Re-add generators as a removed feature and point to the new feature name

Author: oli-obk

compiler/rustc_feature/src/removed.rs

@@ -96,6 +96,10 @@ declare_features! (
     /// Allows `#[doc(include = "some-file")]`.
     (removed, external_doc, "1.54.0", Some(44732), None,
      Some("use #[doc = include_str!(\"filename\")] instead, which handles macro invocations")),
+    /// Allows generators to be cloned.
+    (removed, generator_clone, "1.65.0", Some(95360), None, Some("renamed to `coroutine_clone`")),
+    /// Allows defining generators.
+    (removed, generators, "1.21.0", Some(43122), None, Some("renamed to `coroutine`")),
     /// Allows `impl Trait` in bindings (`let`, `const`, `static`).
     (removed, impl_trait_in_bindings, "1.55.0", Some(63065), None,
      Some("the implementation was not maintainable, the feature may get reintroduced once the current refactorings are done")),

compiler/rustc_span/src/symbol.rs

@@ -819,6 +819,8 @@ symbols! {
         ge,
         gen_future,
         gen_kill,
+        generator_clone,
+        generators,
         generic_arg_infer,
         generic_assert,
         generic_associated_types,