create dummy placeholder crate to prevent compiler from panicing

compiler/rustc_expand/src/base.rs

@@ -5,6 +5,7 @@ use crate::expand::{self, AstFragment, Invocation};
 use crate::module::DirOwnership;
 
 use rustc_ast::attr::MarkedAttrs;
+use rustc_ast::mut_visit::DummyAstNode;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Nonterminal};
 use rustc_ast::tokenstream::TokenStream;
@@ -640,6 +641,10 @@ impl MacResult for DummyResult {
     fn make_variants(self: Box<DummyResult>) -> Option<SmallVec<[ast::Variant; 1]>> {
         Some(SmallVec::new())
     }
+
+    fn make_crate(self: Box<DummyResult>) -> Option<ast::Crate> {
+        Some(DummyAstNode::dummy())
+    }
 }
 
 /// A syntax extension kind.

tests/ui/recursion_limit/issue_21102.rs

@@ -0,0 +1,9 @@
+#![recursion_limit="4"]
+#![invalid_attribute]
+#![invalid_attribute]
+#![invalid_attribute]
+#![invalid_attribute]
+#![invalid_attribute]
+//~^ERROR recursion limit reached while expanding
+
+fn main() {}

tests/ui/recursion_limit/issue_21102.stderr

@@ -0,0 +1,10 @@
+error: recursion limit reached while expanding `#[invalid_attribute]`
+  --> $DIR/issue_21102.rs:6:1
+   |
+LL | #![invalid_attribute]
+   | ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "8"]` attribute to your crate (`issue_21102`)
+
+error: aborting due to previous error
+