Discard overflow obligations in impl_may_apply

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

@@ -5,6 +5,7 @@
 use rustc_middle::ty;
 use rustc_span::{Span, DUMMY_SP};
 
+use crate::traits::query::evaluate_obligation::InferCtxtExt;
 use crate::traits::ObligationCtxt;
 
 #[derive(Debug)]
@@ -52,10 +53,21 @@ pub fn compute_applicable_impls_for_diagnostics<'tcx>(
                 _ => return false,
             }
 
-            let impl_predicates = tcx.predicates_of(impl_def_id).instantiate(tcx, impl_args);
-            ocx.register_obligations(impl_predicates.predicates.iter().map(|&predicate| {
-                Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate)
-            }));
+            let obligations = tcx
+                .predicates_of(impl_def_id)
+                .instantiate(tcx, impl_args)
+                .into_iter()
+                .map(|(predicate, _)| {
+                    Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate)
+                })
+                // Kinda hacky, but let's just throw away obligations that overflow.
+                // This may reduce the accuracy of this check (if the obligation guides
+                // inference or it actually resulted in error after others are processed)
+                // ... but this is diagnostics code.
+                .filter(|obligation| {
+                    infcx.next_trait_solver() || infcx.evaluate_obligation(obligation).is_ok()
+                });
+            ocx.register_obligations(obligations);
 
             ocx.select_where_possible().is_empty()
         })

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

@@ -2398,12 +2398,12 @@ fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>) -> Erro
                 if ambiguities.len() > 5 {
                     let infcx = self.infcx;
                     if !ambiguities.iter().all(|option| match option {
-                        DefId(did) => infcx.fresh_args_for_item(DUMMY_SP, *did).is_empty(),
+                        DefId(did) => infcx.tcx.generics_of(*did).count() == 0,
                         ParamEnv(_) => true,
                     }) {
                         // If not all are blanket impls, we filter blanked impls out.
                         ambiguities.retain(|option| match option {
-                            DefId(did) => infcx.fresh_args_for_item(DUMMY_SP, *did).is_empty(),
+                            DefId(did) => infcx.tcx.generics_of(*did).count() == 0,
                             ParamEnv(_) => true,
                         });
                     }

tests/ui/traits/overflow-computing-ambiguity.rs

@@ -0,0 +1,14 @@
+trait Hello {}
+
+struct Foo<'a, T: ?Sized>(&'a T);
+
+impl<'a, T: ?Sized> Hello for Foo<'a, &'a T> where Foo<'a, T>: Hello {}
+
+impl Hello for Foo<'static, i32> {}
+
+fn hello<T: ?Sized + Hello>() {}
+
+fn main() {
+    hello();
+    //~^ ERROR type annotations needed
+}

tests/ui/traits/overflow-computing-ambiguity.stderr

@@ -0,0 +1,23 @@
+error[E0283]: type annotations needed
+  --> $DIR/overflow-computing-ambiguity.rs:12:5
+   |
+LL |     hello();
+   |     ^^^^^ cannot infer type of the type parameter `T` declared on the function `hello`
+   |
+   = note: cannot satisfy `_: Hello`
+   = help: the following types implement trait `Hello`:
+             Foo<'a, &'a T>
+             Foo<'static, i32>
+note: required by a bound in `hello`
+  --> $DIR/overflow-computing-ambiguity.rs:9:22
+   |
+LL | fn hello<T: ?Sized + Hello>() {}
+   |                      ^^^^^ required by this bound in `hello`
+help: consider specifying the generic argument
+   |
+LL |     hello::<T>();
+   |          +++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0283`.