Give RPITITs variances, so they can (not) capture lifetimes

compiler/rustc_ast_lowering/messages.ftl

@@ -130,9 +130,6 @@ ast_lowering_never_pattern_with_guard =
 
 ast_lowering_no_precise_captures_on_apit = `use<...>` precise capturing syntax not allowed in argument-position `impl Trait`
 
-ast_lowering_no_precise_captures_on_rpitit = `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
-    .note = currently, return-position `impl Trait` in traits and trait implementations capture all lifetimes in scope
-
 ast_lowering_previously_used_here = previously used here
 
 ast_lowering_register1 = register `{$reg1_name}`

compiler/rustc_ast_lowering/src/errors.rs

@@ -424,14 +424,6 @@ pub(crate) struct NoPreciseCapturesOnApit {
     pub span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(ast_lowering_no_precise_captures_on_rpitit)]
-#[note]
-pub(crate) struct NoPreciseCapturesOnRpitit {
-    #[primary_span]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(ast_lowering_yield_in_closure)]
 pub(crate) struct YieldInClosure {

compiler/rustc_ast_lowering/src/lib.rs

@@ -1594,26 +1594,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         };
         debug!(?captured_lifetimes_to_duplicate);
 
-        match fn_kind {
-            // Deny `use<>` on RPITIT in trait/trait-impl for now.
-            Some(FnDeclKind::Trait | FnDeclKind::Impl) => {
-                if let Some(span) = bounds.iter().find_map(|bound| match *bound {
-                    ast::GenericBound::Use(_, span) => Some(span),
-                    _ => None,
-                }) {
-                    self.tcx.dcx().emit_err(errors::NoPreciseCapturesOnRpitit { span });
-                }
-            }
-            None
-            | Some(
-                FnDeclKind::Fn
-                | FnDeclKind::Inherent
-                | FnDeclKind::ExternFn
-                | FnDeclKind::Closure
-                | FnDeclKind::Pointer,
-            ) => {}
-        }
-
         self.lower_opaque_inner(
             opaque_ty_node_id,
             origin,

compiler/rustc_hir_analysis/messages.ftl

@@ -229,6 +229,9 @@ hir_analysis_late_bound_lifetime_in_apit = `impl Trait` can only mention lifetim
 hir_analysis_late_bound_type_in_apit = `impl Trait` can only mention type parameters from an fn or impl
     .label = type parameter declared here
 
+hir_analysis_lifetime_implicitly_captured = `impl Trait` captures lifetime parameter, but it is not mentioned in `use<...>` precise captures list
+    .param_label = all lifetime parameters originating from a trait are captured implicitly
+
 hir_analysis_lifetime_must_be_first = lifetime parameter `{$name}` must be listed before non-lifetime parameters
     .label = move the lifetime before this parameter
 

compiler/rustc_hir_analysis/src/check/check.rs

@@ -593,15 +593,22 @@ fn check_opaque_precise_captures<'tcx>(tcx: TyCtxt<'tcx>, opaque_def_id: LocalDe
                                 param_span: tcx.def_span(def_id),
                             });
                         } else {
-                            // If the `use_span` is actually just the param itself, then we must
-                            // have not duplicated the lifetime but captured the original.
-                            // The "effective" `use_span` will be the span of the opaque itself,
-                            // and the param span will be the def span of the param.
-                            tcx.dcx().emit_err(errors::LifetimeNotCaptured {
-                                opaque_span,
-                                use_span: opaque_span,
-                                param_span: use_span,
-                            });
+                            if tcx.def_kind(tcx.parent(param.def_id)) == DefKind::Trait {
+                                tcx.dcx().emit_err(errors::LifetimeImplicitlyCaptured {
+                                    opaque_span,
+                                    param_span: tcx.def_span(param.def_id),
+                                });
+                            } else {
+                                // If the `use_span` is actually just the param itself, then we must
+                                // have not duplicated the lifetime but captured the original.
+                                // The "effective" `use_span` will be the span of the opaque itself,
+                                // and the param span will be the def span of the param.
+                                tcx.dcx().emit_err(errors::LifetimeNotCaptured {
+                                    opaque_span,
+                                    use_span: opaque_span,
+                                    param_span: use_span,
+                                });
+                            }
                         }
                         continue;
                     }

compiler/rustc_hir_analysis/src/check/compare_impl_item/refine.rs

@@ -6,7 +6,8 @@ use rustc_lint_defs::builtin::{REFINING_IMPL_TRAIT_INTERNAL, REFINING_IMPL_TRAIT
 use rustc_middle::span_bug;
 use rustc_middle::traits::{ObligationCause, Reveal};
 use rustc_middle::ty::{
-    self, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperVisitable, TypeVisitable, TypeVisitor,
+    self, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperVisitable, TypeVisitable,
+    TypeVisitableExt, TypeVisitor,
 };
 use rustc_span::Span;
 use rustc_trait_selection::regions::InferCtxtRegionExt;
@@ -63,6 +64,10 @@ pub(super) fn check_refining_return_position_impl_trait_in_trait<'tcx>(
         return;
     };
 
+    if hidden_tys.items().any(|(_, &ty)| ty.skip_binder().references_error()) {
+        return;
+    }
+
     let mut collector = ImplTraitInTraitCollector { tcx, types: FxIndexSet::default() };
     trait_m_sig.visit_with(&mut collector);
 

compiler/rustc_hir_analysis/src/errors/precise_captures.rs

@@ -33,6 +33,15 @@ pub struct LifetimeNotCaptured {
     pub opaque_span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_lifetime_implicitly_captured)]
+pub struct LifetimeImplicitlyCaptured {
+    #[primary_span]
+    pub opaque_span: Span,
+    #[label(hir_analysis_param_label)]
+    pub param_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(hir_analysis_bad_precise_capture)]
 pub struct BadPreciseCapture {

compiler/rustc_hir_analysis/src/variance/mod.rs

@@ -5,6 +5,7 @@
 
 use itertools::Itertools;
 use rustc_arena::DroplessArena;
+use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::query::Providers;
@@ -61,8 +62,29 @@ fn variances_of(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[ty::Variance] {
             let crate_map = tcx.crate_variances(());
             return crate_map.variances.get(&item_def_id.to_def_id()).copied().unwrap_or(&[]);
         }
+        DefKind::AssocTy => match tcx.opt_rpitit_info(item_def_id.to_def_id()) {
+            Some(ty::ImplTraitInTraitData::Trait { opaque_def_id, .. }) => {
+                return variance_of_opaque(
+                    tcx,
+                    opaque_def_id.expect_local(),
+                    ForceCaptureTraitArgs::Yes,
+                );
+            }
+            None | Some(ty::ImplTraitInTraitData::Impl { .. }) => {}
+        },
         DefKind::OpaqueTy => {
-            return variance_of_opaque(tcx, item_def_id);
+            let force_capture_trait_args = if let hir::OpaqueTyOrigin::FnReturn(fn_def_id) =
+                tcx.hir_node_by_def_id(item_def_id).expect_item().expect_opaque_ty().origin
+                && let Some(ty::AssocItem {
+                    container: ty::AssocItemContainer::TraitContainer, ..
+                }) = tcx.opt_associated_item(fn_def_id.to_def_id())
+            {
+                ForceCaptureTraitArgs::Yes
+            } else {
+                ForceCaptureTraitArgs::No
+            };
+
+            return variance_of_opaque(tcx, item_def_id, force_capture_trait_args);
         }
         _ => {}
     }
@@ -71,8 +93,18 @@ fn variances_of(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[ty::Variance] {
     span_bug!(tcx.def_span(item_def_id), "asked to compute variance for wrong kind of item");
 }
 
+#[derive(Debug, Copy, Clone)]
+enum ForceCaptureTraitArgs {
+    Yes,
+    No,
+}
+
 #[instrument(level = "trace", skip(tcx), ret)]
-fn variance_of_opaque(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[ty::Variance] {
+fn variance_of_opaque(
+    tcx: TyCtxt<'_>,
+    item_def_id: LocalDefId,
+    force_capture_trait_args: ForceCaptureTraitArgs,
+) -> &[ty::Variance] {
     let generics = tcx.generics_of(item_def_id);
 
     // Opaque types may only use regions that are bound. So for
@@ -113,9 +145,7 @@ fn variance_of_opaque(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[ty::Varianc
         #[instrument(level = "trace", skip(self), ret)]
         fn visit_ty(&mut self, t: Ty<'tcx>) {
             match t.kind() {
-                ty::Alias(_, ty::AliasTy { def_id, args, .. })
-                    if matches!(self.tcx.def_kind(*def_id), DefKind::OpaqueTy) =>
-                {
+                ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => {
                     self.visit_opaque(*def_id, args);
                 }
                 _ => t.super_visit_with(self),
@@ -133,6 +163,15 @@ fn variance_of_opaque(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[ty::Varianc
         let mut generics = generics;
         while let Some(def_id) = generics.parent {
             generics = tcx.generics_of(def_id);
+
+            // Don't mark trait params generic if we're in an RPITIT.
+            if matches!(force_capture_trait_args, ForceCaptureTraitArgs::Yes)
+                && generics.parent.is_none()
+            {
+                debug_assert_eq!(tcx.def_kind(def_id), DefKind::Trait);
+                break;
+            }
+
             for param in &generics.own_params {
                 match param.kind {
                     ty::GenericParamDefKind::Lifetime => {

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1089,9 +1089,12 @@ fn should_encode_variances<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, def_kind: Def
         | DefKind::Fn
         | DefKind::Ctor(..)
         | DefKind::AssocFn => true,
+        DefKind::AssocTy => {
+            // Only encode variances for RPITITs (for traits)
+            matches!(tcx.opt_rpitit_info(def_id), Some(ty::ImplTraitInTraitData::Trait { .. }))
+        }
         DefKind::Mod
         | DefKind::Field
-        | DefKind::AssocTy
         | DefKind::AssocConst
         | DefKind::TyParam
         | DefKind::ConstParam

compiler/rustc_middle/src/ty/context.rs

@@ -579,6 +579,10 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
     ) -> Ty<'tcx> {
         placeholder.find_const_ty_from_env(param_env)
     }
+
+    fn is_impl_trait_in_trait(self, def_id: DefId) -> bool {
+        self.is_impl_trait_in_trait(def_id)
+    }
 }
 
 fn trait_lang_item_to_lang_item(lang_item: TraitSolverLangItem) -> LangItem {

compiler/rustc_type_ir/src/interner.rs

@@ -277,6 +277,8 @@ pub trait Interner:
         param_env: Self::ParamEnv,
         placeholder: Self::PlaceholderConst,
     ) -> Self::Ty;
+
+    fn is_impl_trait_in_trait(self, def_id: Self::DefId) -> bool;
 }
 
 /// Imagine you have a function `F: FnOnce(&[T]) -> R`, plus an iterator `iter`

compiler/rustc_type_ir/src/relate.rs

@@ -254,6 +254,16 @@ impl<I: Interner> Relate<I> for ty::AliasTy<I> {
                     b.args,
                     false, // do not fetch `type_of(a_def_id)`, as it will cause a cycle
                 )?,
+                ty::Projection if relation.tcx().is_impl_trait_in_trait(a.def_id) => {
+                    relate_args_with_variances(
+                        relation,
+                        a.def_id,
+                        relation.tcx().variances_of(a.def_id),
+                        a.args,
+                        b.args,
+                        false, // do not fetch `type_of(a_def_id)`, as it will cause a cycle
+                    )?
+                }
                 ty::Projection | ty::Weak | ty::Inherent => {
                     relate_args_invariantly(relation, a.args, b.args)?
                 }

tests/ui/impl-trait/in-trait/variance.rs

@@ -7,14 +7,14 @@ impl<T> Captures<'_> for T {}
 
 trait Foo<'i> {
     fn implicit_capture_early<'a: 'a>() -> impl Sized {}
-    //~^ [o, *, *, o, o]
+    //~^ [o, o, *, o, o]
     // Self, 'i, 'a, 'i_duplicated, 'a_duplicated
 
-    fn explicit_capture_early<'a: 'a>() -> impl Sized + Captures<'a> {} //~ [o, *, *, o, o]
+    fn explicit_capture_early<'a: 'a>() -> impl Sized + Captures<'a> {} //~ [o, o, *, o, o]
 
-    fn implicit_capture_late<'a>(_: &'a ()) -> impl Sized {} //~ [o, *, o, o]
+    fn implicit_capture_late<'a>(_: &'a ()) -> impl Sized {} //~ [o, o, o, o]
 
-    fn explicit_capture_late<'a>(_: &'a ()) -> impl Sized + Captures<'a> {} //~ [o, *, o, o]
+    fn explicit_capture_late<'a>(_: &'a ()) -> impl Sized + Captures<'a> {} //~ [o, o, o, o]
 }
 
 fn main() {}

tests/ui/impl-trait/in-trait/variance.stderr

@@ -1,22 +1,22 @@
-error: [o, *, *, o, o]
+error: [o, o, *, o, o]
   --> $DIR/variance.rs:9:44
    |
 LL |     fn implicit_capture_early<'a: 'a>() -> impl Sized {}
    |                                            ^^^^^^^^^^
 
-error: [o, *, *, o, o]
+error: [o, o, *, o, o]
   --> $DIR/variance.rs:13:44
    |
 LL |     fn explicit_capture_early<'a: 'a>() -> impl Sized + Captures<'a> {}
    |                                            ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: [o, *, o, o]
+error: [o, o, o, o]
   --> $DIR/variance.rs:15:48
    |
 LL |     fn implicit_capture_late<'a>(_: &'a ()) -> impl Sized {}
    |                                                ^^^^^^^^^^
 
-error: [o, *, o, o]
+error: [o, o, o, o]
   --> $DIR/variance.rs:17:48
    |
 LL |     fn explicit_capture_late<'a>(_: &'a ()) -> impl Sized + Captures<'a> {}

tests/ui/impl-trait/precise-capturing/forgot-to-capture-type.rs

@@ -6,7 +6,6 @@ fn type_param<T>() -> impl Sized + use<> {}
 trait Foo {
     fn bar() -> impl Sized + use<>;
     //~^ ERROR `impl Trait` must mention the `Self` type of the trait
-    //~| ERROR `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
 }
 
 fn main() {}

tests/ui/impl-trait/precise-capturing/forgot-to-capture-type.stderr

@@ -1,11 +1,3 @@
-error: `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
-  --> $DIR/forgot-to-capture-type.rs:7:30
-   |
-LL |     fn bar() -> impl Sized + use<>;
-   |                              ^^^^^
-   |
-   = note: currently, return-position `impl Trait` in traits and trait implementations capture all lifetimes in scope
-
 error: `impl Trait` must mention all type parameters in scope in `use<...>`
   --> $DIR/forgot-to-capture-type.rs:3:23
    |
@@ -26,5 +18,5 @@ LL |     fn bar() -> impl Sized + use<>;
    |
    = note: currently, all type parameters are required to be mentioned in the precise captures list
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 

tests/ui/impl-trait/precise-capturing/redundant.rs

@@ -1,27 +1,24 @@
 //@ compile-flags: -Zunstable-options --edition=2024
-//@ revisions: normal rpitit
-//@[normal] check-pass
+//@ check-pass
 
 #![feature(precise_capturing)]
 
 fn hello<'a>() -> impl Sized + use<'a> {}
-//[normal]~^ WARN all possible in-scope parameters are already captured
+//~^ WARN all possible in-scope parameters are already captured
 
 struct Inherent;
 impl Inherent {
     fn inherent(&self) -> impl Sized + use<'_> {}
-    //[normal]~^ WARN all possible in-scope parameters are already captured
+    //~^ WARN all possible in-scope parameters are already captured
 }
 
-#[cfg(rpitit)]
 trait Test<'a> {
     fn in_trait() -> impl Sized + use<'a, Self>;
-    //[rpitit]~^ ERROR `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
+    //~^ WARN all possible in-scope parameters are already captured
 }
-#[cfg(rpitit)]
 impl<'a> Test<'a> for () {
     fn in_trait() -> impl Sized + use<'a> {}
-    //[rpitit]~^ ERROR `use<...>` precise capturing syntax is currently not allowed in return-position `impl Trait` in traits
+    //~^ WARN all possible in-scope parameters are already captured
 }
 
 fn main() {}