Talk about gen fn in diagnostics about gen fn

compiler/rustc_parse/messages.ftl

@@ -278,8 +278,8 @@ parse_found_expr_would_be_stmt = expected expression, found `{$token}`
 parse_function_body_equals_expr = function body cannot be `= expression;`
     .suggestion = surround the expression with `{"{"}` and `{"}"}` instead of `=` and `;`
 
-parse_gen_block = `gen` blocks are not yet implemented
-    .help = only the keyword is reserved for now
+parse_gen_fn = `gen` functions are not yet implemented
+    .help = for now you can use `gen {"{}"}` blocks and return `impl Iterator` instead
 
 parse_generic_args_in_pat_require_turbofish_syntax = generic args in patterns require the turbofish syntax
 

compiler/rustc_parse/src/errors.rs

@@ -521,9 +521,9 @@ pub(crate) struct CatchAfterTry {
 }
 
 #[derive(Diagnostic)]
-#[diag(parse_gen_block)]
+#[diag(parse_gen_fn)]
 #[help]
-pub(crate) struct GenBlock {
+pub(crate) struct GenFn {
     #[primary_span]
     pub span: Span,
 }

compiler/rustc_parse/src/parser/item.rs

@@ -2372,7 +2372,7 @@ impl<'a> Parser<'a> {
         }
 
         if let Gen::Yes { span, .. } = genness {
-            self.sess.emit_err(errors::GenBlock { span });
+            self.sess.emit_err(errors::GenFn { span });
         }
 
         if !self.eat_keyword_case(kw::Fn, case) {

tests/ui/coroutine/gen_fn.e2024.stderr

@@ -1,10 +1,10 @@
-error: `gen` blocks are not yet implemented
+error: `gen` functions are not yet implemented
   --> $DIR/gen_fn.rs:4:1
    |
 LL | gen fn foo() {}
    | ^^^
    |
-   = help: only the keyword is reserved for now
+   = help: for now you can use `gen {}` blocks and return `impl Iterator` instead
 
 error: aborting due to previous error
 

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` blocks are not yet implemented
+//[e2024]~^^ ERROR: `gen` functions are not yet implemented
 
 fn main() {}