gccrs: parser: Fix ICE in AnonConst by handling null macro result

The parse_anon_const function triggered an assertion failure (ICE) when
receiving a success Result containing a null expression pointer. This
occurred because null_denotation_path directly returned the result of
parse_macro_invocation_partial (which returns a raw pointer) without
checking for failure.

When parse_macro_invocation_partial failed (returning nullptr), it was
wrapped into a tl::expected success state containing nullptr, rather than
an error state.

This patch adds a check in null_denotation_path to verify if the parsed
macro invocation is null. If it is, it now returns a tl::unexpected error,
ensuring that invalid expressions are correctly reported as errors
upstream.

Fixes Rust-GCC#4412

gcc/rust/ChangeLog:

	* parse/rust-parse-impl-expr.hxx (null_denotation_path): Check
	if macro invocation returns null and return error.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4412.rs: New test.

Signed-off-by: Jayant Chauhan <0001jayant@gmail.com>

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

@@ -1957,9 +1957,15 @@ Parser<ManagedTokenSource>::null_denotation_path (
   switch (t->get_id ())
     {
     case EXCLAM:
-      // macro
-      return parse_macro_invocation_partial (std::move (path),
-					     std::move (outer_attrs));
+      {
+	// macro
+	auto macro = parse_macro_invocation_partial (std::move (path),
+						     std::move (outer_attrs));
+	if (macro == nullptr)
+	  return tl::unexpected<Parse::Error::Expr> (
+	    Parse::Error::Expr::CHILD_ERROR);
+	return std::unique_ptr<AST::Expr> (std::move (macro));
+      }
     case LEFT_CURLY:
       {
 	bool not_a_block = lexer.peek_token (1)->get_id () == IDENTIFIER

gcc/testsuite/rust/compile/issue-4412.rs

@@ -0,0 +1,9 @@
+// { dg-options "-frust-incomplete-and-experimental-compiler-do-not-use" }
+#![feature(no_core)]
+#![no_core]
+struct Bug([u8; panic!"\t"]);
+// { dg-error "unexpected token" "" { target *-*-* } .-1 }
+// { dg-error "failed to parse" "" { target *-*-* } .-2 }
+// { dg-error "could not parse" "" { target *-*-* } .-3 }
+// { dg-error "expecting" "" { target *-*-* } .-4 }
+fn main() {}