Parsing , pre-lowering support for precise captures

compiler/rustc_ast/src/ast.rs

@@ -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<GenericArgs>>),
     /// No-op; kept solely so that we can pretty-print faithfully.
     Paren(P<Ty>),
     /// Unused for now.

compiler/rustc_ast/src/mut_visit.rs

@@ -518,9 +518,12 @@ 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));
+            visit_opt(precise_capturing, |precise_capturing| {
+                vis.visit_generic_args(precise_capturing);
+            });
         }
         TyKind::MacCall(mac) => vis.visit_mac_call(mac),
         TyKind::AnonStruct(id, fields) | TyKind::AnonUnion(id, fields) => {

compiler/rustc_ast/src/visit.rs

@@ -457,8 +457,9 @@ 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);
+            visit_opt!(visitor, visit_generic_args, precise_capturing);
         }
         TyKind::Typeof(expression) => try_visit!(visitor.visit_anon_const(expression)),
         TyKind::Infer | TyKind::ImplicitSelf | TyKind::Dummy | TyKind::Err(_) => {}

compiler/rustc_ast_lowering/src/lib.rs

@@ -1398,7 +1398,8 @@ 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) => {
+                assert!(precise_capturing.is_none(), "precise captures not supported yet!");
                 let span = t.span;
                 match itctx {
                     ImplTraitContext::OpaqueTy { origin, fn_kind } => self.lower_opaque_impl_trait(

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

@@ -1150,7 +1150,8 @@ impl<'a> State<'a> {
                 }
                 self.print_type_bounds(bounds);
             }
-            ast::TyKind::ImplTrait(_, bounds) => {
+            ast::TyKind::ImplTrait(_, bounds, _precise_capturing) => {
+                // TODO(precise_capturing):
                 self.word_nbsp("impl");
                 self.print_type_bounds(bounds);
             }

compiler/rustc_feature/src/unstable.rs

@@ -535,6 +535,8 @@ declare_features! (
     (unstable, must_not_suspend, "1.57.0", Some(83310)),
     /// Allows `mut ref` and `mut ref mut` identifier patterns.
     (incomplete, mut_ref, "CURRENT_RUSTC_VERSION", Some(123076)),
+    /// 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 using `#[naked]` on functions.
     (unstable, naked_functions, "1.9.0", Some(90957)),
     /// Allows specifying the as-needed link modifier

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_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

@@ -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 +668,26 @@ impl<'a> Parser<'a> {
                 }
             })
         }
+
+        // parse precise captures, if any.
+        let precise_capturing = if self.eat_keyword(kw::Use) {
+            self.expect_lt()?;
+            let use_span = self.prev_token.span;
+            self.psess.gated_spans.gate(sym::precise_capturing, use_span);
+            let lo = self.token.span;
+            let args = self.parse_angle_args(None)?;
+            self.expect_gt()?;
+            Some(ast::AngleBracketedArgs { args, span: lo.to(self.prev_token.span) }.into())
+        } 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))
     }
 
     /// Is a `dyn B0 + ... + Bn` type allowed here?
@@ -957,7 +973,7 @@ 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);

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,