Delay a bug when encountering an impl with unconstrained generics in codegen_select

compiler/rustc_traits/src/codegen.rs

@@ -74,12 +74,21 @@ pub(crate) fn codegen_select_candidate<'tcx>(
     }
 
     let impl_source = infcx.resolve_vars_if_possible(impl_source);
-    let impl_source = infcx.tcx.erase_regions(impl_source);
-    if impl_source.has_infer() {
-        // Unused lifetimes on an impl get replaced with inference vars, but never resolved,
-        // causing the return value of a query to contain inference vars. We do not have a concept
-        // for this and will in fact ICE in stable hashing of the return value. So bail out instead.
-        infcx.tcx.dcx().has_errors().unwrap();
+    let impl_source = tcx.erase_regions(impl_source);
+    if impl_source.has_non_region_infer() {
+        // Unused generic types or consts on an impl get replaced with inference vars,
+        // but never resolved, causing the return value of a query to contain inference
+        // vars. We do not have a concept for this and will in fact ICE in stable hashing
+        // of the return value. So bail out instead.
+        match impl_source {
+            ImplSource::UserDefined(impl_) => {
+                tcx.dcx().span_delayed_bug(
+                    tcx.def_span(impl_.impl_def_id),
+                    "this impl has unconstrained generic parameters",
+                );
+            }
+            _ => unreachable!(),
+        }
         return Err(CodegenObligationError::FulfillmentError);
     }
 

tests/ui/traits/resolve-impl-before-constrain-check.rs

@@ -0,0 +1,20 @@
+// Need a different module so we try to build the mir for `test`
+// before analyzing `mod foo`.
+
+mod foo {
+    pub trait Callable {
+        fn call();
+    }
+
+    impl<V: ?Sized> Callable for () {
+    //~^ ERROR the type parameter `V` is not constrained by the impl trait, self type, or predicates
+        fn call() {}
+    }
+}
+use foo::*;
+
+fn test() -> impl Sized {
+    <() as Callable>::call()
+}
+
+fn main() {}

tests/ui/traits/resolve-impl-before-constrain-check.stderr

@@ -0,0 +1,9 @@
+error[E0207]: the type parameter `V` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/resolve-impl-before-constrain-check.rs:9:10
+   |
+LL |     impl<V: ?Sized> Callable for () {
+   |          ^ unconstrained type parameter
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0207`.