gate gen fn behind gen_blocks

compiler/rustc_parse/src/parser/item.rs

@@ -2410,6 +2410,10 @@ impl<'a> Parser<'a> {
             }
         }
 
+        if let Gen::Yes { span, .. } = genness {
+            self.sess.gated_spans.gate(sym::gen_blocks, span);
+        }
+
         if !self.eat_keyword_case(kw::Fn, case) {
             // It is possible for `expect_one_of` to recover given the contents of
             // `self.expected_tokens`, therefore, do not use `self.unexpected()` which doesn't

tests/ui/coroutine/gen_fn.e2024.stderr

@@ -1,10 +1,12 @@
-error: `gen` functions are not yet implemented
+error[E0658]: gen blocks are experimental
   --> $DIR/gen_fn.rs:4:1
    |
 LL | gen fn foo() {}
    | ^^^
    |
-   = help: for now you can use `gen {}` blocks and return `impl Iterator` instead
+   = note: see issue #117078 <https://github.com/rust-lang/rust/issues/117078> for more information
+   = help: add `#![feature(gen_blocks)]` to the crate attributes to enable
 
 error: aborting due to 1 previous error
 
+For more information about this error, try `rustc --explain E0658`.

tests/ui/coroutine/gen_fn.rs

@@ -3,6 +3,6 @@
 
 gen fn foo() {}
 //[none]~^ ERROR: expected one of `#`, `async`, `const`, `default`, `extern`, `fn`, `pub`, `unsafe`, or `use`, found `gen`
-//[e2024]~^^ ERROR: `gen` functions are not yet implemented
+//[e2024]~^^ ERROR: gen blocks are experimental
 
 fn main() {}