gccrs: Fix segv in errors in type checking an impl item

gcc/rust/typecheck/rust-hir-type-check-implitem.cc

@@ -558,6 +558,8 @@ TypeCheckImplItemWithTrait::visit (HIR::Function &function)
   // normal resolution of the item
   TyTy::BaseType *lookup
     = TypeCheckImplItem::Resolve (parent, function, self, substitutions);
+  if (lookup == nullptr)
+    return;
 
   // map the impl item to the associated trait item
   const auto tref = trait_reference.get ();

gcc/rust/typecheck/rust-hir-type-check.cc

@@ -237,19 +237,20 @@ TraitItemReference::get_type_from_fn (/*const*/ HIR::TraitItemFunc &fn) const
 		      : Mutability::Mut;
 		rust_assert (self_param.has_lifetime ());
 
+		auto region = TyTy::Region::make_anonymous ();
 		auto maybe_region = context->lookup_and_resolve_lifetime (
 		  self_param.get_lifetime ());
-
-		if (!maybe_region.has_value ())
+		if (maybe_region.has_value ())
+		  region = maybe_region.value ();
+		else
 		  {
 		    rust_error_at (self_param.get_locus (),
 				   "failed to resolve lifetime");
-		    return get_error ();
 		  }
+
 		self_type = new TyTy::ReferenceType (
 		  self_param.get_mappings ().get_hirid (),
-		  TyTy::TyVar (self->get_ref ()), mutability,
-		  maybe_region.value ());
+		  TyTy::TyVar (self->get_ref ()), mutability, region);
 	      }
 	      break;
 

gcc/testsuite/rust/compile/issue-4188.rs

@@ -0,0 +1,13 @@
+trait MemoryUnit {
+    extern "C" fn read_dword(&'s self) -> u16 {}
+    // { dg-error {failed to resolve lifetime} "" { target *-*-* } .-1 }
+    // { dg-error {mismatched types} "" { target *-*-* } .-2 }
+}
+
+impl MemoryUnit for MemoryUnit {
+    extern "C" fn read_dword(&'s self) -> u16 {
+        let b16 = self.read_word() as u16;
+
+        b16 << 8
+    }
+}