gccrs: Report an error on an empty path expression

The path parser previously returned an error node silently when the
initial path segment failed to parse. Report a diagnostic for paths
beginning with ::, where the resulting error node could otherwise lead
to an ICE downstream.

Do not apply the guard to bare $, since undefined metavariables already
receive more specific diagnostics downstream. Remove the redundant
parse_stmt_or_expr guard, which otherwise reports the same error twice.

Fixes #4790

gcc/rust/ChangeLog:

	* parse/rust-parse-impl-path.hxx (Parser::parse_path_in_expression):
	Report an error when the initial segment of a scoped path fails to
	parse.
	* parse/rust-parse-impl.hxx (Parser::parse_stmt_or_expr): Remove the
	redundant expected-identifier error guard.

gcc/testsuite/ChangeLog:

	* rust/compile/empty_path2.rs: New test.
	* rust/compile/empty_path3.rs: New test.

Signed-off-by: Utkarsh Bahuguna <utkarshbahuguna10@gmail.com>

Author: u7k4rs6

gcc/rust/parse/rust-parse-impl-path.hxx

@@ -389,8 +389,11 @@ Parser<ManagedTokenSource>::parse_path_in_expression ()
   AST::PathExprSegment initial_segment = parse_path_expr_segment ();
   if (initial_segment.is_error ())
     {
-      // skip after somewhere?
-      // don't necessarily throw error but yeah
+      if (has_opening_scope_resolution)
+	{
+	  Error error (locus, "expected identifier");
+	  add_error (std::move (error));
+	}
       return AST::PathInExpression::create_error ();
     }
   segments.push_back (std::move (initial_segment));

gcc/rust/parse/rust-parse-impl.hxx

@@ -7294,15 +7294,6 @@ Parser<ManagedTokenSource>::parse_stmt_or_expr ()
     case DOLLAR_SIGN:
       {
 	AST::PathInExpression path = parse_path_in_expression ();
-	if (path.is_error ())
-	  {
-	    Error error (t->get_locus (), "expected identifier");
-	    add_error (std::move (error));
-	    skip_after_semicolon ();
-	    return tl::unexpected<Parse::Error::Node> (
-	      Parse::Error::Node::CHILD_ERROR);
-	  }
-
 	tl::expected<std::unique_ptr<AST::Expr>, Parse::Error::Expr>
 	  null_denotation;
 

gcc/testsuite/rust/compile/empty_path2.rs

@@ -0,0 +1,9 @@
+#![feature(no_core)]
+#![no_core]
+
+// A path expression with no segments in a let initialiser segfaulted
+// during type checking, see Rust-GCC/gccrs#4790.
+fn main() {
+    let x = ::;
+    // { dg-error "expected identifier" "" { target *-*-* } .-1 }
+}

gcc/testsuite/rust/compile/empty_path3.rs

@@ -0,0 +1,12 @@
+#![feature(no_core)]
+#![no_core]
+
+// An empty path `::` in pattern position. The parser returned a path with no
+// segments without reporting anything, so type checking dereferenced a null
+// root type and the compiler crashed instead of diagnosing. Same defect as
+// empty_path2.rs, which covers the let initialiser instead of the pattern.
+// See Rust-GCC/gccrs#4790.
+fn main() {
+    let :: = 1;
+    // { dg-error "expected identifier" "" { target *-*-* } .-1 }
+}