No output allowed

Author: compiler-errors

compiler/rustc_ast_lowering/src/lib.rs

@@ -129,6 +129,7 @@ struct LoweringContext<'a, 'hir> {
     /// NodeIds that are lowered inside the current HIR owner.
     node_id_to_local_id: NodeMap<hir::ItemLocalId>,
 
+    allow_fn_once_output: Lrc<[Symbol]>,
     allow_try_trait: Lrc<[Symbol]>,
     allow_gen_future: Lrc<[Symbol]>,
     allow_async_iterator: Lrc<[Symbol]>,
@@ -177,6 +178,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             current_item: None,
             impl_trait_defs: Vec::new(),
             impl_trait_bounds: Vec::new(),
+            allow_fn_once_output: [sym::fn_traits].into(),
             allow_try_trait: [sym::try_trait_v2, sym::yeet_desugar_details].into(),
             allow_gen_future: if tcx.features().async_fn_track_caller {
                 [sym::gen_future, sym::closure_track_caller].into()

compiler/rustc_ast_lowering/src/path.rs

@@ -440,14 +440,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
         // If we have a bound like `async Fn() -> T`, make sure that we mark the
         // `Output = T` associated type bound with the right feature gates.
-        let mut output_span = output_ty.span;
-        if let Some(bound_modifier_allowed_features) = bound_modifier_allowed_features {
-            output_span = self.mark_span_with_reason(
-                DesugaringKind::BoundModifier,
-                output_span,
-                Some(bound_modifier_allowed_features),
-            );
-        }
+        let output_span  = self.mark_span_with_reason(
+            DesugaringKind::BoundModifier,
+            output_ty.span,
+            Some(bound_modifier_allowed_features.unwrap_or_else(|| self.allow_fn_once_output.clone())),
+        );
         let constraint = self.assoc_ty_binding(sym::Output, output_span, output_ty);
 
         (

compiler/rustc_span/src/symbol.rs

@@ -898,6 +898,7 @@ symbols! {
         fn_once_output,
         fn_ptr_addr,
         fn_ptr_trait,
+        fn_traits,
         forbid,
         forget,
         format,

library/core/src/ops/function.rs

@@ -242,7 +242,8 @@ pub trait FnMut<Args: Tuple>: FnOnce<Args> {
 pub trait FnOnce<Args: Tuple> {
     /// The returned type after the call operator is used.
     #[lang = "fn_once_output"]
-    #[stable(feature = "fn_once_output", since = "1.12.0")]
+    #[cfg_attr(bootstrap, stable(feature = "fn_once_output", since = "1.12.0"))]
+    #[cfg_attr(not(bootstrap), unstable(feature = "fn_traits", issue = "29625"))]
     type Output;
 
     /// Performs the call operation.