Do not ICE on generics mismatch with non-local traits

Fixes rust-lang#49841

Author: sinkuu

src/librustc_typeck/check/compare_method.rs

@@ -730,8 +730,7 @@ fn compare_synthetic_generics<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         if impl_ty.synthetic != trait_ty.synthetic {
             let impl_node_id = tcx.hir.as_local_node_id(impl_ty.def_id).unwrap();
             let impl_span = tcx.hir.span(impl_node_id);
-            let trait_node_id = tcx.hir.as_local_node_id(trait_ty.def_id).unwrap();
-            let trait_span = tcx.hir.span(trait_node_id);
+            let trait_span = tcx.def_span(trait_ty.def_id);
             let mut err = struct_span_err!(tcx.sess,
                                            impl_span,
                                            E0643,

src/test/compile-fail/impl-trait/impl-generic-mismatch.rs

@@ -28,4 +28,15 @@ impl Bar for () {
     //~^ Error method `bar` has incompatible signature for trait
 }
 
+// With non-local trait (#49841):
+
+use std::hash::{Hash, Hasher};
+
+struct X;
+
+impl Hash for X {
+    fn hash(&self, hasher: &mut impl Hasher) {}
+    //~^ Error method `hash` has incompatible signature for trait
+}
+
 fn main() {}