Address review comments

compiler/rustc_expand/src/base.rs

@@ -141,6 +141,9 @@ impl Annotatable {
     }
 
     crate fn into_tokens(self, sess: &ParseSess) -> TokenStream {
+        // Tokens of an attribute target may be invalidated by some outer `#[derive]` performing
+        // "full configuration" (attributes following derives on the same item should be the most
+        // common case), that's why synthesizing tokens is allowed.
         nt_to_tokenstream(&self.into_nonterminal(), sess, CanSynthesizeMissingTokens::Yes)
     }
 

src/test/ui/derives/derive-renamed.rs

@@ -0,0 +1,11 @@
+// check-pass
+// edition:2018
+
+use derive as my_derive;
+
+#[my_derive(Debug)]
+struct S;
+
+fn main() {
+    println!("{:?}", S); // OK
+}

src/test/ui/proc-macro/attribute-after-derive-feature-gate.rs

@@ -25,4 +25,13 @@ struct S3 {
     field: [u8; #[identity_attr] 10], //~ ERROR macro attributes in `#[derive]` output are unstable
 }
 
+#[derive(Empty)]
+struct S4 {
+    field: [u8; {
+        #[derive(Empty)] // OK, not gated
+        struct Inner;
+        10
+    }]
+}
+
 fn main() {}

src/test/ui/proc-macro/derive-helper-legacy-limits.rs

@@ -0,0 +1,21 @@
+// Support for legacy derive helpers is limited and heuristic-based
+// (that's exactly the reason why they are deprecated).
+
+// edition:2018
+// aux-build:test-macros.rs
+
+#[macro_use]
+extern crate test_macros;
+
+use derive as my_derive;
+
+#[my_derive(Empty)]
+#[empty_helper] // OK
+struct S1;
+
+// Legacy helper detection doesn't see through `derive` renaming.
+#[empty_helper] //~ ERROR cannot find attribute `empty_helper` in this scope
+#[my_derive(Empty)]
+struct S2;
+
+fn main() {}

src/test/ui/proc-macro/derive-helper-legacy-limits.stderr

@@ -0,0 +1,8 @@
+error: cannot find attribute `empty_helper` in this scope
+  --> $DIR/derive-helper-legacy-limits.rs:17:3
+   |
+LL | #[empty_helper]
+   |   ^^^^^^^^^^^^
+
+error: aborting due to previous error
+