Avoid cycles from projection bounds

Only check the own predicates of associated types when confirming
projection candidates.
Also consider implied bounds when comparing trait and impl methods.

compiler/rustc_trait_selection/src/traits/project.rs

@@ -28,7 +28,6 @@ use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
 use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, WithConstness};
 use rustc_span::symbol::sym;
-use rustc_span::DUMMY_SP;
 
 pub use rustc_middle::traits::Reveal;
 
@@ -1409,6 +1408,7 @@ fn confirm_param_env_candidate<'cx, 'tcx>(
     match infcx.at(cause, param_env).eq(cache_trait_ref, obligation_trait_ref) {
         Ok(InferOk { value: _, obligations }) => {
             nested_obligations.extend(obligations);
+            assoc_ty_own_obligations(selcx, obligation, &mut nested_obligations);
             Progress { ty: cache_entry.ty, obligations: nested_obligations }
         }
         Err(e) => {
@@ -1430,7 +1430,7 @@ fn confirm_impl_candidate<'cx, 'tcx>(
 ) -> Progress<'tcx> {
     let tcx = selcx.tcx();
 
-    let ImplSourceUserDefinedData { impl_def_id, substs, nested } = impl_impl_source;
+    let ImplSourceUserDefinedData { impl_def_id, substs, mut nested } = impl_impl_source;
     let assoc_item_id = obligation.predicate.item_def_id;
     let trait_def_id = tcx.trait_id_of_impl(impl_def_id).unwrap();
 
@@ -1463,15 +1463,48 @@ fn confirm_impl_candidate<'cx, 'tcx>(
     let ty = tcx.type_of(assoc_ty.item.def_id);
     if substs.len() != tcx.generics_of(assoc_ty.item.def_id).count() {
         let err = tcx.ty_error_with_message(
-            DUMMY_SP,
+            obligation.cause.span,
             "impl item and trait item have different parameter counts",
         );
         Progress { ty: err, obligations: nested }
     } else {
+        assoc_ty_own_obligations(selcx, obligation, &mut nested);
         Progress { ty: ty.subst(tcx, substs), obligations: nested }
     }
 }
 
+// Get obligations corresponding to the predicates from the where-clause of the
+// associated type itself.
+// Note: `feature(generic_associated_types)` is required to write such
+// predicates, even for non-generic associcated types.
+fn assoc_ty_own_obligations<'cx, 'tcx>(
+    selcx: &mut SelectionContext<'cx, 'tcx>,
+    obligation: &ProjectionTyObligation<'tcx>,
+    nested: &mut Vec<PredicateObligation<'tcx>>,
+) {
+    let tcx = selcx.tcx();
+    for predicate in tcx
+        .predicates_of(obligation.predicate.item_def_id)
+        .instantiate_own(tcx, obligation.predicate.substs)
+        .predicates
+    {
+        let normalized = normalize_with_depth_to(
+            selcx,
+            obligation.param_env,
+            obligation.cause.clone(),
+            obligation.recursion_depth + 1,
+            &predicate,
+            nested,
+        );
+        nested.push(Obligation::with_depth(
+            obligation.cause.clone(),
+            obligation.recursion_depth + 1,
+            obligation.param_env,
+            normalized,
+        ));
+    }
+}
+
 /// Locate the definition of an associated type in the specialization hierarchy,
 /// starting from the given impl.
 ///

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -157,22 +157,26 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         );
                     }),
             );
-            // Require that the projection is well-formed.
-            let self_ty = self.infcx.replace_bound_vars_with_placeholders(&bound_self_ty);
-            let self_ty = normalize_with_depth_to(
-                self,
-                obligation.param_env,
-                obligation.cause.clone(),
-                obligation.recursion_depth + 1,
-                &self_ty,
-                &mut obligations,
-            );
-            obligations.push(Obligation::with_depth(
-                obligation.cause.clone(),
-                obligation.recursion_depth + 1,
-                obligation.param_env,
-                ty::PredicateKind::WellFormed(self_ty.into()).to_predicate(tcx),
-            ));
+
+            if let ty::Projection(..) = bound_self_ty.skip_binder().kind {
+                for predicate in tcx.predicates_of(def_id).instantiate_own(tcx, substs).predicates {
+                    let normalized = normalize_with_depth_to(
+                        self,
+                        obligation.param_env,
+                        obligation.cause.clone(),
+                        obligation.recursion_depth + 1,
+                        &predicate,
+                        &mut obligations,
+                    );
+                    obligations.push(Obligation::with_depth(
+                        obligation.cause.clone(),
+                        obligation.recursion_depth + 1,
+                        obligation.param_env,
+                        normalized,
+                    ));
+                }
+            }
+
             obligations
         })
     }

compiler/rustc_typeck/src/check/compare_method.rs

@@ -328,7 +328,7 @@ fn compare_predicate_entailment<'tcx>(
         // Finally, resolve all regions. This catches wily misuses of
         // lifetime parameters.
         let fcx = FnCtxt::new(&inh, param_env, impl_m_hir_id);
-        fcx.regionck_item(impl_m_hir_id, impl_m_span, &[]);
+        fcx.regionck_item(impl_m_hir_id, impl_m_span, trait_sig.inputs_and_output);
 
         Ok(())
     })

src/test/ui/associated-types/wf-cycle-2.rs

@@ -0,0 +1,18 @@
+// check-pass
+
+trait IntoIt {
+    type Item;
+}
+
+impl<I> IntoIt for I {
+    type Item = ();
+}
+
+trait BaseGraph
+where
+    <Self::VertexIter as IntoIt>::Item: Sized,
+{
+    type VertexIter: IntoIt;
+}
+
+fn main() {}

src/test/ui/associated-types/wf-cycle.rs

@@ -0,0 +1,13 @@
+// check-pass
+
+trait A {
+    type U: Copy;
+}
+
+trait B where
+    <Self::V as A>::U: Copy,
+{
+    type V: A;
+}
+
+fn main() {}

src/test/ui/regions/regions-trait-1.rs

@@ -1,30 +1,33 @@
-#![feature(box_syntax)]
+// check-pass
 
-struct Ctxt { v: usize }
+struct Ctxt {
+    v: usize,
+}
 
 trait GetCtxt {
     // Here the `&` is bound in the method definition:
     fn get_ctxt(&self) -> &Ctxt;
 }
 
-struct HasCtxt<'a> { c: &'a Ctxt }
-
-impl<'a> GetCtxt for HasCtxt<'a> {
-
-    // Here an error occurs because we used `&self` but
-    // the definition used `&`:
-    fn get_ctxt(&self) -> &'a Ctxt { //~ ERROR method not compatible with trait
-        self.c
-    }
-
+struct HasCtxt<'a> {
+    c: &'a Ctxt,
 }
 
-fn get_v(gc: Box<dyn GetCtxt>) -> usize {
+impl<'a> GetCtxt for HasCtxt<'a> {
+    // Ok: Have implied bound of WF(&'b HasCtxt<'a>)
+    // so know 'a: 'b
+    // so know &'a Ctxt <: &'b Ctxt
+    fn get_ctxt<'b>(&'b self) -> &'a Ctxt {
+        self.c
+    }
+}
+
+fn get_v(gc: Box<dyn GetCtxt + '_>) -> usize {
     gc.get_ctxt().v
 }
 
 fn main() {
     let ctxt = Ctxt { v: 22 };
     let hc = HasCtxt { c: &ctxt };
-    assert_eq!(get_v(box hc as Box<dyn GetCtxt>), 22);
+    assert_eq!(get_v(Box::new(hc) as Box<dyn GetCtxt>), 22);
 }

src/test/ui/regions/regions-trait-1.stderr

@@ -1,22 +0,0 @@
-error[E0308]: method not compatible with trait
-  --> $DIR/regions-trait-1.rs:16:5
-   |
-LL |     fn get_ctxt(&self) -> &'a Ctxt {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
-   |
-   = note: expected fn pointer `fn(&HasCtxt<'a>) -> &Ctxt`
-              found fn pointer `fn(&HasCtxt<'a>) -> &'a Ctxt`
-note: the lifetime `'a` as defined on the impl at 12:6...
-  --> $DIR/regions-trait-1.rs:12:6
-   |
-LL | impl<'a> GetCtxt for HasCtxt<'a> {
-   |      ^^
-note: ...does not necessarily outlive the anonymous lifetime #1 defined on the method body at 16:5
-  --> $DIR/regions-trait-1.rs:16:5
-   |
-LL |     fn get_ctxt(&self) -> &'a Ctxt {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0308`.

src/test/ui/specialization/issue-38091-2.rs

@@ -1,3 +1,6 @@
+// build-fail
+//~^ ERROR overflow evaluating the requirement `i32: Check`
+
 #![feature(specialization)]
 //~^ WARN the feature `specialization` is incomplete
 
@@ -5,7 +8,10 @@ trait Iterate<'a> {
     type Ty: Valid;
     fn iterate(self);
 }
-impl<'a, T> Iterate<'a> for T where T: Check {
+impl<'a, T> Iterate<'a> for T
+where
+    T: Check,
+{
     default type Ty = ();
     default fn iterate(self) {}
 }
@@ -19,5 +25,4 @@ impl Valid for () {}
 
 fn main() {
     Iterate::iterate(0);
-    //~^ ERROR overflow evaluating the requirement `{integer}: Check`
 }

src/test/ui/specialization/issue-38091-2.stderr

@@ -1,5 +1,5 @@
 warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
-  --> $DIR/issue-38091-2.rs:1:12
+  --> $DIR/issue-38091-2.rs:4:12
    |
 LL | #![feature(specialization)]
    |            ^^^^^^^^^^^^^^
@@ -7,16 +7,9 @@ LL | #![feature(specialization)]
    = note: `#[warn(incomplete_features)]` on by default
    = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
 
-error[E0275]: overflow evaluating the requirement `{integer}: Check`
-  --> $DIR/issue-38091-2.rs:21:5
+error[E0275]: overflow evaluating the requirement `i32: Check`
    |
-LL |     fn iterate(self);
-   |     ----------------- required by `Iterate::iterate`
-...
-LL |     Iterate::iterate(0);
-   |     ^^^^^^^^^^^^^^^^
-   |
-   = note: required because of the requirements on the impl of `Iterate<'_>` for `{integer}`
+   = note: required because of the requirements on the impl of `Iterate` for `i32`
 
 error: aborting due to previous error; 1 warning emitted
 

src/test/ui/specialization/issue-38091.rs

@@ -5,7 +5,10 @@ trait Iterate<'a> {
     type Ty: Valid;
     fn iterate(self);
 }
-impl<'a, T> Iterate<'a> for T where T: Check {
+impl<'a, T> Iterate<'a> for T
+where
+    T: Check,
+{
     default type Ty = ();
     //~^ ERROR the trait bound `(): Valid` is not satisfied
     default fn iterate(self) {}
@@ -18,5 +21,4 @@ trait Valid {}
 
 fn main() {
     Iterate::iterate(0);
-    //~^ ERROR overflow evaluating the requirement `{integer}: Check`
 }

src/test/ui/specialization/issue-38091.stderr

@@ -8,7 +8,7 @@ LL | #![feature(specialization)]
    = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
 
 error[E0277]: the trait bound `(): Valid` is not satisfied
-  --> $DIR/issue-38091.rs:9:5
+  --> $DIR/issue-38091.rs:12:5
    |
 LL |     type Ty: Valid;
    |              ----- required by this bound in `Iterate::Ty`
@@ -16,18 +16,6 @@ LL |     type Ty: Valid;
 LL |     default type Ty = ();
    |     ^^^^^^^^^^^^^^^^^^^^^ the trait `Valid` is not implemented for `()`
 
-error[E0275]: overflow evaluating the requirement `{integer}: Check`
-  --> $DIR/issue-38091.rs:20:5
-   |
-LL |     fn iterate(self);
-   |     ----------------- required by `Iterate::iterate`
-...
-LL |     Iterate::iterate(0);
-   |     ^^^^^^^^^^^^^^^^
-   |
-   = note: required because of the requirements on the impl of `Iterate<'_>` for `{integer}`
+error: aborting due to previous error; 1 warning emitted
 
-error: aborting due to 2 previous errors; 1 warning emitted
-
-Some errors have detailed explanations: E0275, E0277.
-For more information about an error, try `rustc --explain E0275`.
+For more information about this error, try `rustc --explain E0277`.