gccrs: fix ICE when we have unimplemented/invalid trait items

When the resulting trait item is in an error state this means the
underlying fields will be null.

Fixes #2478

gcc/rust/ChangeLog:

	* typecheck/rust-hir-type-check-item.cc: add is_error check

gcc/testsuite/ChangeLog:

	* rust/compile/non_member_const.rs: add missing error message
	* rust/compile/issue-2478.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>

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

@@ -615,7 +615,8 @@ TypeCheckItem::validate_trait_impl_block (
 						   impl_item.get (), self,
 						   specified_bound,
 						   substitutions);
-	  trait_item_refs.push_back (trait_item_ref.get_raw_item ());
+	  if (!trait_item_ref.is_error ())
+	    trait_item_refs.push_back (trait_item_ref.get_raw_item ());
 	}
     }
 

gcc/testsuite/rust/compile/issue-2478.rs

@@ -0,0 +1,16 @@
+#[lang = "sized"]
+pub trait Sized {}
+
+struct Bar;
+
+trait Foo {
+    const N: u32;
+
+    fn M();
+}
+
+impl Foo for Bar {
+    // { dg-error "missing N, M in implementation of trait .Foo." "" { target *-*-* } .-1 }
+    fn N() {}
+    // { dg-error "method .N. is not a member of trait .Foo." "" { target *-*-* } .-1 }
+}

gcc/testsuite/rust/compile/non_member_const.rs

@@ -7,9 +7,7 @@ trait Foo {
 
 struct Bar;
 
-impl Foo for Bar {
+impl Foo for Bar {// { dg-error "missing N in implementation of trait .Foo." }
     const N : u32 = 0; // { dg-error "item .N. is an associated const, which does not match its trait .Foo." }
-    // error: item `N` is an associated const, which doesn't match its
-    //        trait `<Bar as Foo>`
 }
-}
+}