Error on using yield without also using #[coroutine] on the closure

And suggest adding the `#[coroutine]` to the closure

Author: oli-obk

compiler/rustc_ast_lowering/messages.ftl

@@ -161,3 +161,6 @@ ast_lowering_underscore_expr_lhs_assign =
     .label = `_` not allowed here
 
 ast_lowering_use_angle_brackets = use angle brackets instead
+ast_lowering_yield_in_closure =
+    `yield` can only be used in `#[coroutine]` closures, or `gen` blocks
+    .suggestion = use `#[coroutine]` to make this closure a coroutine

compiler/rustc_ast_lowering/src/errors.rs

@@ -414,3 +414,12 @@ pub(crate) struct AsyncBoundOnlyForFnTraits {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_lowering_yield_in_closure)]
+pub(crate) struct YieldInClosure {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code = "#[coroutine] ", applicability = "maybe-incorrect", style = "verbose")]
+    pub suggestion: Option<Span>,
+}