Avoid emitting trait bound errors of incoherent traits

Author: oli-obk

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -2364,6 +2364,12 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     return e;
                 }
 
+                if let Err(guar) = self.tcx.ensure().coherent_trait(trait_ref.def_id()) {
+                    // Avoid bogus "type annotations needed `Foo: Bar`" errors on `impl Bar for Foo` in case
+                    // other `Foo` impls are incoherent.
+                    return guar;
+                }
+
                 // This is kind of a hack: it frequently happens that some earlier
                 // error prevents types from being fully inferred, and then we get
                 // a bunch of uninteresting errors saying something like "<generic
@@ -2659,6 +2665,14 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 if let Some(e) = self.tainted_by_errors() {
                     return e;
                 }
+
+                if let Err(guar) =
+                    self.tcx.ensure().coherent_trait(self.tcx.parent(data.projection_ty.def_id))
+                {
+                    // Avoid bogus "type annotations needed `Foo: Bar`" errors on `impl Bar for Foo` in case
+                    // other `Foo` impls are incoherent.
+                    return guar;
+                }
                 let subst = data
                     .projection_ty
                     .args

tests/ui/async-await/in-trait/coherence-constrained.rs

@@ -12,15 +12,13 @@ impl Foo for Bar {
     type T = ();
 
     async fn foo(&self) {}
-    //~^ ERROR type annotations needed: cannot satisfy `<Bar as Foo>::T == ()`
 }
 
 impl Foo for Bar {
     //~^ ERROR conflicting implementations of trait `Foo` for type `Bar`
     type T = ();
 
     async fn foo(&self) {}
-    //~^ ERROR type annotations needed: cannot satisfy `<Bar as Foo>::T == ()`
 }
 
 fn main() {}

tests/ui/async-await/in-trait/coherence-constrained.stderr

@@ -1,25 +1,12 @@
-error[E0284]: type annotations needed: cannot satisfy `<Bar as Foo>::T == ()`
-  --> $DIR/coherence-constrained.rs:14:5
-   |
-LL |     async fn foo(&self) {}
-   |     ^^^^^^^^^^^^^^^^^^^ cannot satisfy `<Bar as Foo>::T == ()`
-
-error[E0284]: type annotations needed: cannot satisfy `<Bar as Foo>::T == ()`
-  --> $DIR/coherence-constrained.rs:22:5
-   |
-LL |     async fn foo(&self) {}
-   |     ^^^^^^^^^^^^^^^^^^^ cannot satisfy `<Bar as Foo>::T == ()`
-
 error[E0119]: conflicting implementations of trait `Foo` for type `Bar`
-  --> $DIR/coherence-constrained.rs:18:1
+  --> $DIR/coherence-constrained.rs:17:1
    |
 LL | impl Foo for Bar {
    | ---------------- first implementation here
 ...
 LL | impl Foo for Bar {
    | ^^^^^^^^^^^^^^^^ conflicting implementation for `Bar`
 
-error: aborting due to 3 previous errors
+error: aborting due to 1 previous error
 
-Some errors have detailed explanations: E0119, E0284.
-For more information about an error, try `rustc --explain E0119`.
+For more information about this error, try `rustc --explain E0119`.