macros: Do not lower macro definitions to HIR

Avoid lowering block statements that should not be lowered, such as
macro-rules definitions

Author: CohenArthur

gcc/rust/hir/rust-ast-lower.cc

@@ -67,6 +67,9 @@ ASTLoweringBlock::visit (AST::BlockExpr &expr)
 
   for (auto &s : expr.get_statements ())
     {
+      if (s->get_ast_kind () == AST::Kind::MACRO_RULES_DEFINITION)
+	continue;
+
       if (block_did_terminate)
 	rust_warning_at (s->get_locus (), 0, "unreachable statement");
 

gcc/testsuite/rust/compile/macro16.rs

@@ -0,0 +1,11 @@
+fn main() {
+    macro_rules! create_type {
+        ($s:ident) => {
+            struct $s(i32);
+        };
+    }
+
+    create_type!(Wrapper);
+
+    let _ = Wrapper(15);
+}