
+            "impl item and trait item have different parameter counts",
+        );
+        Progress {
+            ty: tcx.types.err,
+            obligations: nested,
+        }
+    } else {
+        Progress {
+            ty: ty.subst(tcx, substs),
+            obligations: nested,
+        }
     }
 }
 

src/librustc/traits/structural_impls.rs

@@ -514,6 +514,15 @@ impl<'a, 'tcx> Lift<'tcx> for traits::ObligationCauseCode<'a> {
                 impl_item_def_id,
                 trait_item_def_id,
             }),
+            super::CompareImplTypeObligation {
+                item_name,
+                impl_item_def_id,
+                trait_item_def_id,
+            } => Some(super::CompareImplTypeObligation {
+                item_name,
+                impl_item_def_id,
+                trait_item_def_id,
+            }),
             super::ExprAssignable => Some(super::ExprAssignable),
             super::MatchExpressionArm(box super::MatchExpressionArmCause {
                 arm_span,

src/librustc/ty/mod.rs

@@ -1305,7 +1305,7 @@ impl<'tcx> PolyProjectionPredicate<'tcx> {
     }
 
     #[inline]
-    pub fn to_poly_trait_ref(&self, tcx: TyCtxt<'_>) -> PolyTraitRef<'tcx> {
+    pub fn to_poly_trait_ref(&self, tcx: TyCtxt<'tcx>) -> PolyTraitRef<'tcx> {
         // Note: unlike with `TraitRef::to_poly_trait_ref()`,
         // `self.0.trait_ref` is permitted to have escaping regions.
         // This is because here `self` has a `Binder` and so does our

src/librustc/ty/sty.rs

@@ -1026,11 +1026,11 @@ impl<'tcx> ProjectionTy<'tcx> {
     /// Extracts the underlying trait reference from this projection.
     /// For example, if this is a projection of `<T as Iterator>::Item`,
     /// then this function would return a `T: Iterator` trait reference.
-    pub fn trait_ref(&self, tcx: TyCtxt<'_>) -> ty::TraitRef<'tcx> {
+    pub fn trait_ref(&self, tcx: TyCtxt<'tcx>) -> ty::TraitRef<'tcx> {
         let def_id = tcx.associated_item(self.item_def_id).container.id();
         ty::TraitRef {
             def_id,
-            substs: self.substs,
+            substs: self.substs.truncate_to(tcx, tcx.generics_of(def_id)),
         }
     }
 

src/librustc/ty/subst.rs

@@ -234,7 +234,7 @@ impl<'a, 'tcx> InternalSubsts<'tcx> {
                 ty::GenericParamDefKind::Const => {
                     tcx.mk_const(ty::Const {
                         val: ty::ConstKind::Bound(ty::INNERMOST, ty::BoundVar::from(param.index)),
-                        ty: tcx.type_of(def_id),
+                        ty: tcx.type_of(param.def_id),
                     }).into()
                 }
             }
@@ -533,8 +533,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for SubstFolder<'a, 'tcx> {
                             data.name,
                             self.root_ty,
                             data.index);
-                        self.tcx.sess.delay_span_bug(span, &msg);
-                        r
+                        span_bug!(span, "{}", msg);
                     }
                 }
             }

src/librustc_resolve/late.rs

@@ -1119,17 +1119,16 @@ impl<'a, 'b> LateResolutionVisitor<'a, '_> {
 
                                                 visit::walk_impl_item(this, impl_item);
                                             }
-                                            AssocItemKind::TyAlias(_, Some(ref ty)) => {
+                                            AssocItemKind::TyAlias(_, _) => {
                                                 // If this is a trait impl, ensure the type
                                                 // exists in trait
                                                 this.check_trait_item(impl_item.ident,
                                                                       TypeNS,
                                                                       impl_item.span,
                                                     |n, s| TypeNotMemberOfTrait(n, s));
 
-                                                this.visit_ty(ty);
+                                                visit::walk_impl_item(this, impl_item);
                                             }
-                                            AssocItemKind::TyAlias(_, None) => {}
                                             AssocItemKind::Macro(_) =>
                                                 panic!("unexpanded macro in resolve!"),
                                         }

src/librustc_typeck/astconv.rs

@@ -88,6 +88,7 @@ pub trait AstConv<'tcx> {
     fn projected_ty_from_poly_trait_ref(&self,
                                         span: Span,
                                         item_def_id: DefId,
+                                        item_segment: &hir::PathSegment,
                                         poly_trait_ref: ty::PolyTraitRef<'tcx>)
                                         -> Ty<'tcx>;
 
@@ -205,6 +206,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let (substs, assoc_bindings, _) = self.create_substs_for_ast_path(
             span,
             def_id,
+            &[],
             item_segment.generic_args(),
             item_segment.infer_args,
             None,
@@ -615,9 +617,21 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     ///    `Output = u32` are returned in the `Vec<ConvertedBinding...>` result.
     ///
     /// Note that the type listing given here is *exactly* what the user provided.
+    ///
+    /// For (generic) associated types
+    ///
+    /// ```
+    /// <Vec<u8> as Iterable<u8>>::Iter::<'a>
+    /// ```
+    ///
+    /// We have the parent substs are the substs for the parent trait:
+    /// `[Vec<u8>, u8]` and `generic_args` are the arguments for the associated
+    /// type itself: `['a]`. The returned `SubstsRef` concatenates these two
+    /// lists: `[Vec<u8>, u8, 'a]`.
     fn create_substs_for_ast_path<'a>(&self,
         span: Span,
         def_id: DefId,
+        parent_substs: &[subst::GenericArg<'tcx>],
         generic_args: &'a hir::GenericArgs,
         infer_args: bool,
         self_ty: Option<Ty<'tcx>>)
@@ -633,17 +647,26 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let tcx = self.tcx();
         let generic_params = tcx.generics_of(def_id);
 
-        // If a self-type was declared, one should be provided.
-        assert_eq!(generic_params.has_self, self_ty.is_some());
+        if generic_params.has_self {
+            if generic_params.parent.is_some() {
+                // The parent is a trait so it should have at least one subst
+                // for the `Self` type.
+                assert!(!parent_substs.is_empty())
+            } else {
+                // This item (presumably a trait) needs a self-type.
+                assert!(self_ty.is_some());
+            }
+        } else {
+            assert!(self_ty.is_none() && parent_substs.is_empty());
+        }
 
-        let has_self = generic_params.has_self;
         let (_, potential_assoc_types) = Self::check_generic_arg_count(
             tcx,
             span,
             &generic_params,
             &generic_args,
             GenericArgPosition::Type,
-            has_self,
+            self_ty.is_some(),
             infer_args,
         );
 
@@ -652,7 +675,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         });
         let default_needs_object_self = |param: &ty::GenericParamDef| {
             if let GenericParamDefKind::Type { has_default, .. } = param.kind {
-                if is_object && has_default && has_self {
+                if is_object && has_default {
                     let self_param = tcx.types.self_param;
                     if tcx.at(span).type_of(param.def_id).walk().any(|ty| ty == self_param) {
                         // There is no suitable inference default for a type parameter
@@ -668,7 +691,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let substs = Self::create_substs_for_generic_args(
             tcx,
             def_id,
-            &[][..],
+            parent_substs,
             self_ty.is_some(),
             self_ty,
             // Provide the generic args, and whether types should be inferred.
@@ -780,6 +803,30 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         (substs, assoc_bindings, potential_assoc_types)
     }
 
+    crate fn create_substs_for_associated_item(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        span: Span,
+        item_def_id: DefId,
+        item_segment: &hir::PathSegment,
+        parent_substs: SubstsRef<'tcx>,
+    ) -> SubstsRef<'tcx> {
+        if tcx.generics_of(item_def_id).params.is_empty() {
+            self.prohibit_generics(slice::from_ref(item_segment));
+
+            parent_substs
+        } else {
+            self.create_substs_for_ast_path(
+                span,
+                item_def_id,
+                parent_substs,
+                item_segment.generic_args(),
+                item_segment.infer_args,
+                None,
+            ).0
+        }
+    }
+
     /// Instantiates the path for the given trait reference, assuming that it's
     /// bound to a valid trait type. Returns the `DefId` of the defining trait.
     /// The type _cannot_ be a type other than a trait type.
@@ -919,6 +966,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
         self.create_substs_for_ast_path(span,
                                         trait_def_id,
+                                        &[],
                                         trait_segment.generic_args(),
                                         trait_segment.infer_args,
                                         Some(self_ty))
@@ -1665,8 +1713,6 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
         debug!("associated_path_to_ty: {:?}::{}", qself_ty, assoc_ident);
 
-        self.prohibit_generics(slice::from_ref(assoc_segment));
-
         // Check if we have an enum variant.
         let mut variant_resolution = None;
         if let ty::Adt(adt_def, _) = qself_ty.kind {
@@ -1677,6 +1723,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 if let Some(variant_def) = variant_def {
                     if permit_variants {
                         tcx.check_stability(variant_def.def_id, Some(hir_ref_id), span);
+                        self.prohibit_generics(slice::from_ref(assoc_segment));
                         return Ok((qself_ty, DefKind::Variant, variant_def.def_id));
                     } else {
                         variant_resolution = Some(variant_def.def_id);
@@ -1767,7 +1814,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 i.ident.modern() == assoc_ident
         }).expect("missing associated type");
 
-        let ty = self.projected_ty_from_poly_trait_ref(span, item.def_id, bound);
+        let ty = self.projected_ty_from_poly_trait_ref(span, item.def_id, assoc_segment, bound);
         let ty = self.normalize_ty(span, ty);
 
         let kind = DefKind::AssocTy;
@@ -1818,8 +1865,6 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
         debug!("qpath_to_ty: trait_def_id={:?}", trait_def_id);
 
-        self.prohibit_generics(slice::from_ref(item_segment));
-
         let self_ty = if let Some(ty) = opt_self_ty {
             ty
         } else {
@@ -1861,9 +1906,17 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                                                         self_ty,
                                                         trait_segment);
 
+        let item_substs = self.create_substs_for_associated_item(
+            tcx,
+            span,
+            item_def_id,
+            item_segment,
+            trait_ref.substs,
+        );
+
         debug!("qpath_to_ty: trait_ref={:?}", trait_ref);
 
-        self.normalize_ty(span, tcx.mk_projection(item_def_id, trait_ref.substs))
+        self.normalize_ty(span, tcx.mk_projection(item_def_id, item_substs))
     }
 
     pub fn prohibit_generics<'a, T: IntoIterator<Item = &'a hir::PathSegment>>(
@@ -2518,21 +2571,22 @@ impl<'tcx> Bounds<'tcx> {
         // If it could be sized, and is, add the `Sized` predicate.
         let sized_predicate = self.implicitly_sized.and_then(|span| {
             tcx.lang_items().sized_trait().map(|sized| {
-                let trait_ref = ty::TraitRef {
+                let trait_ref = ty::Binder::bind(ty::TraitRef {
                     def_id: sized,
                     substs: tcx.mk_substs_trait(param_ty, &[])
-                };
+                });
                 (trait_ref.to_predicate(), span)
             })
         });
 
         sized_predicate.into_iter().chain(
             self.region_bounds.iter().map(|&(region_bound, span)| {
                 // Account for the binder being introduced below; no need to shift `param_ty`
-                // because, at present at least, it can only refer to early-bound regions.
+                // because, at present at least, it either only refers to early-bound regions,
+                // or it's a generic associated type that deliberately has escaping bound vars.
                 let region_bound = ty::fold::shift_region(tcx, region_bound, 1);
                 let outlives = ty::OutlivesPredicate(param_ty, region_bound);
-                (ty::Binder::dummy(outlives).to_predicate(), span)
+                (ty::Binder::bind(outlives).to_predicate(), span)
             }).chain(
                 self.trait_bounds.iter().map(|&(bound_trait_ref, span)| {
                     (bound_trait_ref.to_predicate(), span)