Auto merge of #123468 - compiler-errors:precise-capturing, r=oli-obk

Implement syntax for `impl Trait` to specify its captures explicitly (`feature(precise_capturing)`)

Implements `impl use<'a, 'b, T, U> Sized` syntax that allows users to explicitly list the captured parameters for an opaque, rather than inferring it from the opaque's bounds (or capturing *all* lifetimes under 2024-edition capture rules). This allows us to exclude some implicit captures, so this syntax may be used as a migration strategy for changes due to #117587.

We represent this list of captured params as `PreciseCapturingArg` in AST and HIR, resolving them between `rustc_resolve` and `resolve_bound_vars`. Later on, we validate that the opaques only capture the parameters in this list.

We artificially limit the feature to *require* mentioning all type and const parameters, since we don't currently have support for non-lifetime bivariant generics. This can be relaxed in the future.

We also may need to limit this to require naming *all* lifetime parameters for RPITIT, since GATs have no variance. I have to investigate this. This can also be relaxed in the future.

r? `@oli-obk`

Tracking issue:

- https://github.com/rust-lang/rust/issues/123432

compiler/rustc_ast/src/ast.rs

@@ -63,7 +63,7 @@ impl fmt::Debug for Label {
 
 /// A "Lifetime" is an annotation of the scope in which variable
 /// can be used, e.g. `'a` in `&'a i32`.
-#[derive(Clone, Encodable, Decodable, Copy, PartialEq, Eq)]
+#[derive(Clone, Encodable, Decodable, Copy, PartialEq, Eq, Hash)]
 pub struct Lifetime {
     pub id: NodeId,
     pub ident: Ident,
@@ -2132,7 +2132,7 @@ pub enum TyKind {
     /// The `NodeId` exists to prevent lowering from having to
     /// generate `NodeId`s on the fly, which would complicate
     /// the generation of opaque `type Foo = impl Trait` items significantly.
-    ImplTrait(NodeId, GenericBounds),
+    ImplTrait(NodeId, GenericBounds, Option<P<(ThinVec<PreciseCapturingArg>, Span)>>),
     /// No-op; kept solely so that we can pretty-print faithfully.
     Paren(P<Ty>),
     /// Unused for now.
@@ -2188,6 +2188,14 @@ pub enum TraitObjectSyntax {
     None,
 }
 
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum PreciseCapturingArg {
+    /// Lifetime parameter
+    Lifetime(Lifetime),
+    /// Type or const parameter
+    Arg(Path, NodeId),
+}
+
 /// Inline assembly operand explicit register or register class.
 ///
 /// E.g., `"eax"` as in `asm!("mov eax, 2", out("eax") result)`.

compiler/rustc_ast/src/mut_visit.rs

@@ -259,6 +259,10 @@ pub trait MutVisitor: Sized {
         noop_visit_param_bound(tpb, self);
     }
 
+    fn visit_precise_capturing_arg(&mut self, arg: &mut PreciseCapturingArg) {
+        noop_visit_precise_capturing_arg(arg, self);
+    }
+
     fn visit_mt(&mut self, mt: &mut MutTy) {
         noop_visit_mt(mt, self);
     }
@@ -518,9 +522,14 @@ pub fn noop_visit_ty<T: MutVisitor>(ty: &mut P<Ty>, vis: &mut T) {
         TyKind::TraitObject(bounds, _syntax) => {
             visit_vec(bounds, |bound| vis.visit_param_bound(bound))
         }
-        TyKind::ImplTrait(id, bounds) => {
+        TyKind::ImplTrait(id, bounds, precise_capturing) => {
             vis.visit_id(id);
             visit_vec(bounds, |bound| vis.visit_param_bound(bound));
+            if let Some((precise_capturing, _span)) = precise_capturing.as_deref_mut() {
+                for arg in precise_capturing {
+                    vis.visit_precise_capturing_arg(arg);
+                }
+            }
         }
         TyKind::MacCall(mac) => vis.visit_mac_call(mac),
         TyKind::AnonStruct(id, fields) | TyKind::AnonUnion(id, fields) => {
@@ -914,6 +923,18 @@ pub fn noop_visit_param_bound<T: MutVisitor>(pb: &mut GenericBound, vis: &mut T)
     }
 }
 
+pub fn noop_visit_precise_capturing_arg<T: MutVisitor>(arg: &mut PreciseCapturingArg, vis: &mut T) {
+    match arg {
+        PreciseCapturingArg::Lifetime(lt) => {
+            vis.visit_lifetime(lt);
+        }
+        PreciseCapturingArg::Arg(path, id) => {
+            vis.visit_path(path);
+            vis.visit_id(id);
+        }
+    }
+}
+
 pub fn noop_flat_map_generic_param<T: MutVisitor>(
     mut param: GenericParam,
     vis: &mut T,

compiler/rustc_ast/src/visit.rs

@@ -184,6 +184,9 @@ pub trait Visitor<'ast>: Sized {
     fn visit_param_bound(&mut self, bounds: &'ast GenericBound, _ctxt: BoundKind) -> Self::Result {
         walk_param_bound(self, bounds)
     }
+    fn visit_precise_capturing_arg(&mut self, arg: &'ast PreciseCapturingArg) {
+        walk_precise_capturing_arg(self, arg);
+    }
     fn visit_poly_trait_ref(&mut self, t: &'ast PolyTraitRef) -> Self::Result {
         walk_poly_trait_ref(self, t)
     }
@@ -457,8 +460,13 @@ pub fn walk_ty<'a, V: Visitor<'a>>(visitor: &mut V, typ: &'a Ty) -> V::Result {
         TyKind::TraitObject(bounds, ..) => {
             walk_list!(visitor, visit_param_bound, bounds, BoundKind::TraitObject);
         }
-        TyKind::ImplTrait(_, bounds) => {
+        TyKind::ImplTrait(_, bounds, precise_capturing) => {
             walk_list!(visitor, visit_param_bound, bounds, BoundKind::Impl);
+            if let Some((precise_capturing, _span)) = precise_capturing.as_deref() {
+                for arg in precise_capturing {
+                    try_visit!(visitor.visit_precise_capturing_arg(arg));
+                }
+            }
         }
         TyKind::Typeof(expression) => try_visit!(visitor.visit_anon_const(expression)),
         TyKind::Infer | TyKind::ImplicitSelf | TyKind::Dummy | TyKind::Err(_) => {}
@@ -637,6 +645,20 @@ pub fn walk_param_bound<'a, V: Visitor<'a>>(visitor: &mut V, bound: &'a GenericB
     }
 }
 
+pub fn walk_precise_capturing_arg<'a, V: Visitor<'a>>(
+    visitor: &mut V,
+    arg: &'a PreciseCapturingArg,
+) {
+    match arg {
+        PreciseCapturingArg::Lifetime(lt) => {
+            visitor.visit_lifetime(lt, LifetimeCtxt::GenericArg);
+        }
+        PreciseCapturingArg::Arg(path, id) => {
+            visitor.visit_path(path, *id);
+        }
+    }
+}
+
 pub fn walk_generic_param<'a, V: Visitor<'a>>(
     visitor: &mut V,
     param: &'a GenericParam,

compiler/rustc_ast_lowering/messages.ftl

@@ -127,6 +127,8 @@ ast_lowering_never_pattern_with_guard =
     a guard on a never pattern will never be run
     .suggestion = remove this guard
 
+ast_lowering_no_precise_captures_on_apit = `use<...>` precise capturing syntax not allowed on argument-position `impl Trait`
+
 ast_lowering_previously_used_here = previously used here
 
 ast_lowering_register1 = register `{$reg1_name}`

compiler/rustc_ast_lowering/src/errors.rs

@@ -414,3 +414,10 @@ pub(crate) struct AsyncBoundOnlyForFnTraits {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_lowering_no_precise_captures_on_apit)]
+pub(crate) struct NoPreciseCapturesOnApit {
+    #[primary_span]
+    pub span: Span,
+}

compiler/rustc_ast_lowering/src/index.rs

@@ -385,4 +385,21 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
     fn visit_pattern_type_pattern(&mut self, p: &'hir hir::Pat<'hir>) {
         self.visit_pat(p)
     }
+
+    fn visit_precise_capturing_arg(
+        &mut self,
+        arg: &'hir PreciseCapturingArg<'hir>,
+    ) -> Self::Result {
+        match arg {
+            PreciseCapturingArg::Lifetime(_) => {
+                // This is represented as a `Node::Lifetime`, intravisit will get to it below.
+            }
+            PreciseCapturingArg::Param(param) => self.insert(
+                param.ident.span,
+                param.hir_id,
+                Node::PreciseCapturingNonLifetimeArg(param),
+            ),
+        }
+        intravisit::walk_precise_capturing_arg(self, arg);
+    }
 }

compiler/rustc_ast_lowering/src/lib.rs

@@ -48,6 +48,7 @@ use rustc_ast::{self as ast, *};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fingerprint::Fingerprint;
+use rustc_data_structures::fx::FxIndexSet;
 use rustc_data_structures::sorted_map::SortedMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync::Lrc;
@@ -1398,7 +1399,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 });
                 hir::TyKind::TraitObject(bounds, lifetime_bound, *kind)
             }
-            TyKind::ImplTrait(def_node_id, bounds) => {
+            TyKind::ImplTrait(def_node_id, bounds, precise_capturing) => {
                 let span = t.span;
                 match itctx {
                     ImplTraitContext::OpaqueTy { origin, fn_kind } => self.lower_opaque_impl_trait(
@@ -1408,8 +1409,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         bounds,
                         fn_kind,
                         itctx,
+                        precise_capturing.as_deref().map(|(args, _)| args.as_slice()),
                     ),
                     ImplTraitContext::Universal => {
+                        if let Some(&(_, span)) = precise_capturing.as_deref() {
+                            self.tcx.dcx().emit_err(errors::NoPreciseCapturesOnApit { span });
+                        };
                         let span = t.span;
 
                         // HACK: pprust breaks strings with newlines when the type
@@ -1520,6 +1525,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         bounds: &GenericBounds,
         fn_kind: Option<FnDeclKind>,
         itctx: ImplTraitContext,
+        precise_capturing_args: Option<&[PreciseCapturingArg]>,
     ) -> hir::TyKind<'hir> {
         // Make sure we know that some funky desugaring has been going on here.
         // This is a first: there is code in other places like for loop
@@ -1528,7 +1534,23 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         // frequently opened issues show.
         let opaque_ty_span = self.mark_span_with_reason(DesugaringKind::OpaqueTy, span, None);
 
-        let captured_lifetimes_to_duplicate = match origin {
+        let captured_lifetimes_to_duplicate =
+            if let Some(precise_capturing) = precise_capturing_args {
+                // We'll actually validate these later on; all we need is the list of
+                // lifetimes to duplicate during this portion of lowering.
+                precise_capturing
+                    .iter()
+                    .filter_map(|arg| match arg {
+                        PreciseCapturingArg::Lifetime(lt) => Some(*lt),
+                        PreciseCapturingArg::Arg(..) => None,
+                    })
+                    // Add in all the lifetimes mentioned in the bounds. We will error
+                    // them out later, but capturing them here is important to make sure
+                    // they actually get resolved in resolve_bound_vars.
+                    .chain(lifetime_collector::lifetimes_in_bounds(self.resolver, bounds))
+                    .collect()
+            } else {
+                match origin {
                     hir::OpaqueTyOrigin::TyAlias { .. } => {
                         // type alias impl trait and associated type position impl trait were
                         // decided to capture all in-scope lifetimes, which we collect for
@@ -1563,6 +1585,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     hir::OpaqueTyOrigin::AsyncFn(..) => {
                         unreachable!("should be using `lower_async_fn_ret_ty`")
                     }
+                }
             };
         debug!(?captured_lifetimes_to_duplicate);
 
@@ -1573,6 +1596,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             captured_lifetimes_to_duplicate,
             span,
             opaque_ty_span,
+            precise_capturing_args,
             |this| this.lower_param_bounds(bounds, itctx),
         )
     }
@@ -1582,9 +1606,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         opaque_ty_node_id: NodeId,
         origin: hir::OpaqueTyOrigin,
         in_trait: bool,
-        captured_lifetimes_to_duplicate: Vec<Lifetime>,
+        captured_lifetimes_to_duplicate: FxIndexSet<Lifetime>,
         span: Span,
         opaque_ty_span: Span,
+        precise_capturing_args: Option<&[PreciseCapturingArg]>,
         lower_item_bounds: impl FnOnce(&mut Self) -> &'hir [hir::GenericBound<'hir>],
     ) -> hir::TyKind<'hir> {
         let opaque_ty_def_id = self.create_def(
@@ -1671,8 +1696,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             // Install the remapping from old to new (if any). This makes sure that
             // any lifetimes that would have resolved to the def-id of captured
             // lifetimes are remapped to the new *synthetic* lifetimes of the opaque.
-            let bounds = this
+            let (bounds, precise_capturing_args) =
-                .with_remapping(captured_to_synthesized_mapping, |this| lower_item_bounds(this));
+                this.with_remapping(captured_to_synthesized_mapping, |this| {
+                    (
+                        lower_item_bounds(this),
+                        precise_capturing_args.map(|precise_capturing| {
+                            this.lower_precise_capturing_args(precise_capturing)
+                        }),
+                    )
+                });
 
             let generic_params =
                 this.arena.alloc_from_iter(synthesized_lifetime_definitions.iter().map(
@@ -1717,6 +1749,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 origin,
                 lifetime_mapping,
                 in_trait,
+                precise_capturing_args,
             };
 
             // Generate an `type Foo = impl Trait;` declaration.
@@ -1749,6 +1782,30 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         )
     }
 
+    fn lower_precise_capturing_args(
+        &mut self,
+        precise_capturing_args: &[PreciseCapturingArg],
+    ) -> &'hir [hir::PreciseCapturingArg<'hir>] {
+        self.arena.alloc_from_iter(precise_capturing_args.iter().map(|arg| match arg {
+            PreciseCapturingArg::Lifetime(lt) => {
+                hir::PreciseCapturingArg::Lifetime(self.lower_lifetime(lt))
+            }
+            PreciseCapturingArg::Arg(path, id) => {
+                let [segment] = path.segments.as_slice() else {
+                    panic!();
+                };
+                let res = self.resolver.get_partial_res(*id).map_or(Res::Err, |partial_res| {
+                    partial_res.full_res().expect("no partial res expected for precise capture arg")
+                });
+                hir::PreciseCapturingArg::Param(hir::PreciseCapturingNonLifetimeArg {
+                    hir_id: self.lower_node_id(*id),
+                    ident: self.lower_ident(segment.ident),
+                    res: self.lower_res(res),
+                })
+            }
+        }))
+    }
+
     fn lower_fn_params_to_names(&mut self, decl: &FnDecl) -> &'hir [Ident] {
         self.arena.alloc_from_iter(decl.inputs.iter().map(|param| match param.pat.kind {
             PatKind::Ident(_, ident, _) => self.lower_ident(ident),
@@ -1889,7 +1946,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let opaque_ty_span =
             self.mark_span_with_reason(DesugaringKind::Async, span, allowed_features);
 
-        let captured_lifetimes: Vec<_> = self
+        let captured_lifetimes = self
             .resolver
             .take_extra_lifetime_params(opaque_ty_node_id)
             .into_iter()
@@ -1903,6 +1960,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             captured_lifetimes,
             span,
             opaque_ty_span,
+            None,
             |this| {
                 let bound = this.lower_coroutine_fn_output_type_to_bound(
                     output,

compiler/rustc_ast_lowering/src/lifetime_collector.rs

@@ -1,6 +1,7 @@
 use super::ResolverAstLoweringExt;
 use rustc_ast::visit::{self, BoundKind, LifetimeCtxt, Visitor};
 use rustc_ast::{GenericBounds, Lifetime, NodeId, PathSegment, PolyTraitRef, Ty, TyKind};
+use rustc_data_structures::fx::FxIndexSet;
 use rustc_hir::def::{DefKind, LifetimeRes, Res};
 use rustc_middle::span_bug;
 use rustc_middle::ty::ResolverAstLowering;
@@ -10,27 +11,23 @@ use rustc_span::Span;
 struct LifetimeCollectVisitor<'ast> {
     resolver: &'ast ResolverAstLowering,
     current_binders: Vec<NodeId>,
-    collected_lifetimes: Vec<Lifetime>,
+    collected_lifetimes: FxIndexSet<Lifetime>,
 }
 
 impl<'ast> LifetimeCollectVisitor<'ast> {
     fn new(resolver: &'ast ResolverAstLowering) -> Self {
-        Self { resolver, current_binders: Vec::new(), collected_lifetimes: Vec::new() }
+        Self { resolver, current_binders: Vec::new(), collected_lifetimes: FxIndexSet::default() }
     }
 
     fn record_lifetime_use(&mut self, lifetime: Lifetime) {
         match self.resolver.get_lifetime_res(lifetime.id).unwrap_or(LifetimeRes::Error) {
             LifetimeRes::Param { binder, .. } | LifetimeRes::Fresh { binder, .. } => {
                 if !self.current_binders.contains(&binder) {
-                    if !self.collected_lifetimes.contains(&lifetime) {
+                    self.collected_lifetimes.insert(lifetime);
-                        self.collected_lifetimes.push(lifetime);
-                    }
                 }
             }
             LifetimeRes::Static | LifetimeRes::Error => {
-                if !self.collected_lifetimes.contains(&lifetime) {
+                self.collected_lifetimes.insert(lifetime);
-                    self.collected_lifetimes.push(lifetime);
-                }
             }
             LifetimeRes::Infer => {}
             res => {
@@ -111,7 +108,7 @@ impl<'ast> Visitor<'ast> for LifetimeCollectVisitor<'ast> {
 pub(crate) fn lifetimes_in_bounds(
     resolver: &ResolverAstLowering,
     bounds: &GenericBounds,
-) -> Vec<Lifetime> {
+) -> FxIndexSet<Lifetime> {
     let mut visitor = LifetimeCollectVisitor::new(resolver);
     for bound in bounds {
         visitor.visit_param_bound(bound, BoundKind::Bound);

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -737,7 +737,7 @@ impl<'a> AstValidator<'a> {
                     }
                 }
             }
-            TyKind::ImplTrait(_, bounds) => {
+            TyKind::ImplTrait(_, bounds, _) => {
                 if self.is_impl_trait_banned {
                     self.dcx().emit_err(errors::ImplTraitPath { span: ty.span });
                 }

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -569,6 +569,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(fn_delegation, "functions delegation is not yet fully implemented");
     gate_all!(postfix_match, "postfix match is experimental");
     gate_all!(mut_ref, "mutable by-reference bindings are experimental");
+    gate_all!(precise_capturing, "precise captures on `impl Trait` are experimental");
 
     if !visitor.features.never_patterns {
         if let Some(spans) = spans.get(&sym::never_patterns) {

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1150,8 +1150,17 @@ impl<'a> State<'a> {
                 }
                 self.print_type_bounds(bounds);
             }
-            ast::TyKind::ImplTrait(_, bounds) => {
+            ast::TyKind::ImplTrait(_, bounds, precise_capturing_args) => {
                 self.word_nbsp("impl");
+                if let Some((precise_capturing_args, ..)) = precise_capturing_args.as_deref() {
+                    self.word("use");
+                    self.word("<");
+                    self.commasep(Inconsistent, precise_capturing_args, |s, arg| match arg {
+                        ast::PreciseCapturingArg::Arg(p, _) => s.print_path(p, false, 0),
+                        ast::PreciseCapturingArg::Lifetime(lt) => s.print_lifetime(*lt),
+                    });
+                    self.word(">")
+                }
                 self.print_type_bounds(bounds);
             }
             ast::TyKind::Array(ty, length) => {

compiler/rustc_feature/src/unstable.rs

@@ -567,6 +567,8 @@ declare_features! (
     (unstable, optimize_attribute, "1.34.0", Some(54882)),
     /// Allows postfix match `expr.match { ... }`
     (unstable, postfix_match, "CURRENT_RUSTC_VERSION", Some(121618)),
+    /// Allows `use<'a, 'b, A, B>` in `impl use<...> Trait` for precise capture of generic args.
+    (incomplete, precise_capturing, "CURRENT_RUSTC_VERSION", Some(123432)),
     /// Allows macro attributes on expressions, statements and non-inline modules.
     (unstable, proc_macro_hygiene, "1.30.0", Some(54727)),
     /// Allows `&raw const $place_expr` and `&raw mut $place_expr` expressions.

compiler/rustc_hir/src/hir.rs

@@ -2557,6 +2557,27 @@ pub struct OpaqueTy<'hir> {
     /// originating from a trait method. This makes it so that the opaque is
     /// lowered as an associated type.
     pub in_trait: bool,
+    /// List of arguments captured via `impl use<'a, P, ...> Trait` syntax.
+    pub precise_capturing_args: Option<&'hir [PreciseCapturingArg<'hir>]>,
+}
+
+#[derive(Debug, Clone, Copy, HashStable_Generic)]
+pub enum PreciseCapturingArg<'hir> {
+    Lifetime(&'hir Lifetime),
+    /// Non-lifetime argument (type or const)
+    Param(PreciseCapturingNonLifetimeArg),
+}
+
+/// We need to have a [`Node`] for the [`HirId`] that we attach the type/const param
+/// resolution to. Lifetimes don't have this problem, and for them, it's actually
+/// kind of detrimental to use a custom node type versus just using [`Lifetime`],
+/// since resolve_bound_vars operates on `Lifetime`s.
+// FIXME(precise_capturing): Investigate storing this as a path instead?
+#[derive(Debug, Clone, Copy, HashStable_Generic)]
+pub struct PreciseCapturingNonLifetimeArg {
+    pub hir_id: HirId,
+    pub ident: Ident,
+    pub res: Res,
 }
 
 /// From whence the opaque type came.
@@ -3535,6 +3556,7 @@ pub enum Node<'hir> {
     WhereBoundPredicate(&'hir WhereBoundPredicate<'hir>),
     // FIXME: Merge into `Node::Infer`.
     ArrayLenInfer(&'hir InferArg),
+    PreciseCapturingNonLifetimeArg(&'hir PreciseCapturingNonLifetimeArg),
     // Created by query feeding
     Synthetic,
     // Span by reference to minimize `Node`'s size
@@ -3571,6 +3593,7 @@ impl<'hir> Node<'hir> {
             Node::TypeBinding(b) => Some(b.ident),
             Node::PatField(f) => Some(f.ident),
             Node::ExprField(f) => Some(f.ident),
+            Node::PreciseCapturingNonLifetimeArg(a) => Some(a.ident),
             Node::Param(..)
             | Node::AnonConst(..)
             | Node::ConstBlock(..)

compiler/rustc_hir/src/intravisit.rs

@@ -413,6 +413,9 @@ pub trait Visitor<'v>: Sized {
     fn visit_param_bound(&mut self, bounds: &'v GenericBound<'v>) -> Self::Result {
         walk_param_bound(self, bounds)
     }
+    fn visit_precise_capturing_arg(&mut self, arg: &'v PreciseCapturingArg<'v>) -> Self::Result {
+        walk_precise_capturing_arg(self, arg)
+    }
     fn visit_poly_trait_ref(&mut self, t: &'v PolyTraitRef<'v>) -> Self::Result {
         walk_poly_trait_ref(self, t)
     }
@@ -526,10 +529,15 @@ pub fn walk_item<'v, V: Visitor<'v>>(visitor: &mut V, item: &'v Item<'v>) -> V::
             try_visit!(visitor.visit_ty(ty));
             try_visit!(visitor.visit_generics(generics));
         }
-        ItemKind::OpaqueTy(&OpaqueTy { generics, bounds, .. }) => {
+        ItemKind::OpaqueTy(&OpaqueTy { generics, bounds, precise_capturing_args, .. }) => {
             try_visit!(visitor.visit_id(item.hir_id()));
             try_visit!(walk_generics(visitor, generics));
             walk_list!(visitor, visit_param_bound, bounds);
+            if let Some(precise_capturing_args) = precise_capturing_args {
+                for arg in precise_capturing_args {
+                    try_visit!(visitor.visit_precise_capturing_arg(arg));
+                }
+            }
         }
         ItemKind::Enum(ref enum_definition, ref generics) => {
             try_visit!(visitor.visit_generics(generics));
@@ -1137,6 +1145,16 @@ pub fn walk_param_bound<'v, V: Visitor<'v>>(
     }
 }
 
+pub fn walk_precise_capturing_arg<'v, V: Visitor<'v>>(
+    visitor: &mut V,
+    arg: &'v PreciseCapturingArg<'v>,
+) -> V::Result {
+    match *arg {
+        PreciseCapturingArg::Lifetime(lt) => visitor.visit_lifetime(lt),
+        PreciseCapturingArg::Param(param) => visitor.visit_id(param.hir_id),
+    }
+}
+
 pub fn walk_poly_trait_ref<'v, V: Visitor<'v>>(
     visitor: &mut V,
     trait_ref: &'v PolyTraitRef<'v>,

compiler/rustc_hir_analysis/messages.ftl

@@ -37,6 +37,8 @@ hir_analysis_auto_deref_reached_recursion_limit = reached the recursion limit wh
     .label = deref recursion limit reached
     .help = consider increasing the recursion limit by adding a `#![recursion_limit = "{$suggested_limit}"]` attribute to your crate (`{$crate_name}`)
 
+hir_analysis_bad_precise_capture = expected {$kind} parameter in `use<...>` precise captures list, found {$found}
+
 hir_analysis_cannot_capture_late_bound_const =
     cannot capture late-bound const parameter in {$what}
     .label = parameter defined here
@@ -111,6 +113,9 @@ hir_analysis_drop_impl_on_wrong_item =
 
 hir_analysis_drop_impl_reservation = reservation `Drop` impls are not supported
 
+hir_analysis_duplicate_precise_capture = cannot capture parameter `{$name}` twice
+    .label = parameter captured again here
+
 hir_analysis_empty_specialization = specialization impl does not specialize any associated items
     .note = impl is a specialization of this impl
 
@@ -214,6 +219,13 @@ 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_must_be_first = lifetime parameter `{$name}` must be listed before non-lifetime parameters
+    .label = move the lifetime before this parameter
+
+hir_analysis_lifetime_not_captured = `impl Trait` captures lifetime parameter, but it is not mentioned in `use<...>` precise captures list
+    .label = lifetime captured due to being mentioned in the bounds of the `impl Trait`
+    .param_label = this lifetime parameter is captured
+
 hir_analysis_lifetimes_or_bounds_mismatch_on_trait =
     lifetime parameters or bounds on {$item_kind} `{$ident}` do not match the trait declaration
     .label = lifetimes do not match {$item_kind} in trait
@@ -339,6 +351,10 @@ hir_analysis_param_in_ty_of_assoc_const_binding =
         *[normal] the {$param_def_kind} `{$param_name}` is defined here
     }
 
+hir_analysis_param_not_captured = `impl Trait` must mention all {$kind} parameters in scope
+    .label = {$kind} parameter is implicitly captured by this `impl Trait`
+    .note = currently, all {$kind} parameters are required to be mentioned in the precise captures list
+
 hir_analysis_paren_sugar_attribute = the `#[rustc_paren_sugar]` attribute is a temporary means of controlling which traits can use parenthetical notation
     .help = add `#![feature(unboxed_closures)]` to the crate attributes to use it
 
@@ -355,6 +371,9 @@ hir_analysis_pattern_type_wild_pat = "wildcard patterns are not permitted for pa
 hir_analysis_placeholder_not_allowed_item_signatures = the placeholder `_` is not allowed within types on item signatures for {$kind}
     .label = not allowed in type signatures
 
+hir_analysis_precise_capture_self_alias = `Self` can't be captured in `use<...>` precise captures list, since it is an alias
+    .label = `Self` is not a generic argument, but an alias to the type of the {$what}
+
 hir_analysis_redundant_lifetime_args = unnecessary lifetime parameter `{$victim}`
     .note = you can use the `{$candidate}` lifetime directly, in place of `{$victim}`
 

compiler/rustc_hir_analysis/src/check/check.rs

@@ -1,10 +1,10 @@
 use crate::check::intrinsicck::InlineAsmCtxt;
-use crate::errors::LinkageType;
 
 use super::compare_impl_item::check_type_bounds;
 use super::compare_impl_item::{compare_impl_method, compare_impl_ty};
 use super::*;
 use rustc_attr as attr;
+use rustc_data_structures::unord::{UnordMap, UnordSet};
 use rustc_errors::{codes::*, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, DefKind};
@@ -12,6 +12,7 @@ use rustc_hir::Node;
 use rustc_infer::infer::{RegionVariableOrigin, TyCtxtInferExt};
 use rustc_infer::traits::{Obligation, TraitEngineExt as _};
 use rustc_lint_defs::builtin::REPR_TRANSPARENT_EXTERNAL_PRIVATE_FIELDS;
+use rustc_middle::middle::resolve_bound_vars::ResolvedArg;
 use rustc_middle::middle::stability::EvalResult;
 use rustc_middle::traits::ObligationCauseCode;
 use rustc_middle::ty::fold::BottomUpFolder;
@@ -474,6 +475,133 @@ fn sanity_check_found_hidden_type<'tcx>(
     }
 }
 
+/// Check that the opaque's precise captures list is valid (if present).
+/// We check this for regular `impl Trait`s and also RPITITs, even though the latter
+/// are technically GATs.
+///
+/// This function is responsible for:
+/// 1. Checking that all type/const params are mention in the captures list.
+/// 2. Checking that all lifetimes that are implicitly captured are mentioned.
+/// 3. Asserting that all parameters mentioned in the captures list are invariant.
+fn check_opaque_precise_captures<'tcx>(tcx: TyCtxt<'tcx>, opaque_def_id: LocalDefId) {
+    let hir::OpaqueTy { precise_capturing_args, .. } =
+        *tcx.hir_node_by_def_id(opaque_def_id).expect_item().expect_opaque_ty();
+    let Some(precise_capturing_args) = precise_capturing_args else {
+        // No precise capturing args; nothing to validate
+        return;
+    };
+
+    let mut expected_captures = UnordSet::default();
+    let mut seen_params = UnordMap::default();
+    let mut prev_non_lifetime_param = None;
+    for arg in precise_capturing_args {
+        let (hir_id, ident) = match *arg {
+            hir::PreciseCapturingArg::Param(hir::PreciseCapturingNonLifetimeArg {
+                hir_id,
+                ident,
+                ..
+            }) => {
+                if prev_non_lifetime_param.is_none() {
+                    prev_non_lifetime_param = Some(ident);
+                }
+                (hir_id, ident)
+            }
+            hir::PreciseCapturingArg::Lifetime(&hir::Lifetime { hir_id, ident, .. }) => {
+                if let Some(prev_non_lifetime_param) = prev_non_lifetime_param {
+                    tcx.dcx().emit_err(errors::LifetimesMustBeFirst {
+                        lifetime_span: ident.span,
+                        name: ident.name,
+                        other_span: prev_non_lifetime_param.span,
+                    });
+                }
+                (hir_id, ident)
+            }
+        };
+
+        let ident = ident.normalize_to_macros_2_0();
+        if let Some(span) = seen_params.insert(ident, ident.span) {
+            tcx.dcx().emit_err(errors::DuplicatePreciseCapture {
+                name: ident.name,
+                first_span: span,
+                second_span: ident.span,
+            });
+        }
+
+        match tcx.named_bound_var(hir_id) {
+            Some(ResolvedArg::EarlyBound(def_id)) => {
+                expected_captures.insert(def_id);
+            }
+            _ => {
+                tcx.dcx().span_delayed_bug(
+                    tcx.hir().span(hir_id),
+                    "parameter should have been resolved",
+                );
+            }
+        }
+    }
+
+    let variances = tcx.variances_of(opaque_def_id);
+    let mut def_id = Some(opaque_def_id.to_def_id());
+    while let Some(generics) = def_id {
+        let generics = tcx.generics_of(generics);
+        def_id = generics.parent;
+
+        for param in &generics.params {
+            if expected_captures.contains(&param.def_id) {
+                assert_eq!(
+                    variances[param.index as usize],
+                    ty::Invariant,
+                    "precise captured param should be invariant"
+                );
+                continue;
+            }
+
+            match param.kind {
+                ty::GenericParamDefKind::Lifetime => {
+                    // Check if the lifetime param was captured but isn't named in the precise captures list.
+                    if variances[param.index as usize] == ty::Invariant {
+                        let param_span =
+                            if let ty::ReEarlyParam(ty::EarlyParamRegion { def_id, .. })
+                            | ty::ReLateParam(ty::LateParamRegion {
+                                bound_region: ty::BoundRegionKind::BrNamed(def_id, _),
+                                ..
+                            }) = *tcx
+                                .map_opaque_lifetime_to_parent_lifetime(param.def_id.expect_local())
+                            {
+                                Some(tcx.def_span(def_id))
+                            } else {
+                                None
+                            };
+                        // FIXME(precise_capturing): Structured suggestion for this would be useful
+                        tcx.dcx().emit_err(errors::LifetimeNotCaptured {
+                            use_span: tcx.def_span(param.def_id),
+                            param_span,
+                            opaque_span: tcx.def_span(opaque_def_id),
+                        });
+                        continue;
+                    }
+                }
+                ty::GenericParamDefKind::Type { .. } => {
+                    // FIXME(precise_capturing): Structured suggestion for this would be useful
+                    tcx.dcx().emit_err(errors::ParamNotCaptured {
+                        param_span: tcx.def_span(param.def_id),
+                        opaque_span: tcx.def_span(opaque_def_id),
+                        kind: "type",
+                    });
+                }
+                ty::GenericParamDefKind::Const { .. } => {
+                    // FIXME(precise_capturing): Structured suggestion for this would be useful
+                    tcx.dcx().emit_err(errors::ParamNotCaptured {
+                        param_span: tcx.def_span(param.def_id),
+                        opaque_span: tcx.def_span(opaque_def_id),
+                        kind: "const",
+                    });
+                }
+            }
+        }
+    }
+}
+
 fn is_enum_of_nonnullable_ptr<'tcx>(
     tcx: TyCtxt<'tcx>,
     adt_def: AdtDef<'tcx>,
@@ -499,7 +627,7 @@ fn check_static_linkage(tcx: TyCtxt<'_>, def_id: LocalDefId) {
             ty::Adt(adt_def, args) => !is_enum_of_nonnullable_ptr(tcx, *adt_def, *args),
             _ => true,
         } {
-            tcx.dcx().emit_err(LinkageType { span: tcx.def_span(def_id) });
+            tcx.dcx().emit_err(errors::LinkageType { span: tcx.def_span(def_id) });
         }
     }
 }
@@ -566,6 +694,8 @@ pub(crate) fn check_item_type(tcx: TyCtxt<'_>, def_id: LocalDefId) {
             check_union(tcx, def_id);
         }
         DefKind::OpaqueTy => {
+            check_opaque_precise_captures(tcx, def_id);
+
             let origin = tcx.opaque_type_origin(def_id);
             if let hir::OpaqueTyOrigin::FnReturn(fn_def_id)
             | hir::OpaqueTyOrigin::AsyncFn(fn_def_id) = origin

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -557,6 +557,50 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
         }
     }
 
+    fn visit_precise_capturing_arg(
+        &mut self,
+        arg: &'tcx hir::PreciseCapturingArg<'tcx>,
+    ) -> Self::Result {
+        match *arg {
+            hir::PreciseCapturingArg::Lifetime(lt) => match lt.res {
+                LifetimeName::Param(def_id) => {
+                    self.resolve_lifetime_ref(def_id, lt);
+                }
+                LifetimeName::Error => {}
+                LifetimeName::ImplicitObjectLifetimeDefault
+                | LifetimeName::Infer
+                | LifetimeName::Static => {
+                    self.tcx.dcx().emit_err(errors::BadPreciseCapture {
+                        span: lt.ident.span,
+                        kind: "lifetime",
+                        found: format!("`{}`", lt.ident.name),
+                    });
+                }
+            },
+            hir::PreciseCapturingArg::Param(param) => match param.res {
+                Res::Def(DefKind::TyParam | DefKind::ConstParam, def_id)
+                | Res::SelfTyParam { trait_: def_id } => {
+                    self.resolve_type_ref(def_id.expect_local(), param.hir_id);
+                }
+                Res::Err => {}
+                Res::SelfTyAlias { alias_to, .. } => {
+                    self.tcx.dcx().emit_err(errors::PreciseCaptureSelfAlias {
+                        span: param.ident.span,
+                        self_span: self.tcx.def_span(alias_to),
+                        what: self.tcx.def_descr(alias_to),
+                    });
+                }
+                res => {
+                    self.tcx.dcx().emit_err(errors::BadPreciseCapture {
+                        span: param.ident.span,
+                        kind: "type or const",
+                        found: res.descr().to_string(),
+                    });
+                }
+            },
+        }
+    }
+
     fn visit_foreign_item(&mut self, item: &'tcx hir::ForeignItem<'tcx>) {
         match item.kind {
             hir::ForeignItemKind::Fn(_, _, generics) => {

compiler/rustc_hir_analysis/src/errors.rs

@@ -10,6 +10,9 @@ use rustc_span::{symbol::Ident, Span, Symbol};
 mod pattern_types;
 pub use pattern_types::*;
 
+mod precise_captures;
+pub(crate) use precise_captures::*;
+
 #[derive(Diagnostic)]
 #[diag(hir_analysis_ambiguous_assoc_item)]
 pub struct AmbiguousAssocItem<'a> {

compiler/rustc_hir_analysis/src/errors/precise_captures.rs

@@ -0,0 +1,63 @@
+use rustc_macros::Diagnostic;
+use rustc_span::{Span, Symbol};
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_param_not_captured)]
+#[note]
+pub struct ParamNotCaptured {
+    #[primary_span]
+    pub param_span: Span,
+    #[label]
+    pub opaque_span: Span,
+    pub kind: &'static str,
+}
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_lifetime_not_captured)]
+pub struct LifetimeNotCaptured {
+    #[primary_span]
+    pub use_span: Span,
+    #[label(hir_analysis_param_label)]
+    pub param_span: Option<Span>,
+    #[label]
+    pub opaque_span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_bad_precise_capture)]
+pub struct BadPreciseCapture {
+    #[primary_span]
+    pub span: Span,
+    pub kind: &'static str,
+    pub found: String,
+}
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_precise_capture_self_alias)]
+pub struct PreciseCaptureSelfAlias {
+    #[primary_span]
+    pub span: Span,
+    #[label]
+    pub self_span: Span,
+    pub what: &'static str,
+}
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_duplicate_precise_capture)]
+pub struct DuplicatePreciseCapture {
+    #[primary_span]
+    pub first_span: Span,
+    pub name: Symbol,
+    #[label]
+    pub second_span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_lifetime_must_be_first)]
+pub struct LifetimesMustBeFirst {
+    #[primary_span]
+    pub lifetime_span: Span,
+    pub name: Symbol,
+    #[label]
+    pub other_span: Span,
+}

compiler/rustc_hir_pretty/src/lib.rs

@@ -95,6 +95,7 @@ impl<'a> State<'a> {
             Node::PatField(a) => self.print_patfield(a),
             Node::Arm(a) => self.print_arm(a),
             Node::Infer(_) => self.word("_"),
+            Node::PreciseCapturingNonLifetimeArg(param) => self.print_ident(param.ident),
             Node::Block(a) => {
                 // Containing cbox, will be closed by print-block at `}`.
                 self.cbox(INDENT_UNIT);

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs

@@ -283,6 +283,7 @@ pub fn suggest_new_region_bound(
             continue;
         }
         match fn_return.kind {
+            // FIXME(precise_captures): Suggest adding to `use<...>` list instead.
             TyKind::OpaqueDef(item_id, _, _) => {
                 let item = tcx.hir().item(item_id);
                 let ItemKind::OpaqueTy(opaque) = &item.kind else {

compiler/rustc_lint/src/unused.rs

@@ -1235,7 +1235,7 @@ impl EarlyLintPass for UnusedParens {
                     ast::TyKind::TraitObject(..) => {}
                     ast::TyKind::BareFn(b)
                         if self.with_self_ty_parens && b.generic_params.len() > 0 => {}
-                    ast::TyKind::ImplTrait(_, bounds) if bounds.len() > 1 => {}
+                    ast::TyKind::ImplTrait(_, bounds, _) if bounds.len() > 1 => {}
                     _ => {
                         let spans = if !ty.span.from_expansion() {
                             r.span

compiler/rustc_middle/src/hir/map/mod.rs

@@ -910,6 +910,7 @@ impl<'hir> Map<'hir> {
             Node::Crate(item) => item.spans.inner_span,
             Node::WhereBoundPredicate(pred) => pred.span,
             Node::ArrayLenInfer(inf) => inf.span,
+            Node::PreciseCapturingNonLifetimeArg(param) => param.ident.span,
             Node::Synthetic => unreachable!(),
             Node::Err(span) => *span,
         }
@@ -1183,6 +1184,7 @@ fn hir_id_to_string(map: Map<'_>, id: HirId) -> String {
         Node::ArrayLenInfer(_) => node_str("array len infer"),
         Node::Synthetic => unreachable!(),
         Node::Err(_) => node_str("error"),
+        Node::PreciseCapturingNonLifetimeArg(_param) => node_str("parameter"),
     }
 }
 

compiler/rustc_parse/src/parser/generics.rs

@@ -62,7 +62,7 @@ impl<'a> Parser<'a> {
                 let snapshot = self.create_snapshot_for_diagnostic();
                 match self.parse_ty() {
                     Ok(p) => {
-                        if let TyKind::ImplTrait(_, bounds) = &p.kind {
+                        if let TyKind::ImplTrait(_, bounds, None) = &p.kind {
                             let span = impl_span.to(self.token.span.shrink_to_lo());
                             let mut err = self.dcx().struct_span_err(
                                 span,

compiler/rustc_parse/src/parser/item.rs

@@ -625,7 +625,7 @@ impl<'a> Parser<'a> {
                     // This notably includes paths passed through `ty` macro fragments (#46438).
                     TyKind::Path(None, path) => path,
                     other => {
-                        if let TyKind::ImplTrait(_, bounds) = other
+                        if let TyKind::ImplTrait(_, bounds, None) = other
                             && let [bound] = bounds.as_slice()
                         {
                             // Suggest removing extra `impl` keyword:

compiler/rustc_parse/src/parser/ty.rs

@@ -1,4 +1,4 @@
-use super::{Parser, PathStyle, TokenType, Trailing};
+use super::{Parser, PathStyle, SeqSep, TokenType, Trailing};
 
 use crate::errors::{
     self, DynAfterMut, ExpectedFnPathFoundFnKeyword, ExpectedMutOrConstInRawPointerType,
@@ -14,7 +14,7 @@ use rustc_ast::util::case::Case;
 use rustc_ast::{
     self as ast, BareFnTy, BoundAsyncness, BoundConstness, BoundPolarity, FnRetTy, GenericBound,
     GenericBounds, GenericParam, Generics, Lifetime, MacCall, MutTy, Mutability, PolyTraitRef,
-    TraitBoundModifiers, TraitObjectSyntax, Ty, TyKind, DUMMY_NODE_ID,
+    PreciseCapturingArg, TraitBoundModifiers, TraitObjectSyntax, Ty, TyKind, DUMMY_NODE_ID,
 };
 use rustc_errors::{Applicability, PResult};
 use rustc_span::symbol::{kw, sym, Ident};
@@ -316,7 +316,7 @@ impl<'a> Parser<'a> {
                             TyKind::TraitObject(bounds, TraitObjectSyntax::Dyn)
                         }
                         (TyKind::TraitObject(bounds, _), kw::Impl) => {
-                            TyKind::ImplTrait(ast::DUMMY_NODE_ID, bounds)
+                            TyKind::ImplTrait(ast::DUMMY_NODE_ID, bounds, None)
                         }
                         _ => return Err(err),
                     };
@@ -655,7 +655,6 @@ impl<'a> Parser<'a> {
 
     /// Parses an `impl B0 + ... + Bn` type.
     fn parse_impl_ty(&mut self, impl_dyn_multi: &mut bool) -> PResult<'a, TyKind> {
-        // Always parse bounds greedily for better error recovery.
         if self.token.is_lifetime() {
             self.look_ahead(1, |t| {
                 if let token::Ident(sym, _) = t.kind {
@@ -669,9 +668,53 @@ impl<'a> Parser<'a> {
                 }
             })
         }
+
+        // parse precise captures, if any. This is `use<'lt, 'lt, P, P>`; a list of
+        // lifetimes and ident params (including SelfUpper). These are validated later
+        // for order, duplication, and whether they actually reference params.
+        let precise_capturing = if self.eat_keyword(kw::Use) {
+            let use_span = self.prev_token.span;
+            self.psess.gated_spans.gate(sym::precise_capturing, use_span);
+            let args = self.parse_precise_capturing_args()?;
+            Some(P((args, use_span)))
+        } else {
+            None
+        };
+
+        // Always parse bounds greedily for better error recovery.
         let bounds = self.parse_generic_bounds()?;
+
         *impl_dyn_multi = bounds.len() > 1 || self.prev_token.kind == TokenKind::BinOp(token::Plus);
-        Ok(TyKind::ImplTrait(ast::DUMMY_NODE_ID, bounds))
+
+        Ok(TyKind::ImplTrait(ast::DUMMY_NODE_ID, bounds, precise_capturing))
+    }
+
+    fn parse_precise_capturing_args(&mut self) -> PResult<'a, ThinVec<PreciseCapturingArg>> {
+        Ok(self
+            .parse_unspanned_seq(
+                &TokenKind::Lt,
+                &TokenKind::Gt,
+                SeqSep::trailing_allowed(token::Comma),
+                |self_| {
+                    if self_.check_keyword(kw::SelfUpper) {
+                        self_.bump();
+                        Ok(PreciseCapturingArg::Arg(
+                            ast::Path::from_ident(self_.prev_token.ident().unwrap().0),
+                            DUMMY_NODE_ID,
+                        ))
+                    } else if self_.check_ident() {
+                        Ok(PreciseCapturingArg::Arg(
+                            ast::Path::from_ident(self_.parse_ident()?),
+                            DUMMY_NODE_ID,
+                        ))
+                    } else if self_.check_lifetime() {
+                        Ok(PreciseCapturingArg::Lifetime(self_.expect_lifetime()))
+                    } else {
+                        self_.unexpected_any()
+                    }
+                },
+            )?
+            .0)
     }
 
     /// Is a `dyn B0 + ... + Bn` type allowed here?
@@ -957,7 +1000,7 @@ impl<'a> Parser<'a> {
                             Applicability::MaybeIncorrect,
                         )
                     }
-                    TyKind::ImplTrait(_, bounds)
+                    TyKind::ImplTrait(_, bounds, None)
                         if let [GenericBound::Trait(tr, ..), ..] = bounds.as_slice() =>
                     {
                         (

compiler/rustc_resolve/src/late.rs

@@ -793,7 +793,7 @@ impl<'a: 'ast, 'ast, 'tcx> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast,
                 self.r.record_partial_res(ty.id, PartialRes::new(res));
                 visit::walk_ty(self, ty)
             }
-            TyKind::ImplTrait(node_id, _) => {
+            TyKind::ImplTrait(node_id, _, _) => {
                 let candidates = self.lifetime_elision_candidates.take();
                 visit::walk_ty(self, ty);
                 self.record_lifetime_params_for_impl_trait(*node_id);
@@ -1047,10 +1047,39 @@ impl<'a: 'ast, 'ast, 'tcx> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast,
         });
         self.diag_metadata.current_function = previous_value;
     }
+
     fn visit_lifetime(&mut self, lifetime: &'ast Lifetime, use_ctxt: visit::LifetimeCtxt) {
         self.resolve_lifetime(lifetime, use_ctxt)
     }
 
+    fn visit_precise_capturing_arg(&mut self, arg: &'ast PreciseCapturingArg) {
+        match arg {
+            // Lower the lifetime regularly; we'll resolve the lifetime and check
+            // it's a parameter later on in HIR lowering.
+            PreciseCapturingArg::Lifetime(_) => {}
+
+            PreciseCapturingArg::Arg(path, id) => {
+                // we want `impl use<C>` to try to resolve `C` as both a type parameter or
+                // a const parameter. Since the resolver specifically doesn't allow having
+                // two generic params with the same name, even if they're a different namespace,
+                // it doesn't really matter which we try resolving first, but just like
+                // `Ty::Param` we just fall back to the value namespace only if it's missing
+                // from the type namespace.
+                let mut check_ns = |ns| {
+                    self.maybe_resolve_ident_in_lexical_scope(path.segments[0].ident, ns).is_some()
+                };
+                // Like `Ty::Param`, we try resolving this as both a const and a type.
+                if !check_ns(TypeNS) && check_ns(ValueNS) {
+                    self.smart_resolve_path(*id, &None, path, PathSource::Expr(None));
+                } else {
+                    self.smart_resolve_path(*id, &None, path, PathSource::Type);
+                }
+            }
+        }
+
+        visit::walk_precise_capturing_arg(self, arg)
+    }
+
     fn visit_generics(&mut self, generics: &'ast Generics) {
         self.visit_generic_params(&generics.params, self.diag_metadata.current_self_item.is_some());
         for p in &generics.where_clause.predicates {

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -3121,7 +3121,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
                                 .inputs
                                 .iter()
                                 .filter_map(|param| match &param.ty.kind {
-                                    TyKind::ImplTrait(_, bounds) => Some(bounds),
+                                    TyKind::ImplTrait(_, bounds, _) => Some(bounds),
                                     _ => None,
                                 })
                                 .flat_map(|bounds| bounds.into_iter())

compiler/rustc_span/src/symbol.rs

@@ -1375,6 +1375,7 @@ symbols! {
         powif32,
         powif64,
         pre_dash_lto: "pre-lto",
+        precise_capturing,
         precise_pointer_size_matching,
         pref_align_of,
         prefetch_read_data,

src/tools/clippy/clippy_utils/src/ast_utils.rs

@@ -709,7 +709,8 @@ pub fn eq_ty(l: &Ty, r: &Ty) -> bool {
         (Tup(l), Tup(r)) => over(l, r, |l, r| eq_ty(l, r)),
         (Path(lq, lp), Path(rq, rp)) => both(lq, rq, eq_qself) && eq_path(lp, rp),
         (TraitObject(lg, ls), TraitObject(rg, rs)) => ls == rs && over(lg, rg, eq_generic_bound),
-        (ImplTrait(_, lg), ImplTrait(_, rg)) => over(lg, rg, eq_generic_bound),
+        (ImplTrait(_, lg, lc), ImplTrait(_, rg, rc)) =>
+            over(lg, rg, eq_generic_bound) && both(lc, rc, |lc, rc| over(lc.0.as_slice(), rc.0.as_slice(), eq_precise_capture)),
         (Typeof(l), Typeof(r)) => eq_expr(&l.value, &r.value),
         (MacCall(l), MacCall(r)) => eq_mac_call(l, r),
         _ => false,
@@ -770,6 +771,14 @@ pub fn eq_generic_bound(l: &GenericBound, r: &GenericBound) -> bool {
     }
 }
 
+pub fn eq_precise_capture(l: &PreciseCapturingArg, r: &PreciseCapturingArg) -> bool {
+    match (l, r) {
+        (PreciseCapturingArg::Lifetime(l), PreciseCapturingArg::Lifetime(r)) => l.ident == r.ident,
+        (PreciseCapturingArg::Arg(l, _), PreciseCapturingArg::Arg(r, _)) => l.segments[0].ident == r.segments[0].ident,
+        _ => false,
+    }
+}
+
 fn eq_term(l: &Term, r: &Term) -> bool {
     match (l, r) {
         (Term::Ty(l), Term::Ty(r)) => eq_ty(l, r),

src/tools/rustfmt/src/types.rs

@@ -843,7 +843,11 @@ impl Rewrite for ast::Ty {
                 rewrite_macro(mac, None, context, shape, MacroPosition::Expression)
             }
             ast::TyKind::ImplicitSelf => Some(String::from("")),
-            ast::TyKind::ImplTrait(_, ref it) => {
+            ast::TyKind::ImplTrait(_, ref it, ref captures) => {
+                // FIXME(precise_capturing): Implement formatting.
+                if captures.is_some() {
+                    return None;
+                }
                 // Empty trait is not a parser error.
                 if it.is_empty() {
                     return Some("impl".to_owned());
@@ -1106,7 +1110,8 @@ fn join_bounds_inner(
 
 pub(crate) fn opaque_ty(ty: &Option<ptr::P<ast::Ty>>) -> Option<&ast::GenericBounds> {
     ty.as_ref().and_then(|t| match &t.kind {
-        ast::TyKind::ImplTrait(_, bounds) => Some(bounds),
+        // FIXME(precise_capturing): Implement support here
+        ast::TyKind::ImplTrait(_, bounds, _) => Some(bounds),
         _ => None,
     })
 }

tests/ui/feature-gates/feature-gate-precise-capturing.rs

@@ -0,0 +1,4 @@
+fn hello() -> impl use<> Sized {}
+//~^ ERROR precise captures on `impl Trait` are experimental
+
+fn main() {}

tests/ui/feature-gates/feature-gate-precise-capturing.stderr

@@ -0,0 +1,13 @@
+error[E0658]: precise captures on `impl Trait` are experimental
+  --> $DIR/feature-gate-precise-capturing.rs:1:20
+   |
+LL | fn hello() -> impl use<> Sized {}
+   |                    ^^^
+   |
+   = note: see issue #123432 <https://github.com/rust-lang/rust/issues/123432> for more information
+   = help: add `#![feature(precise_capturing)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.

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

@@ -0,0 +1,7 @@
+#![feature(precise_capturing)]
+//~^ WARN the feature `precise_capturing` is incomplete
+
+fn hello(_: impl use<> Sized) {}
+//~^ ERROR `use<...>` precise capturing syntax not allowed on argument-position `impl Trait`
+
+fn main() {}

tests/ui/impl-trait/precise-capturing/apit.stderr

@@ -0,0 +1,17 @@
+warning: the feature `precise_capturing` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/apit.rs:1:12
+   |
+LL | #![feature(precise_capturing)]
+   |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #123432 <https://github.com/rust-lang/rust/issues/123432> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: `use<...>` precise capturing syntax not allowed on argument-position `impl Trait`
+  --> $DIR/apit.rs:4:18
+   |
+LL | fn hello(_: impl use<> Sized) {}
+   |                  ^^^
+
+error: aborting due to 1 previous error; 1 warning emitted
+

tests/ui/impl-trait/precise-capturing/bad-lifetimes.rs

@@ -0,0 +1,14 @@
+#![feature(precise_capturing)]
+//~^ WARN the feature `precise_capturing` is incomplete
+
+fn no_elided_lt() -> impl use<'_> Sized {}
+//~^ ERROR missing lifetime specifier
+//~| ERROR expected lifetime parameter in `use<...>` precise captures list, found `'_`
+
+fn static_lt() -> impl use<'static> Sized {}
+//~^ ERROR expected lifetime parameter in `use<...>` precise captures list, found `'static`
+
+fn missing_lt() -> impl use<'missing> Sized {}
+//~^ ERROR use of undeclared lifetime name `'missing`
+
+fn main() {}

tests/ui/impl-trait/precise-capturing/bad-lifetimes.stderr

@@ -0,0 +1,45 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/bad-lifetimes.rs:4:31
+   |
+LL | fn no_elided_lt() -> impl use<'_> Sized {}
+   |                               ^^ expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
+help: consider using the `'static` lifetime, but this is uncommon unless you're returning a borrowed value from a `const` or a `static`, or if you will only have owned values
+   |
+LL | fn no_elided_lt() -> impl use<'static> Sized {}
+   |                               ~~~~~~~
+
+error[E0261]: use of undeclared lifetime name `'missing`
+  --> $DIR/bad-lifetimes.rs:11:29
+   |
+LL | fn missing_lt() -> impl use<'missing> Sized {}
+   |              -              ^^^^^^^^ undeclared lifetime
+   |              |
+   |              help: consider introducing lifetime `'missing` here: `<'missing>`
+
+warning: the feature `precise_capturing` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/bad-lifetimes.rs:1:12
+   |
+LL | #![feature(precise_capturing)]
+   |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #123432 <https://github.com/rust-lang/rust/issues/123432> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: expected lifetime parameter in `use<...>` precise captures list, found `'_`
+  --> $DIR/bad-lifetimes.rs:4:31
+   |
+LL | fn no_elided_lt() -> impl use<'_> Sized {}
+   |                               ^^
+
+error: expected lifetime parameter in `use<...>` precise captures list, found `'static`
+  --> $DIR/bad-lifetimes.rs:8:28
+   |
+LL | fn static_lt() -> impl use<'static> Sized {}
+   |                            ^^^^^^^
+
+error: aborting due to 4 previous errors; 1 warning emitted
+
+Some errors have detailed explanations: E0106, E0261.
+For more information about an error, try `rustc --explain E0106`.

tests/ui/impl-trait/precise-capturing/bad-params.rs

@@ -0,0 +1,19 @@
+#![feature(precise_capturing)]
+//~^ WARN the feature `precise_capturing` is incomplete
+
+fn missing() -> impl use<T> Sized {}
+//~^ ERROR cannot find type `T` in this scope
+
+fn missing_self() -> impl use<Self> Sized {}
+//~^ ERROR cannot find type `Self` in this scope
+
+struct MyType;
+impl MyType {
+    fn self_is_not_param() -> impl use<Self> Sized {}
+    //~^ ERROR `Self` can't be captured in `use<...>` precise captures list, since it is an alias
+}
+
+fn hello() -> impl use<hello> Sized {}
+//~^ ERROR expected type or const parameter in `use<...>` precise captures list, found function
+
+fn main() {}

tests/ui/impl-trait/precise-capturing/bad-params.stderr

@@ -0,0 +1,46 @@
+error[E0412]: cannot find type `T` in this scope
+  --> $DIR/bad-params.rs:4:26
+   |
+LL | fn missing() -> impl use<T> Sized {}
+   |                          ^ not found in this scope
+   |
+help: you might be missing a type parameter
+   |
+LL | fn missing<T>() -> impl use<T> Sized {}
+   |           +++
+
+error[E0411]: cannot find type `Self` in this scope
+  --> $DIR/bad-params.rs:7:31
+   |
+LL | fn missing_self() -> impl use<Self> Sized {}
+   |    ------------               ^^^^ `Self` is only available in impls, traits, and type definitions
+   |    |
+   |    `Self` not allowed in a function
+
+warning: the feature `precise_capturing` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/bad-params.rs:1:12
+   |
+LL | #![feature(precise_capturing)]
+   |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #123432 <https://github.com/rust-lang/rust/issues/123432> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: `Self` can't be captured in `use<...>` precise captures list, since it is an alias
+  --> $DIR/bad-params.rs:12:40
+   |
+LL | impl MyType {
+   | ----------- `Self` is not a generic argument, but an alias to the type of the implementation
+LL |     fn self_is_not_param() -> impl use<Self> Sized {}
+   |                                        ^^^^
+
+error: expected type or const parameter in `use<...>` precise captures list, found function
+  --> $DIR/bad-params.rs:16:24
+   |
+LL | fn hello() -> impl use<hello> Sized {}
+   |                        ^^^^^
+
+error: aborting due to 4 previous errors; 1 warning emitted
+
+Some errors have detailed explanations: E0411, E0412.
+For more information about an error, try `rustc --explain E0411`.

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

@@ -0,0 +1,8 @@
+//@ check-pass
+
+#![feature(precise_capturing)]
+//~^ WARN the feature `precise_capturing` is incomplete
+
+fn elided(x: &()) -> impl use<'_> Sized { x }
+
+fn main() {}

tests/ui/impl-trait/precise-capturing/elided.stderr

@@ -0,0 +1,11 @@
+warning: the feature `precise_capturing` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/elided.rs:3:12
+   |
+LL | #![feature(precise_capturing)]
+   |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #123432 <https://github.com/rust-lang/rust/issues/123432> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+

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

@@ -0,0 +1,7 @@
+#![feature(precise_capturing)]
+//~^ WARN the feature `precise_capturing` is incomplete
+
+fn constant<const C: usize>() -> impl use<> Sized {}
+//~^ ERROR `impl Trait` must mention all const parameters in scope
+
+fn main() {}

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

@@ -0,0 +1,19 @@
+warning: the feature `precise_capturing` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/forgot-to-capture-const.rs:1:12
+   |
+LL | #![feature(precise_capturing)]
+   |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #123432 <https://github.com/rust-lang/rust/issues/123432> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: `impl Trait` must mention all const parameters in scope
+  --> $DIR/forgot-to-capture-const.rs:4:13
+   |
+LL | fn constant<const C: usize>() -> impl use<> Sized {}
+   |             ^^^^^^^^^^^^^^       ---------------- const parameter is implicitly captured by this `impl Trait`
+   |
+   = note: currently, all const parameters are required to be mentioned in the precise captures list
+
+error: aborting due to 1 previous error; 1 warning emitted
+

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

@@ -0,0 +1,10 @@
+#![feature(precise_capturing)]
+//~^ WARN the feature `precise_capturing` is incomplete
+
+fn lifetime_in_bounds<'a>(x: &'a ()) -> impl use<> Into<&'a ()> { x }
+//~^ ERROR `impl Trait` captures lifetime parameter, but it is not mentioned in `use<...>` precise captures list
+
+fn lifetime_in_hidden<'a>(x: &'a ()) -> impl use<> Sized { x }
+//~^ ERROR hidden type for `impl Sized` captures lifetime that does not appear in bounds
+
+fn main() {}

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

@@ -0,0 +1,35 @@
+warning: the feature `precise_capturing` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/forgot-to-capture-lifetime.rs:1:12
+   |
+LL | #![feature(precise_capturing)]
+   |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #123432 <https://github.com/rust-lang/rust/issues/123432> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: `impl Trait` captures lifetime parameter, but it is not mentioned in `use<...>` precise captures list
+  --> $DIR/forgot-to-capture-lifetime.rs:4:58
+   |
+LL | fn lifetime_in_bounds<'a>(x: &'a ()) -> impl use<> Into<&'a ()> { x }
+   |                       --                -----------------^^----
+   |                       |                 |
+   |                       |                 lifetime captured due to being mentioned in the bounds of the `impl Trait`
+   |                       this lifetime parameter is captured
+
+error[E0700]: hidden type for `impl Sized` captures lifetime that does not appear in bounds
+  --> $DIR/forgot-to-capture-lifetime.rs:7:60
+   |
+LL | fn lifetime_in_hidden<'a>(x: &'a ()) -> impl use<> Sized { x }
+   |                       --                ----------------   ^
+   |                       |                 |
+   |                       |                 opaque type defined here
+   |                       hidden type `&'a ()` captures the lifetime `'a` as defined here
+   |
+help: to declare that `impl Sized` captures `'a`, you can add an explicit `'a` lifetime bound
+   |
+LL | fn lifetime_in_hidden<'a>(x: &'a ()) -> impl use<> Sized + 'a { x }
+   |                                                          ++++
+
+error: aborting due to 2 previous errors; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0700`.

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

@@ -0,0 +1,12 @@
+#![feature(precise_capturing)]
+//~^ WARN the feature `precise_capturing` is incomplete
+
+fn type_param<T>() -> impl use<> Sized {}
+//~^ ERROR `impl Trait` must mention all type parameters in scope
+
+trait Foo {
+//~^ ERROR `impl Trait` must mention all type parameters in scope
+    fn bar() -> impl use<> Sized;
+}
+
+fn main() {}

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

@@ -0,0 +1,30 @@
+warning: the feature `precise_capturing` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/forgot-to-capture-type.rs:1:12
+   |
+LL | #![feature(precise_capturing)]
+   |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #123432 <https://github.com/rust-lang/rust/issues/123432> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: `impl Trait` must mention all type parameters in scope
+  --> $DIR/forgot-to-capture-type.rs:4:15
+   |
+LL | fn type_param<T>() -> impl use<> Sized {}
+   |               ^       ---------------- type parameter is implicitly captured by this `impl Trait`
+   |
+   = note: currently, all type parameters are required to be mentioned in the precise captures list
+
+error: `impl Trait` must mention all type parameters in scope
+  --> $DIR/forgot-to-capture-type.rs:7:1
+   |
+LL | trait Foo {
+   | ^^^^^^^^^
+LL |
+LL |     fn bar() -> impl use<> Sized;
+   |                 ---------------- type parameter is implicitly captured by this `impl Trait`
+   |
+   = note: currently, all type parameters are required to be mentioned in the precise captures list
+
+error: aborting due to 2 previous errors; 1 warning emitted
+

tests/ui/impl-trait/precise-capturing/higher-ranked.rs

@@ -0,0 +1,18 @@
+//@ check-pass
+
+// Show how precise captures allow us to skip capturing a higher-ranked lifetime
+
+#![feature(lifetime_capture_rules_2024, precise_capturing)]
+//~^ WARN the feature `precise_capturing` is incomplete
+
+trait Trait<'a> {
+    type Item;
+}
+
+impl Trait<'_> for () {
+    type Item = Vec<()>;
+}
+
+fn hello() -> impl for<'a> Trait<'a, Item = impl use<> IntoIterator> {}
+
+fn main() {}

tests/ui/impl-trait/precise-capturing/higher-ranked.stderr

@@ -0,0 +1,11 @@
+warning: the feature `precise_capturing` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/higher-ranked.rs:5:41
+   |
+LL | #![feature(lifetime_capture_rules_2024, precise_capturing)]
+   |                                         ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #123432 <https://github.com/rust-lang/rust/issues/123432> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+

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

@@ -0,0 +1,16 @@
+#![feature(precise_capturing)]
+//~^ WARN the feature `precise_capturing` is incomplete
+
+fn lt<'a>() -> impl use<'a, 'a> Sized {}
+//~^ ERROR cannot capture parameter `'a` twice
+
+fn ty<T>() -> impl use<T, T> Sized {}
+//~^ ERROR cannot capture parameter `T` twice
+
+fn ct<const N: usize>() -> impl use<N, N> Sized {}
+//~^ ERROR cannot capture parameter `N` twice
+
+fn ordering<'a, T>() -> impl use<T, 'a> Sized {}
+//~^ ERROR lifetime parameter `'a` must be listed before non-lifetime parameters
+
+fn main() {}

tests/ui/impl-trait/precise-capturing/ordering.stderr

@@ -0,0 +1,37 @@
+warning: the feature `precise_capturing` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/ordering.rs:1:12
+   |
+LL | #![feature(precise_capturing)]
+   |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #123432 <https://github.com/rust-lang/rust/issues/123432> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: cannot capture parameter `'a` twice
+  --> $DIR/ordering.rs:4:25
+   |
+LL | fn lt<'a>() -> impl use<'a, 'a> Sized {}
+   |                         ^^  -- parameter captured again here
+
+error: cannot capture parameter `T` twice
+  --> $DIR/ordering.rs:7:24
+   |
+LL | fn ty<T>() -> impl use<T, T> Sized {}
+   |                        ^  - parameter captured again here
+
+error: cannot capture parameter `N` twice
+  --> $DIR/ordering.rs:10:37
+   |
+LL | fn ct<const N: usize>() -> impl use<N, N> Sized {}
+   |                                     ^  - parameter captured again here
+
+error: lifetime parameter `'a` must be listed before non-lifetime parameters
+  --> $DIR/ordering.rs:13:37
+   |
+LL | fn ordering<'a, T>() -> impl use<T, 'a> Sized {}
+   |                                  -  ^^
+   |                                  |
+   |                                  move the lifetime before this parameter
+
+error: aborting due to 4 previous errors; 1 warning emitted
+

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

@@ -0,0 +1,16 @@
+//@ check-pass
+
+// Show that precise captures allow us to skip a lifetime param for outlives
+
+#![feature(lifetime_capture_rules_2024, precise_capturing)]
+//~^ WARN the feature `precise_capturing` is incomplete
+
+fn hello<'a: 'a, 'b: 'b>() -> impl use<'a> Sized { }
+
+fn outlives<'a, T: 'a>(_: T) {}
+
+fn test<'a, 'b>() {
+    outlives::<'a, _>(hello::<'a, 'b>());
+}
+
+fn main() {}

tests/ui/impl-trait/precise-capturing/outlives.stderr

@@ -0,0 +1,11 @@
+warning: the feature `precise_capturing` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/outlives.rs:5:41
+   |
+LL | #![feature(lifetime_capture_rules_2024, precise_capturing)]
+   |                                         ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #123432 <https://github.com/rust-lang/rust/issues/123432> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/impl-trait/precise-capturing/self-capture.rs

@@ -0,0 +1,10 @@
+//@ check-pass
+
+#![feature(precise_capturing)]
+//~^ WARN the feature `precise_capturing` is incomplete
+
+trait Foo {
+    fn bar<'a>() -> impl use<Self> Sized;
+}
+
+fn main() {}

tests/ui/impl-trait/precise-capturing/self-capture.stderr

@@ -0,0 +1,11 @@
+warning: the feature `precise_capturing` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/self-capture.rs:3:12
+   |
+LL | #![feature(precise_capturing)]
+   |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #123432 <https://github.com/rust-lang/rust/issues/123432> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+warning: 1 warning emitted
+