Error on using yield without also using gen on the closure

And suggest adding the `gen` to the closure or function

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 `gen` closures, functions, or blocks
+    .suggestion = use `gen` to make this item 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 = "gen ", applicability = "maybe-incorrect", style = "verbose")]
+    pub suggestion: Option<Span>,
+}

compiler/rustc_ast_lowering/src/expr.rs

@@ -8,6 +8,7 @@ use super::errors::{
 };
 use super::ResolverAstLoweringExt;
 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
+use crate::errors::YieldInClosure;
 use crate::{FnDeclKind, ImplTraitPosition};
 use rustc_ast::ptr::P as AstP;
 use rustc_ast::*;
@@ -1578,7 +1579,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     )
                     .emit();
                 }
+                if self.tcx.sess.edition().at_least_rust_2024() {
+                    let suggestion = self.current_item.map(|s| s.shrink_to_lo());
+                    self.dcx().emit_err(YieldInClosure { span, suggestion });
+                }
                 self.coroutine_kind = Some(hir::CoroutineKind::Coroutine(Movability::Movable));
+
                 false
             }
         };

compiler/rustc_ast_lowering/src/item.rs

@@ -203,7 +203,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 body,
                 ..
             }) => {
-                self.with_new_scopes(ident.span, |this| {
+                self.with_new_scopes(*fn_sig_span, |this| {
                     // Note: we don't need to change the return type from `T` to
                     // `impl Future<Output = T>` here because lower_body
                     // only cares about the input argument patterns in the function

tests/ui/async-await/async-outside-of-await-issue-121096.stderr

@@ -2,7 +2,7 @@ error[E0728]: `await` is only allowed inside `async` functions and blocks
   --> $DIR/async-outside-of-await-issue-121096.rs:7:7
    |
 LL | fn main() {
-   |    ---- this is not `async`
+   | --------- this is not `async`
 ...
 LL |     }.await
    |       ^^^^^ only allowed inside `async` functions and blocks

tests/ui/async-await/await-keyword/incorrect-syntax-suggestions.stderr

@@ -141,39 +141,39 @@ error[E0728]: `await` is only allowed inside `async` functions and blocks
   --> $DIR/incorrect-syntax-suggestions.rs:68:19
    |
 LL | fn foo13() -> Result<(), ()> {
-   |    ----- this is not `async`
+   | ---------------------------- this is not `async`
 LL |     let _ = bar().await();
    |                   ^^^^^ only allowed inside `async` functions and blocks
 
 error[E0728]: `await` is only allowed inside `async` functions and blocks
   --> $DIR/incorrect-syntax-suggestions.rs:73:19
    |
 LL | fn foo14() -> Result<(), ()> {
-   |    ----- this is not `async`
+   | ---------------------------- this is not `async`
 LL |     let _ = bar().await()?;
    |                   ^^^^^ only allowed inside `async` functions and blocks
 
 error[E0728]: `await` is only allowed inside `async` functions and blocks
   --> $DIR/incorrect-syntax-suggestions.rs:78:19
    |
 LL | fn foo15() -> Result<(), ()> {
-   |    ----- this is not `async`
+   | ---------------------------- this is not `async`
 LL |     let _ = bar().await;
    |                   ^^^^^ only allowed inside `async` functions and blocks
 
 error[E0728]: `await` is only allowed inside `async` functions and blocks
   --> $DIR/incorrect-syntax-suggestions.rs:82:19
    |
 LL | fn foo16() -> Result<(), ()> {
-   |    ----- this is not `async`
+   | ---------------------------- this is not `async`
 LL |     let _ = bar().await?;
    |                   ^^^^^ only allowed inside `async` functions and blocks
 
 error[E0728]: `await` is only allowed inside `async` functions and blocks
   --> $DIR/incorrect-syntax-suggestions.rs:87:23
    |
 LL |     fn foo() -> Result<(), ()> {
-   |        --- this is not `async`
+   |     -------------------------- this is not `async`
 LL |         let _ = bar().await?;
    |                       ^^^^^ only allowed inside `async` functions and blocks
 

tests/ui/async-await/issues/issue-51751.stderr

@@ -2,7 +2,7 @@ error[E0728]: `await` is only allowed inside `async` functions and blocks
   --> $DIR/issue-51751.rs:9:27
    |
 LL | fn main() {
-   |    ---- this is not `async`
+   | --------- this is not `async`
 LL |     let result = inc(10000);
 LL |     let finished = result.await;
    |                           ^^^^^ only allowed inside `async` functions and blocks

tests/ui/async-await/issues/issue-62009-1.stderr

@@ -2,15 +2,15 @@ error[E0728]: `await` is only allowed inside `async` functions and blocks
   --> $DIR/issue-62009-1.rs:6:23
    |
 LL | fn main() {
-   |    ---- this is not `async`
+   | --------- this is not `async`
 LL |     async { let (); }.await;
    |                       ^^^^^ only allowed inside `async` functions and blocks
 
 error[E0728]: `await` is only allowed inside `async` functions and blocks
   --> $DIR/issue-62009-1.rs:10:7
    |
 LL | fn main() {
-   |    ---- this is not `async`
+   | --------- this is not `async`
 ...
 LL |     }.await;
    |       ^^^^^ only allowed inside `async` functions and blocks
@@ -19,7 +19,7 @@ error[E0728]: `await` is only allowed inside `async` functions and blocks
   --> $DIR/issue-62009-1.rs:12:16
    |
 LL | fn main() {
-   |    ---- this is not `async`
+   | --------- this is not `async`
 ...
 LL |     (|_| 2333).await;
    |                ^^^^^ only allowed inside `async` functions and blocks

tests/ui/async-await/issues/issue-62009-2.stderr

@@ -2,7 +2,7 @@ error[E0728]: `await` is only allowed inside `async` functions and blocks
   --> $DIR/issue-62009-2.rs:8:23
    |
 LL | fn main() {
-   |    ---- this is not `async`
+   | --------- this is not `async`
 LL |     (async || 2333)().await;
    |                       ^^^^^ only allowed inside `async` functions and blocks
 

tests/ui/async-await/issues/non-async-enclosing-span.stderr

@@ -2,7 +2,7 @@ error[E0728]: `await` is only allowed inside `async` functions and blocks
   --> $DIR/non-async-enclosing-span.rs:9:28
    |
 LL | fn main() {
-   |    ---- this is not `async`
+   | --------- this is not `async`
 LL |     let x = move || {};
 LL |     let y = do_the_thing().await;
    |                            ^^^^^ only allowed inside `async` functions and blocks