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

When we typecheck a trait impl block item, at the end we validate it
against the trait definition by doing a final unify but if the type check
fails on the the impl item it returns NULL here. The other issue was that
if we fail to resolve the specified lifetime we return error early, this
changes the typechecking to default to an anon lifetime so we can continue
typechecking.

Fixes #4188

gcc/rust/ChangeLog:

	* typecheck/rust-hir-type-check-implitem.cc (TypeCheckImplItemWithTrait::visit): null guard
	* typecheck/rust-hir-type-check.cc (TraitItemReference::get_type_from_fn): default to anon

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4188.rs: New test.

Signed-off-by: Philip Herron <[email protected]>

Author: philberty

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

@@ -582,6 +582,8 @@ 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
+    }
+}