Use delay_span_bug for "Failed to unify obligation"

src/librustc/traits/project.rs

@@ -1454,13 +1454,18 @@ fn confirm_param_env_candidate<'cx, 'gcx, 'tcx>(
             }
         }
         Err(e) => {
-            span_bug!(
+            let msg = format!(
-                obligation.cause.span,
+                "Failed to unify obligation `{:?}` with poly_projection `{:?}`: {:?}",
-                "Failed to unify obligation `{:?}` \
-                 with poly_projection `{:?}`: {:?}",
                 obligation,
                 poly_cache_entry,
-                e);
+                e,
+            );
+            debug!("confirm_param_env_candidate: {}", msg);
+            infcx.tcx.sess.delay_span_bug(obligation.cause.span, &msg);
+            Progress {
+                ty: infcx.tcx.types.err,
+                obligations: vec![],
+            }
         }
     }
 }

src/test/ui/issues/issue-60283.rs

@@ -0,0 +1,17 @@
+pub trait Trait<'a> {
+    type Item;
+}
+
+impl<'a> Trait<'a> for () {
+    type Item = ();
+}
+
+pub fn foo<T, F>(_: T, _: F)
+where T: for<'a> Trait<'a>,
+      F: for<'a> FnMut(<T as Trait<'a>>::Item) {}
+
+fn main() {
+    foo((), drop)
+    //~^ ERROR type mismatch in function arguments
+    //~| ERROR type mismatch resolving
+}

src/test/ui/issues/issue-60283.stderr

@@ -0,0 +1,34 @@
+error[E0631]: type mismatch in function arguments
+  --> $DIR/issue-60283.rs:14:5
+   |
+LL |     foo((), drop)
+   |     ^^^
+   |     |
+   |     expected signature of `for<'a> fn(<() as Trait<'a>>::Item) -> _`
+   |     found signature of `fn(_) -> _`
+   |
+note: required by `foo`
+  --> $DIR/issue-60283.rs:9:1
+   |
+LL | / pub fn foo<T, F>(_: T, _: F)
+LL | | where T: for<'a> Trait<'a>,
+LL | |       F: for<'a> FnMut(<T as Trait<'a>>::Item) {}
+   | |_________________________________________________^
+
+error[E0271]: type mismatch resolving `for<'a> <fn(_) {std::mem::drop::<_>} as std::ops::FnOnce<(<() as Trait<'a>>::Item,)>>::Output == ()`
+  --> $DIR/issue-60283.rs:14:5
+   |
+LL |     foo((), drop)
+   |     ^^^ expected bound lifetime parameter 'a, found concrete lifetime
+   |
+note: required by `foo`
+  --> $DIR/issue-60283.rs:9:1
+   |
+LL | / pub fn foo<T, F>(_: T, _: F)
+LL | | where T: for<'a> Trait<'a>,
+LL | |       F: for<'a> FnMut(<T as Trait<'a>>::Item) {}
+   | |_________________________________________________^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0271`.