Ensure valid TraitRefs are created for GATs

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -914,6 +914,7 @@ impl<'a> State<'a> {
 
     pub fn print_assoc_constraint(&mut self, constraint: &ast::AssocTyConstraint) {
         self.print_ident(constraint.ident);
+        constraint.gen_args.as_ref().map(|args| self.print_generic_args(args, false));
         self.s.space();
         match &constraint.kind {
             ast::AssocTyConstraintKind::Equality { ty } => {

compiler/rustc_hir/src/lang_items.rs

@@ -238,6 +238,7 @@ language_item_table! {
 
     Deref,                   sym::deref,               deref_trait,                Target::Trait;
     DerefMut,                sym::deref_mut,           deref_mut_trait,            Target::Trait;
+    DerefTarget,             sym::deref_target,        deref_target,               Target::AssocTy;
     Receiver,                sym::receiver,            receiver_trait,             Target::Trait;
 
     Fn,                      kw::Fn,                   fn_trait,                   Target::Trait;

compiler/rustc_infer/src/infer/at.rs

@@ -55,6 +55,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
 pub trait ToTrace<'tcx>: Relate<'tcx> + Copy {
     fn to_trace(
+        tcx: TyCtxt<'tcx>,
         cause: &ObligationCause<'tcx>,
         a_is_expected: bool,
         a: Self,
@@ -178,7 +179,7 @@ impl<'a, 'tcx> At<'a, 'tcx> {
     where
         T: ToTrace<'tcx>,
     {
-        let trace = ToTrace::to_trace(self.cause, a_is_expected, a, b);
+        let trace = ToTrace::to_trace(self.infcx.tcx, self.cause, a_is_expected, a, b);
         Trace { at: self, trace, a_is_expected }
     }
 }
@@ -251,6 +252,7 @@ impl<'a, 'tcx> Trace<'a, 'tcx> {
 
 impl<'tcx> ToTrace<'tcx> for Ty<'tcx> {
     fn to_trace(
+        _: TyCtxt<'tcx>,
         cause: &ObligationCause<'tcx>,
         a_is_expected: bool,
         a: Self,
@@ -262,6 +264,7 @@ impl<'tcx> ToTrace<'tcx> for Ty<'tcx> {
 
 impl<'tcx> ToTrace<'tcx> for ty::Region<'tcx> {
     fn to_trace(
+        _: TyCtxt<'tcx>,
         cause: &ObligationCause<'tcx>,
         a_is_expected: bool,
         a: Self,
@@ -273,6 +276,7 @@ impl<'tcx> ToTrace<'tcx> for ty::Region<'tcx> {
 
 impl<'tcx> ToTrace<'tcx> for &'tcx Const<'tcx> {
     fn to_trace(
+        _: TyCtxt<'tcx>,
         cause: &ObligationCause<'tcx>,
         a_is_expected: bool,
         a: Self,
@@ -284,6 +288,7 @@ impl<'tcx> ToTrace<'tcx> for &'tcx Const<'tcx> {
 
 impl<'tcx> ToTrace<'tcx> for ty::TraitRef<'tcx> {
     fn to_trace(
+        _: TyCtxt<'tcx>,
         cause: &ObligationCause<'tcx>,
         a_is_expected: bool,
         a: Self,
@@ -298,6 +303,7 @@ impl<'tcx> ToTrace<'tcx> for ty::TraitRef<'tcx> {
 
 impl<'tcx> ToTrace<'tcx> for ty::PolyTraitRef<'tcx> {
     fn to_trace(
+        _: TyCtxt<'tcx>,
         cause: &ObligationCause<'tcx>,
         a_is_expected: bool,
         a: Self,
@@ -309,3 +315,20 @@ impl<'tcx> ToTrace<'tcx> for ty::PolyTraitRef<'tcx> {
         }
     }
 }
+
+impl<'tcx> ToTrace<'tcx> for ty::ProjectionTy<'tcx> {
+    fn to_trace(
+        tcx: TyCtxt<'tcx>,
+        cause: &ObligationCause<'tcx>,
+        a_is_expected: bool,
+        a: Self,
+        b: Self,
+    ) -> TypeTrace<'tcx> {
+        let a_ty = tcx.mk_projection(a.item_def_id, a.substs);
+        let b_ty = tcx.mk_projection(b.item_def_id, b.substs);
+        TypeTrace {
+            cause: cause.clone(),
+            values: Types(ExpectedFound::new(a_is_expected, a_ty, b_ty)),
+        }
+    }
+}

compiler/rustc_middle/src/ty/error.rs

@@ -1,5 +1,6 @@
 use crate::traits::{ObligationCause, ObligationCauseCode};
 use crate::ty::diagnostics::suggest_constraining_type_param;
+use crate::ty::print::{FmtPrinter, Printer};
 use crate::ty::{self, BoundRegionKind, Region, Ty, TyCtxt};
 use rustc_errors::Applicability::{MachineApplicable, MaybeIncorrect};
 use rustc_errors::{pluralize, DiagnosticBuilder};
@@ -400,14 +401,22 @@ impl<'tcx> TyCtxt<'tcx> {
                         {
                             // Synthesize the associated type restriction `Add<Output = Expected>`.
                             // FIXME: extract this logic for use in other diagnostics.
-                            let trait_ref = proj.trait_ref(self);
+                            let (trait_ref, assoc_substs) = proj.trait_ref_and_own_substs(self);
                             let path =
                                 self.def_path_str_with_substs(trait_ref.def_id, trait_ref.substs);
                             let item_name = self.item_name(proj.item_def_id);
+                            let item_args = self.format_generic_args(assoc_substs);
+
                             let path = if path.ends_with('>') {
-                                format!("{}, {} = {}>", &path[..path.len() - 1], item_name, p)
+                                format!(
+                                    "{}, {}{} = {}>",
+                                    &path[..path.len() - 1],
+                                    item_name,
+                                    item_args,
+                                    p
+                                )
                             } else {
-                                format!("{}<{} = {}>", path, item_name, p)
+                                format!("{}<{}{} = {}>", path, item_name, item_args, p)
                             };
                             note = !suggest_constraining_type_param(
                                 self,
@@ -556,7 +565,7 @@ impl<T> Trait<T> for X {
         ty: Ty<'tcx>,
     ) -> bool {
         let assoc = self.associated_item(proj_ty.item_def_id);
-        let trait_ref = proj_ty.trait_ref(self);
+        let (trait_ref, assoc_substs) = proj_ty.trait_ref_and_own_substs(self);
         if let Some(item) = self.hir().get_if_local(body_owner_def_id) {
             if let Some(hir_generics) = item.generics() {
                 // Get the `DefId` for the type parameter corresponding to `A` in `<A as T>::Foo`.
@@ -590,6 +599,7 @@ impl<T> Trait<T> for X {
                             &trait_ref,
                             pred.bounds,
                             &assoc,
+                            assoc_substs,
                             ty,
                             msg,
                         ) {
@@ -607,6 +617,7 @@ impl<T> Trait<T> for X {
                             &trait_ref,
                             param.bounds,
                             &assoc,
+                            assoc_substs,
                             ty,
                             msg,
                         );
@@ -692,6 +703,7 @@ impl<T> Trait<T> for X {
                 db,
                 self.def_span(def_id),
                 &assoc,
+                proj_ty.trait_ref_and_own_substs(self).1,
                 values.found,
                 &msg,
             ) {
@@ -856,6 +868,7 @@ fn foo(&self) -> Self::T { String::new() }
         trait_ref: &ty::TraitRef<'tcx>,
         bounds: hir::GenericBounds<'_>,
         assoc: &ty::AssocItem,
+        assoc_substs: &[ty::GenericArg<'tcx>],
         ty: Ty<'tcx>,
         msg: &str,
     ) -> bool {
@@ -865,7 +878,12 @@ fn foo(&self) -> Self::T { String::new() }
                 // Relate the type param against `T` in `<A as T>::Foo`.
                 ptr.trait_ref.trait_def_id() == Some(trait_ref.def_id)
                     && self.constrain_associated_type_structured_suggestion(
-                        db, ptr.span, assoc, ty, msg,
+                        db,
+                        ptr.span,
+                        assoc,
+                        assoc_substs,
+                        ty,
+                        msg,
                     )
             }
             _ => false,
@@ -879,6 +897,7 @@ fn foo(&self) -> Self::T { String::new() }
         db: &mut DiagnosticBuilder<'_>,
         span: Span,
         assoc: &ty::AssocItem,
+        assoc_substs: &[ty::GenericArg<'tcx>],
         ty: Ty<'tcx>,
         msg: &str,
     ) -> bool {
@@ -890,11 +909,20 @@ fn foo(&self) -> Self::T { String::new() }
                 let span = Span::new(pos, pos, span.ctxt());
                 (span, format!(", {} = {}", assoc.ident, ty))
             } else {
-                (span.shrink_to_hi(), format!("<{} = {}>", assoc.ident, ty))
+                let item_args = self.format_generic_args(assoc_substs);
+                (span.shrink_to_hi(), format!("<{}{} = {}>", assoc.ident, item_args, ty))
             };
             db.span_suggestion_verbose(span, msg, sugg, MaybeIncorrect);
             return true;
         }
         false
     }
+
+    fn format_generic_args(self, args: &[ty::GenericArg<'tcx>]) -> String {
+        let mut item_args = String::new();
+        FmtPrinter::new(self, &mut item_args, hir::def::Namespace::TypeNS)
+            .path_generic_args(Ok, args)
+            .expect("could not write to `String`.");
+        item_args
+    }
 }

compiler/rustc_middle/src/ty/mod.rs

@@ -1289,8 +1289,22 @@ impl<'tcx> PolyProjectionPredicate<'tcx> {
         self.skip_binder().projection_ty.item_def_id
     }
 
+    /// Returns the `DefId` of the trait of the associated item being projected.
     #[inline]
-    pub fn to_poly_trait_ref(&self, tcx: TyCtxt<'tcx>) -> PolyTraitRef<'tcx> {
+    pub fn trait_def_id(&self, tcx: TyCtxt<'tcx>) -> DefId {
+        self.skip_binder().projection_ty.trait_def_id(tcx)
+    }
+
+    #[inline]
+    pub fn projection_self_ty(&self) -> Binder<Ty<'tcx>> {
+        self.map_bound(|predicate| predicate.projection_ty.self_ty())
+    }
+
+    /// Get the [PolyTraitRef] required for this projection to be well formed.
+    /// Note that for generic associated types the predicates of the associated
+    /// type also need to be checked.
+    #[inline]
+    pub fn required_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

compiler/rustc_middle/src/ty/sty.rs

@@ -17,7 +17,7 @@ use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_index::vec::Idx;
 use rustc_macros::HashStable;
-use rustc_span::symbol::{kw, Ident, Symbol};
+use rustc_span::symbol::{kw, Symbol};
 use rustc_target::abi::VariantIdx;
 use rustc_target::spec::abi;
 use std::borrow::Cow;
@@ -1112,36 +1112,35 @@ pub struct ProjectionTy<'tcx> {
 }
 
 impl<'tcx> ProjectionTy<'tcx> {
-    /// Construct a `ProjectionTy` by searching the trait from `trait_ref` for the
-    /// associated item named `item_name`.
-    pub fn from_ref_and_name(
-        tcx: TyCtxt<'_>,
-        trait_ref: ty::TraitRef<'tcx>,
-        item_name: Ident,
-    ) -> ProjectionTy<'tcx> {
-        let item_def_id = tcx
-            .associated_items(trait_ref.def_id)
-            .find_by_name_and_kind(tcx, item_name, ty::AssocKind::Type, trait_ref.def_id)
-            .unwrap()
-            .def_id;
+    pub fn trait_def_id(&self, tcx: TyCtxt<'tcx>) -> DefId {
+        tcx.associated_item(self.item_def_id).container.id()
+    }
 
-        ProjectionTy { substs: trait_ref.substs, item_def_id }
+    /// Extracts the underlying trait reference and own substs from this projection.
+    /// For example, if this is a projection of `<T as StreamingIterator>::Item<'a>`,
+    /// then this function would return a `T: Iterator` trait reference and `['a]` as the own substs
+    pub fn trait_ref_and_own_substs(
+        &self,
+        tcx: TyCtxt<'tcx>,
+    ) -> (ty::TraitRef<'tcx>, &'tcx [ty::GenericArg<'tcx>]) {
+        let def_id = tcx.associated_item(self.item_def_id).container.id();
+        let trait_generics = tcx.generics_of(def_id);
+        (
+            ty::TraitRef { def_id, substs: self.substs.truncate_to(tcx, trait_generics) },
+            &self.substs[trait_generics.count()..],
+        )
     }
 
     /// 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.
+    ///
+    /// WARNING: This will drop the substs for generic associated types
+    /// consider calling [Self::trait_ref_and_own_substs] to get those
+    /// as well.
     pub fn trait_ref(&self, tcx: TyCtxt<'tcx>) -> ty::TraitRef<'tcx> {
-        // FIXME: This method probably shouldn't exist at all, since it's not
-        // clear what this method really intends to do. Be careful when
-        // using this method since the resulting TraitRef additionally
-        // contains the substs for the assoc_item, which strictly speaking
-        // is not correct
-        let def_id = tcx.associated_item(self.item_def_id).container.id();
-        // Include substitutions for generic arguments of associated types
-        let assoc_item = tcx.associated_item(self.item_def_id);
-        let substs_assoc_item = self.substs.truncate_to(tcx, tcx.generics_of(assoc_item.def_id));
-        ty::TraitRef { def_id, substs: substs_assoc_item }
+        let def_id = self.trait_def_id(tcx);
+        ty::TraitRef { def_id, substs: self.substs.truncate_to(tcx, tcx.generics_of(def_id)) }
     }
 
     pub fn self_ty(&self) -> Ty<'tcx> {
@@ -1493,12 +1492,11 @@ impl<'tcx> ExistentialProjection<'tcx> {
     /// For example, if this is a projection of `exists T. <T as Iterator>::Item == X`,
     /// then this function would return a `exists T. T: Iterator` existential trait
     /// reference.
-    pub fn trait_ref(&self, tcx: TyCtxt<'_>) -> ty::ExistentialTraitRef<'tcx> {
-        // FIXME(generic_associated_types): substs is the substs of the
-        // associated type, which should be truncated to get the correct substs
-        // for the trait.
+    pub fn trait_ref(&self, tcx: TyCtxt<'tcx>) -> ty::ExistentialTraitRef<'tcx> {
         let def_id = tcx.associated_item(self.item_def_id).container.id();
-        ty::ExistentialTraitRef { def_id, substs: self.substs }
+        let subst_count = tcx.generics_of(def_id).count() - 1;
+        let substs = tcx.intern_substs(&self.substs[..subst_count]);
+        ty::ExistentialTraitRef { def_id, substs }
     }
 
     pub fn with_self_ty(
@@ -1517,6 +1515,20 @@ impl<'tcx> ExistentialProjection<'tcx> {
             ty: self.ty,
         }
     }
+
+    pub fn erase_self_ty(
+        tcx: TyCtxt<'tcx>,
+        projection_predicate: ty::ProjectionPredicate<'tcx>,
+    ) -> Self {
+        // Assert there is a Self.
+        projection_predicate.projection_ty.substs.type_at(0);
+
+        Self {
+            item_def_id: projection_predicate.projection_ty.item_def_id,
+            substs: tcx.intern_substs(&projection_predicate.projection_ty.substs[1..]),
+            ty: projection_predicate.ty,
+        }
+    }
 }
 
 impl<'tcx> PolyExistentialProjection<'tcx> {