@@ -406,6 +406,12 @@ pub trait TypeErrCtxtExt<'tcx> {
406406 candidate_impls : & [ ImplCandidate < ' tcx > ] ,
407407 span : Span ,
408408 ) ;
409+
410+ fn suggest_desugaring_async_fn_in_trait (
411+ & self ,
412+ err : & mut Diagnostic ,
413+ trait_ref : ty:: PolyTraitRef < ' tcx > ,
414+ ) ;
409415}
410416
411417fn predicate_constraint ( generics : & hir:: Generics < ' _ > , pred : ty:: Predicate < ' _ > ) -> ( Span , String ) {
@@ -4027,6 +4033,124 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
40274033 }
40284034 }
40294035 }
4036+
4037+ fn suggest_desugaring_async_fn_in_trait (
4038+ & self ,
4039+ err : & mut Diagnostic ,
4040+ trait_ref : ty:: PolyTraitRef < ' tcx > ,
4041+ ) {
4042+ // Don't suggest if RTN is active -- we should prefer a where-clause bound instead.
4043+ if self . tcx . features ( ) . return_type_notation {
4044+ return ;
4045+ }
4046+
4047+ let trait_def_id = trait_ref. def_id ( ) ;
4048+
4049+ // Only suggest specifying auto traits
4050+ if !self . tcx . trait_is_auto ( trait_def_id) {
4051+ return ;
4052+ }
4053+
4054+ // Look for an RPITIT
4055+ let ty:: Alias ( ty:: Projection , alias_ty) = trait_ref. self_ty ( ) . skip_binder ( ) . kind ( ) else {
4056+ return ;
4057+ } ;
4058+ let Some ( ty:: ImplTraitInTraitData :: Trait { fn_def_id, opaque_def_id } ) =
4059+ self . tcx . opt_rpitit_info ( alias_ty. def_id )
4060+ else {
4061+ return ;
4062+ } ;
4063+
4064+ // ... which is a local function
4065+ let Some ( fn_def_id) = fn_def_id. as_local ( ) else {
4066+ return ;
4067+ } ;
4068+ let Some ( hir:: Node :: TraitItem ( item) ) = self . tcx . hir ( ) . find_by_def_id ( fn_def_id) else {
4069+ return ;
4070+ } ;
4071+
4072+ // ... whose signature is `async` (i.e. this is an AFIT)
4073+ let ( sig, body) = item. expect_fn ( ) ;
4074+ let hir:: IsAsync :: Async ( async_span) = sig. header . asyncness else {
4075+ return ;
4076+ } ;
4077+ let Ok ( async_span) =
4078+ self . tcx . sess . source_map ( ) . span_extend_while ( async_span, |c| c. is_whitespace ( ) )
4079+ else {
4080+ return ;
4081+ } ;
4082+ let hir:: FnRetTy :: Return ( hir:: Ty { kind : hir:: TyKind :: OpaqueDef ( def, ..) , .. } ) =
4083+ sig. decl . output
4084+ else {
4085+ // This should never happen, but let's not ICE.
4086+ return ;
4087+ } ;
4088+
4089+ // Check that this is *not* a nested `impl Future` RPIT in an async fn
4090+ // (i.e. `async fn foo() -> impl Future`)
4091+ if def. owner_id . to_def_id ( ) != opaque_def_id {
4092+ return ;
4093+ }
4094+
4095+ let future = self . tcx . hir ( ) . item ( * def) . expect_opaque_ty ( ) ;
4096+ let Some ( hir:: GenericBound :: LangItemTrait ( _, _, _, generics) ) = future. bounds . get ( 0 ) else {
4097+ // `async fn` should always lower to a lang item bound... but don't ICE.
4098+ return ;
4099+ } ;
4100+ let Some ( hir:: TypeBindingKind :: Equality { term : hir:: Term :: Ty ( future_output_ty) } ) =
4101+ generics. bindings . get ( 0 ) . map ( |binding| binding. kind )
4102+ else {
4103+ // Also should never happen.
4104+ return ;
4105+ } ;
4106+
4107+ let function_name = self . tcx . def_path_str ( fn_def_id) ;
4108+ let auto_trait = self . tcx . def_path_str ( trait_def_id) ;
4109+
4110+ let mut sugg = if future_output_ty. span . is_empty ( ) {
4111+ vec ! [
4112+ ( async_span, String :: new( ) ) ,
4113+ (
4114+ future_output_ty. span,
4115+ format!( " -> impl std::future::Future<Output = ()> + {auto_trait}" ) ,
4116+ ) ,
4117+ ]
4118+ } else {
4119+ vec ! [
4120+ (
4121+ future_output_ty. span. shrink_to_lo( ) ,
4122+ "impl std::future::Future<Output = " . to_owned( ) ,
4123+ ) ,
4124+ ( future_output_ty. span. shrink_to_hi( ) , format!( "> + {auto_trait}" ) ) ,
4125+ ( async_span, String :: new( ) ) ,
4126+ ]
4127+ } ;
4128+
4129+ // If there's a body, we also need to wrap it in `async {}`
4130+ if let hir:: TraitFn :: Provided ( body) = body {
4131+ let body = self . tcx . hir ( ) . body ( * body) ;
4132+ let body_span = body. value . span ;
4133+ let body_span_without_braces =
4134+ body_span. with_lo ( body_span. lo ( ) + BytePos ( 1 ) ) . with_hi ( body_span. hi ( ) - BytePos ( 1 ) ) ;
4135+ if body_span_without_braces. is_empty ( ) {
4136+ sugg. push ( ( body_span_without_braces, " async {} " . to_owned ( ) ) ) ;
4137+ } else {
4138+ sugg. extend ( [
4139+ ( body_span_without_braces. shrink_to_lo ( ) , "async {" . to_owned ( ) ) ,
4140+ ( body_span_without_braces. shrink_to_hi ( ) , "} " . to_owned ( ) ) ,
4141+ ] ) ;
4142+ }
4143+ }
4144+
4145+ err. multipart_suggestion (
4146+ format ! (
4147+ "`{auto_trait}` can be made part of the associated future's \
4148+
4149+ ) ,
4150+ sugg,
4151+ Applicability :: MachineApplicable ,
4152+ ) ;
4153+ }
40304154}
40314155
40324156/// Add a hint to add a missing borrow or remove an unnecessary one.
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