gccrs: Fix ICE with invalid init expression in TuplePattern LetStmt

The compiler was throwing an ICE when a Type Alias was used as a value
in a LetStmt (e.g., `let () = ::X = ();`). This occurred because the
backend pattern compiler received an `error_mark_node` for the
initialization expression and failed to handle it.

This patch adds a check in `CompileStmt` to detect if the initialization
expression is an `error_mark_node`. To prevent regression failures in
other tests (excess errors), this check is restricted specifically to
`TuplePattern`, which was the source of the crash. If detected, we now
emit a proper error message and exit early.

gcc/rust/ChangeLog:

	* backend/rust-compile-stmt.cc (CompileStmt::visit): Check for
	error_mark_node in init expression for TuplePatterns and exit
	early if found.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4161.rs: New test.

Signed-off-by: Harishankar P P <harishankarpp7@gmail.com>

Author: Harishankar14

gcc/rust/backend/rust-compile-stmt.cc

@@ -74,6 +74,17 @@ CompileStmt::visit (HIR::LetStmt &stmt)
   tree init = CompileExpr::Compile (stmt.get_init_expr (), ctx);
   // FIXME use error_mark_node, check that CompileExpr returns error_mark_node
   // on failure and make this an assertion
+  if (init == error_mark_node)
+    {
+      if (stmt.get_pattern ().get_pattern_type ()
+	  == HIR::Pattern::PatternType::TUPLE)
+	{
+	  if (!seen_error ())
+	    rust_error_at (stmt.get_init_expr ().get_locus (),
+			   "expected value, found invalid expression");
+	  return;
+	}
+    }
   if (init == nullptr)
     return;
 

gcc/rust/checks/lints/rust-lint-marklive.cc

@@ -268,8 +268,6 @@ MarkLive::visit (HIR::TypeAlias &alias)
 
   if (auto hid = mappings.lookup_node_to_hir (ast_node_id))
     mark_hir_id (*hid);
-  else
-    rust_unreachable ();
 }
 
 void

gcc/testsuite/rust/compile/issue-4161.rs

@@ -0,0 +1,6 @@
+
+type X = ();
+
+fn main() {
+    let () = ::X = (); // { dg-error "expected value, found invalid expression" }
+}