gccrs: fix a compiler crash when path expression contains nothing

gcc/rust/ChangeLog:

	* ast/rust-path.h: allows error nodes to specify the source
	location, so that it could be used in diagnostics later.
	* parse/rust-parse-impl.h: creates the PathInExpression error node
	with source location.
	* typecheck/rust-hir-type-check-path.cc: properly checks if the
	PathInExpression is an error node and prints proper diagnostics if
	so.

gcc/testsuite/ChangeLog:

	* rust/compile/paamayim-nekudotayim.rs: add a test for testing
	proper error recovery logic when there is no path names.

gcc/rust/ast/rust-path.h

@@ -727,9 +727,9 @@ class PathInExpression : public Pattern, public ExprWithoutBlock
   }
 
   // Creates an error state path in expression.
-  static PathInExpression create_error ()
+  static PathInExpression create_error (location_t locus = UNDEF_LOCATION)
   {
-    return PathInExpression ({}, {}, UNDEF_LOCATION);
+    return PathInExpression ({}, {}, locus);
   }
 
   // Returns whether path in expression is in an error state.

gcc/rust/parse/rust-parse-impl.h

@@ -6715,7 +6715,7 @@ Parser<ManagedTokenSource>::parse_path_in_expression ()
     {
       // skip after somewhere?
       // don't necessarily throw error but yeah
-      return AST::PathInExpression::create_error ();
+      return AST::PathInExpression::create_error (locus);
     }
   segments.push_back (std::move (initial_segment));
 

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

@@ -342,6 +342,13 @@ TypeCheckExpr::resolve_root_path (HIR::PathInExpression &expr, size_t *offset,
       root_tyty = lookup;
     }
 
+  if (!root_tyty)
+    {
+      // then it must be an empty `::` path
+      rust_error_at (expr.get_locus (), "expected identifier");
+      return new TyTy::ErrorType (expr.get_mappings ().get_hirid ());
+    }
+
   return root_tyty;
 }
 

gcc/testsuite/rust/compile/paamayim-nekudotayim.rs

@@ -0,0 +1,5 @@
+// http://phpsadness.com/sad/1
+fn main() {
+    ::;
+    // { dg-error "expected identifier" "" { target *-*-* } .-1 }
+}