Auto merge of #98279 - cjgillot:all-fresh-nofn, r=petrochenkov

Create elided lifetime parameters for function-like types

Split from https://github.com/rust-lang/rust/pull/97720

This PR refactor lifetime generic parameters in bare function types and parenthesized traits to introduce the additional required lifetimes as fresh parameters in a `for<>` bound.

This PR does the same to lifetimes appearing in closure signatures, and as-if introducing `for<>` bounds on closures (without the associated change in semantics).

r? `@petrochenkov`

compiler/rustc_ast_lowering/src/expr.rs

@@ -165,6 +165,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     if let Async::Yes { closure_id, .. } = asyncness {
                         self.lower_expr_async_closure(
                             capture_clause,
+                            e.id,
                             closure_id,
                             decl,
                             body,
@@ -173,6 +174,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     } else {
                         self.lower_expr_closure(
                             capture_clause,
+                            e.id,
                             movability,
                             decl,
                             body,
@@ -604,6 +606,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // `static |_task_context| -> <ret_ty> { body }`:
         let generator_kind = hir::ExprKind::Closure {
             capture_clause,
+            bound_generic_params: &[],
             fn_decl,
             body,
             fn_decl_span: self.lower_span(span),
@@ -828,6 +831,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     fn lower_expr_closure(
         &mut self,
         capture_clause: CaptureBy,
+        closure_id: NodeId,
         movability: Movability,
         decl: &FnDecl,
         body: &Expr,
@@ -848,16 +852,19 @@ impl<'hir> LoweringContext<'_, 'hir> {
             (body_id, generator_option)
         });
 
-        // Lower outside new scope to preserve `is_in_loop_condition`.
+        self.with_lifetime_binder(closure_id, &[], |this, bound_generic_params| {
-        let fn_decl = self.lower_fn_decl(decl, None, FnDeclKind::Closure, None);
+            // Lower outside new scope to preserve `is_in_loop_condition`.
+            let fn_decl = this.lower_fn_decl(decl, None, FnDeclKind::Closure, None);
 
-        hir::ExprKind::Closure {
+            hir::ExprKind::Closure {
-            capture_clause,
+                capture_clause,
-            fn_decl,
+                bound_generic_params,
-            body,
+                fn_decl,
-            fn_decl_span: self.lower_span(fn_decl_span),
+                body,
-            movability: generator_option,
+                fn_decl_span: this.lower_span(fn_decl_span),
-        }
+                movability: generator_option,
+            }
+        })
     }
 
     fn generator_movability_for_fn(
@@ -897,6 +904,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         &mut self,
         capture_clause: CaptureBy,
         closure_id: NodeId,
+        inner_closure_id: NodeId,
         decl: &FnDecl,
         body: &Expr,
         fn_decl_span: Span,
@@ -927,7 +935,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     if let FnRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
                 let async_body = this.make_async_expr(
                     capture_clause,
-                    closure_id,
+                    inner_closure_id,
                     async_ret_ty,
                     body.span,
                     hir::AsyncGeneratorKind::Closure,
@@ -938,18 +946,21 @@ impl<'hir> LoweringContext<'_, 'hir> {
             body_id
         });
 
-        // We need to lower the declaration outside the new scope, because we
+        self.with_lifetime_binder(closure_id, &[], |this, bound_generic_params| {
-        // have to conserve the state of being inside a loop condition for the
+            // We need to lower the declaration outside the new scope, because we
-        // closure argument types.
+            // have to conserve the state of being inside a loop condition for the
-        let fn_decl = self.lower_fn_decl(&outer_decl, None, FnDeclKind::Closure, None);
+            // closure argument types.
+            let fn_decl = this.lower_fn_decl(&outer_decl, None, FnDeclKind::Closure, None);
 
-        hir::ExprKind::Closure {
+            hir::ExprKind::Closure {
-            capture_clause,
+                capture_clause,
-            fn_decl,
+                bound_generic_params,
-            body,
+                fn_decl,
-            fn_decl_span: self.lower_span(fn_decl_span),
+                body,
-            movability: None,
+                fn_decl_span: this.lower_span(fn_decl_span),
-        }
+                movability: None,
+            }
+        })
     }
 
     /// Destructure the LHS of complex assignments.

compiler/rustc_ast_lowering/src/item.rs

@@ -1386,16 +1386,19 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         let mut params: SmallVec<[hir::GenericParam<'hir>; 4]> =
             self.lower_generic_params_mut(&generics.params).collect();
+
+        // Introduce extra lifetimes if late resolution tells us to.
+        let extra_lifetimes = self.resolver.take_extra_lifetime_params(parent_node_id);
+        params.extend(extra_lifetimes.into_iter().filter_map(|(ident, node_id, res)| {
+            self.lifetime_res_to_generic_param(ident, node_id, res)
+        }));
+
         let has_where_clause_predicates = !generics.where_clause.predicates.is_empty();
         let where_clause_span = self.lower_span(generics.where_clause.span);
         let span = self.lower_span(generics.span);
         let res = f(self);
 
-        let extra_lifetimes = self.resolver.take_extra_lifetime_params(parent_node_id);
         let impl_trait_defs = std::mem::take(&mut self.impl_trait_defs);
-        params.extend(extra_lifetimes.into_iter().filter_map(|(ident, node_id, res)| {
-            self.lifetime_res_to_generic_param(ident, node_id, res)
-        }));
         params.extend(impl_trait_defs.into_iter());
 
         let impl_trait_bounds = std::mem::take(&mut self.impl_trait_bounds);

compiler/rustc_ast_lowering/src/lib.rs

@@ -223,6 +223,12 @@ impl ResolverAstLoweringExt for ResolverAstLowering {
     }
 
     /// Obtain the list of lifetimes parameters to add to an item.
+    ///
+    /// Extra lifetime parameters should only be added in places that can appear
+    /// as a `binder` in `LifetimeRes`.
+    ///
+    /// The extra lifetimes that appear from the parenthesized `Fn`-trait desugaring
+    /// should appear at the enclosing `PolyTraitRef`.
     fn take_extra_lifetime_params(&mut self, id: NodeId) -> Vec<(Ident, NodeId, LifetimeRes)> {
         self.extra_lifetime_params_map.remove(&id).unwrap_or_default()
     }
@@ -721,6 +727,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     /// Converts a lifetime into a new generic parameter.
+    #[tracing::instrument(level = "debug", skip(self))]
     fn lifetime_res_to_generic_param(
         &mut self,
         ident: Ident,
@@ -731,7 +738,17 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             LifetimeRes::Param { .. } => {
                 (hir::ParamName::Plain(ident), hir::LifetimeParamKind::Explicit)
             }
-            LifetimeRes::Fresh { .. } => (hir::ParamName::Fresh, hir::LifetimeParamKind::Elided),
+            LifetimeRes::Fresh { param, .. } => {
+                // Late resolution delegates to us the creation of the `LocalDefId`.
+                let _def_id = self.create_def(
+                    self.current_hir_id_owner,
+                    param,
+                    DefPathData::LifetimeNs(kw::UnderscoreLifetime),
+                );
+                debug!(?_def_id);
+
+                (hir::ParamName::Fresh, hir::LifetimeParamKind::Elided)
+            }
             LifetimeRes::Static | LifetimeRes::Error => return None,
             res => panic!(
                 "Unexpected lifetime resolution {:?} for {:?} at {:?}",
@@ -777,11 +794,25 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     /// Register a binder to be ignored for lifetime capture.
     #[tracing::instrument(level = "debug", skip(self, f))]
     #[inline]
-    fn with_lifetime_binder<T>(&mut self, binder: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
+    fn with_lifetime_binder<T>(
+        &mut self,
+        binder: NodeId,
+        generic_params: &[GenericParam],
+        f: impl FnOnce(&mut Self, &'hir [hir::GenericParam<'hir>]) -> T,
+    ) -> T {
+        let mut generic_params: Vec<_> = self.lower_generic_params_mut(generic_params).collect();
+        let extra_lifetimes = self.resolver.take_extra_lifetime_params(binder);
+        debug!(?extra_lifetimes);
+        generic_params.extend(extra_lifetimes.into_iter().filter_map(|(ident, node_id, res)| {
+            self.lifetime_res_to_generic_param(ident, node_id, res)
+        }));
+        let generic_params = self.arena.alloc_from_iter(generic_params);
+        debug!(?generic_params);
+
         if let Some(ctxt) = &mut self.captured_lifetimes {
             ctxt.binders_to_ignore.insert(binder);
         }
-        let ret = f(self);
+        let ret = f(self, generic_params);
         if let Some(ctxt) = &mut self.captured_lifetimes {
             ctxt.binders_to_ignore.remove(&binder);
         }
@@ -1178,15 +1209,17 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 let lifetime = self.lower_lifetime(&region);
                 hir::TyKind::Rptr(lifetime, self.lower_mt(mt, itctx))
             }
-            TyKind::BareFn(ref f) => self.with_lifetime_binder(t.id, |this| {
+            TyKind::BareFn(ref f) => {
-                hir::TyKind::BareFn(this.arena.alloc(hir::BareFnTy {
+                self.with_lifetime_binder(t.id, &f.generic_params, |this, generic_params| {
-                    generic_params: this.lower_generic_params(&f.generic_params),
+                    hir::TyKind::BareFn(this.arena.alloc(hir::BareFnTy {
-                    unsafety: this.lower_unsafety(f.unsafety),
+                        generic_params,
-                    abi: this.lower_extern(f.ext),
+                        unsafety: this.lower_unsafety(f.unsafety),
-                    decl: this.lower_fn_decl(&f.decl, None, FnDeclKind::Pointer, None),
+                        abi: this.lower_extern(f.ext),
-                    param_names: this.lower_fn_params_to_names(&f.decl),
+                        decl: this.lower_fn_decl(&f.decl, None, FnDeclKind::Pointer, None),
-                }))
+                        param_names: this.lower_fn_params_to_names(&f.decl),
-            }),
+                    }))
+                })
+            }
             TyKind::Never => hir::TyKind::Never,
             TyKind::Tup(ref tys) => hir::TyKind::Tup(
                 self.arena.alloc_from_iter(tys.iter().map(|ty| self.lower_ty_direct(ty, itctx))),
@@ -1814,8 +1847,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 }
                 hir::LifetimeName::Param(param, p_name)
             }
-            LifetimeRes::Fresh { mut param, binder } => {
+            LifetimeRes::Fresh { param, binder } => {
                 debug_assert_eq!(ident.name, kw::UnderscoreLifetime);
+                let mut param = self.local_def_id(param);
                 if let Some(mut captured_lifetimes) = self.captured_lifetimes.take() {
                     if !captured_lifetimes.binders_to_ignore.contains(&binder) {
                         match captured_lifetimes.captures.entry(param) {
@@ -1952,13 +1986,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         p: &PolyTraitRef,
         itctx: ImplTraitContext,
     ) -> hir::PolyTraitRef<'hir> {
-        let bound_generic_params = self.lower_generic_params(&p.bound_generic_params);
+        self.with_lifetime_binder(
-
+            p.trait_ref.ref_id,
-        let trait_ref = self.with_lifetime_binder(p.trait_ref.ref_id, |this| {
+            &p.bound_generic_params,
-            this.lower_trait_ref(&p.trait_ref, itctx)
+            |this, bound_generic_params| {
-        });
+                let trait_ref = this.lower_trait_ref(&p.trait_ref, itctx);
-
+                hir::PolyTraitRef { bound_generic_params, trait_ref, span: this.lower_span(p.span) }
-        hir::PolyTraitRef { bound_generic_params, trait_ref, span: self.lower_span(p.span) }
+            },
+        )
     }
 
     fn lower_mt(&mut self, mt: &MutTy, itctx: ImplTraitContext) -> hir::MutTy<'hir> {

compiler/rustc_ast_lowering/src/path.rs

@@ -191,9 +191,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     self.lower_angle_bracketed_parameter_data(data, param_mode, itctx)
                 }
                 GenericArgs::Parenthesized(ref data) => match parenthesized_generic_args {
-                    ParenthesizedGenericArgs::Ok => {
+                    ParenthesizedGenericArgs::Ok => self.lower_parenthesized_parameter_data(data),
-                        self.lower_parenthesized_parameter_data(segment.id, data)
-                    }
                     ParenthesizedGenericArgs::Err => {
                         let mut err = struct_span_err!(self.sess, data.span, E0214, "{}", msg);
                         err.span_label(data.span, "only `Fn` traits may use parentheses");
@@ -351,7 +349,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
     fn lower_parenthesized_parameter_data(
         &mut self,
-        id: NodeId,
         data: &ParenthesizedArgs,
     ) -> (GenericArgsCtor<'hir>, bool) {
         // Switch to `PassThrough` mode for anonymous lifetimes; this
@@ -359,31 +356,27 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         // a hidden lifetime parameter. This is needed for backwards
         // compatibility, even in contexts like an impl header where
         // we generally don't permit such things (see #51008).
-        self.with_lifetime_binder(id, |this| {
+        let ParenthesizedArgs { span, inputs, inputs_span, output } = data;
-            let ParenthesizedArgs { span, inputs, inputs_span, output } = data;
+        let inputs = self.arena.alloc_from_iter(inputs.iter().map(|ty| {
-            let inputs = this.arena.alloc_from_iter(inputs.iter().map(|ty| {
+            self.lower_ty_direct(ty, ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitParam))
-                this.lower_ty_direct(
+        }));
-                    ty,
+        let output_ty = match output {
-                    ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitParam),
+            FnRetTy::Ty(ty) => {
-                )
+                self.lower_ty(&ty, ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitReturn))
-            }));
+            }
-            let output_ty = match output {
+            FnRetTy::Default(_) => self.arena.alloc(self.ty_tup(*span, &[])),
-                FnRetTy::Ty(ty) => this
+        };
-                    .lower_ty(&ty, ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitReturn)),
+        let args = smallvec![GenericArg::Type(self.ty_tup(*inputs_span, inputs))];
-                FnRetTy::Default(_) => this.arena.alloc(this.ty_tup(*span, &[])),
+        let binding = self.output_ty_binding(output_ty.span, output_ty);
-            };
+        (
-            let args = smallvec![GenericArg::Type(this.ty_tup(*inputs_span, inputs))];
+            GenericArgsCtor {
-            let binding = this.output_ty_binding(output_ty.span, output_ty);
+                args,
-            (
+                bindings: arena_vec![self; binding],
-                GenericArgsCtor {
+                parenthesized: true,
-                    args,
+                span: data.inputs_span,
-                    bindings: arena_vec![this; binding],
+            },
-                    parenthesized: true,
+            false,
-                    span: data.inputs_span,
+        )
-                },
-                false,
-            )
-        })
     }
 
     /// An associated type binding `Output = $ty`.

compiler/rustc_borrowck/src/diagnostics/region_name.rs

@@ -254,6 +254,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
     /// *user* has a name for. In that case, we'll be able to map
     /// `fr` to a `Region<'tcx>`, and that region will be one of
     /// named variants.
+    #[tracing::instrument(level = "trace", skip(self))]
     fn give_name_from_error_region(&self, fr: RegionVid) -> Option<RegionName> {
         let error_region = self.to_error_region(fr)?;
 
@@ -290,7 +291,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
                     if free_region.bound_region.is_named() {
                         // A named region that is actually named.
                         Some(RegionName { name, source: RegionNameSource::NamedFreeRegion(span) })
-                    } else {
+                    } else if let hir::IsAsync::Async = tcx.asyncness(self.mir_hir_id().owner) {
                         // If we spuriously thought that the region is named, we should let the
                         // system generate a true name for error messages. Currently this can
                         // happen if we have an elided name in an async fn for example: the
@@ -301,6 +302,8 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
                             name,
                             source: RegionNameSource::AnonRegionFromAsyncFn(span),
                         })
+                    } else {
+                        None
                     }
                 }
 
@@ -362,6 +365,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
     ///  | fn foo(x: &u32) { .. }
     ///           ------- fully elaborated type of `x` is `&'1 u32`
     /// ```
+    #[tracing::instrument(level = "trace", skip(self))]
     fn give_name_if_anonymous_region_appears_in_arguments(
         &self,
         fr: RegionVid,
@@ -651,6 +655,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
     ///  | let x = Some(&22);
     ///        - fully elaborated type of `x` is `Option<&'1 u32>`
     /// ```
+    #[tracing::instrument(level = "trace", skip(self))]
     fn give_name_if_anonymous_region_appears_in_upvars(&self, fr: RegionVid) -> Option<RegionName> {
         let upvar_index = self.regioncx.get_upvar_index_for_region(self.infcx.tcx, fr)?;
         let (upvar_name, upvar_span) = self.regioncx.get_upvar_name_and_span_for_region(
@@ -670,6 +675,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
     /// must be a closure since, in a free fn, such an argument would
     /// have to either also appear in an argument (if using elision)
     /// or be early bound (named, not in argument).
+    #[tracing::instrument(level = "trace", skip(self))]
     fn give_name_if_anonymous_region_appears_in_output(&self, fr: RegionVid) -> Option<RegionName> {
         let tcx = self.infcx.tcx;
         let hir = tcx.hir();
@@ -801,6 +807,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
         }
     }
 
+    #[tracing::instrument(level = "trace", skip(self))]
     fn give_name_if_anonymous_region_appears_in_yield_ty(
         &self,
         fr: RegionVid,

compiler/rustc_hir/src/def.rs

@@ -722,8 +722,7 @@ pub enum LifetimeRes {
         /// Id of the introducing place. That can be:
         /// - an item's id, for the item's generic parameters;
         /// - a TraitRef's ref_id, identifying the `for<...>` binder;
-        /// - a BareFn type's id;
+        /// - a BareFn type's id.
-        /// - a Path's id when this path has parenthesized generic args.
         ///
         /// This information is used for impl-trait lifetime captures, to know when to or not to
         /// capture any given lifetime.
@@ -732,7 +731,9 @@ pub enum LifetimeRes {
     /// Created a generic parameter for an anonymous lifetime.
     Fresh {
         /// Id of the generic parameter that introduced it.
-        param: LocalDefId,
+        ///
+        /// Creating the associated `LocalDefId` is the responsibility of lowering.
+        param: NodeId,
         /// Id of the introducing place. See `Param`.
         binder: NodeId,
     },

compiler/rustc_hir/src/hir.rs

@@ -1932,6 +1932,7 @@ pub enum ExprKind<'hir> {
     /// `Option<Movability>`.
     Closure {
         capture_clause: CaptureBy,
+        bound_generic_params: &'hir [GenericParam<'hir>],
         fn_decl: &'hir FnDecl<'hir>,
         body: BodyId,
         fn_decl_span: Span,
@@ -3480,7 +3481,7 @@ impl<'hir> Node<'hir> {
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
 mod size_asserts {
     rustc_data_structures::static_assert_size!(super::Block<'static>, 48);
-    rustc_data_structures::static_assert_size!(super::Expr<'static>, 56);
+    rustc_data_structures::static_assert_size!(super::Expr<'static>, 64);
     rustc_data_structures::static_assert_size!(super::Pat<'static>, 88);
     rustc_data_structures::static_assert_size!(super::QPath<'static>, 24);
     rustc_data_structures::static_assert_size!(super::Ty<'static>, 72);

compiler/rustc_hir/src/intravisit.rs

@@ -1169,12 +1169,16 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr<'v>)
             walk_list!(visitor, visit_arm, arms);
         }
         ExprKind::Closure {
+            bound_generic_params,
             ref fn_decl,
             body,
             capture_clause: _,
             fn_decl_span: _,
             movability: _,
-        } => visitor.visit_fn(FnKind::Closure, fn_decl, body, expression.span, expression.hir_id),
+        } => {
+            walk_list!(visitor, visit_generic_param, bound_generic_params);
+            visitor.visit_fn(FnKind::Closure, fn_decl, body, expression.span, expression.hir_id)
+        }
         ExprKind::Block(ref block, ref opt_label) => {
             walk_list!(visitor, visit_label, opt_label);
             visitor.visit_block(block);

compiler/rustc_hir_pretty/src/lib.rs

@@ -1459,11 +1459,13 @@ impl<'a> State<'a> {
             }
             hir::ExprKind::Closure {
                 capture_clause,
+                bound_generic_params,
                 ref fn_decl,
                 body,
                 fn_decl_span: _,
                 movability: _,
             } => {
+                self.print_formal_generic_params(bound_generic_params);
                 self.print_capture_clause(capture_clause);
 
                 self.print_closure_params(&fn_decl, body);

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -2242,9 +2242,18 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
             ]
             .contains(&Some(did))
             {
-                let (span, span_type) = match &trait_ref.bound_generic_params {
+                let (span, span_type) = if let Some(bound) =
-                    [] => (trait_ref.span.shrink_to_lo(), ForLifetimeSpanType::BoundEmpty),
+                    trait_ref.bound_generic_params.iter().rfind(|param| {
-                    [.., bound] => (bound.span.shrink_to_hi(), ForLifetimeSpanType::BoundTail),
+                        matches!(
+                            param.kind,
+                            hir::GenericParamKind::Lifetime {
+                                kind: hir::LifetimeParamKind::Explicit
+                            }
+                        )
+                    }) {
+                    (bound.span.shrink_to_hi(), ForLifetimeSpanType::BoundTail)
+                } else {
+                    (trait_ref.span.shrink_to_lo(), ForLifetimeSpanType::BoundEmpty)
                 };
                 self.missing_named_lifetime_spots
                     .push(MissingLifetimeSpot::HigherRanked { span, span_type });

compiler/rustc_resolve/src/late/lifetimes.rs

@@ -29,8 +29,6 @@ use std::cell::Cell;
 use std::fmt;
 use std::mem::take;
 
-use tracing::{debug, span, Level};
-
 trait RegionExt {
     fn early(hir_map: Map<'_>, index: &mut u32, param: &GenericParam<'_>) -> (LocalDefId, Region);
 
@@ -572,41 +570,38 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
         });
     }
 
-    fn visit_fn(
+    fn visit_expr(&mut self, e: &'tcx hir::Expr<'tcx>) {
-        &mut self,
+        if let hir::ExprKind::Closure { bound_generic_params, .. } = e.kind {
-        fk: intravisit::FnKind<'tcx>,
+            let next_early_index = self.next_early_index();
-        fd: &'tcx hir::FnDecl<'tcx>,
+            let (lifetimes, binders): (FxIndexMap<LocalDefId, Region>, Vec<_>) =
-        b: hir::BodyId,
+                bound_generic_params
-        s: rustc_span::Span,
+                    .iter()
-        hir_id: hir::HirId,
+                    .filter(|param| matches!(param.kind, GenericParamKind::Lifetime { .. }))
-    ) {
+                    .enumerate()
-        let name = match fk {
+                    .map(|(late_bound_idx, param)| {
-            intravisit::FnKind::ItemFn(id, _, _) => id.name,
+                        let pair = Region::late(late_bound_idx as u32, self.tcx.hir(), param);
-            intravisit::FnKind::Method(id, _) => id.name,
+                        let r = late_region_as_bound_region(self.tcx, &pair.1);
-            intravisit::FnKind::Closure => sym::closure,
+                        (pair, r)
-        };
+                    })
-        let name = name.as_str();
+                    .unzip();
-        let span = span!(Level::DEBUG, "visit_fn", name);
+            self.map.late_bound_vars.insert(e.hir_id, binders);
-        let _enter = span.enter();
+            let scope = Scope::Binder {
-        match fk {
+                hir_id: e.hir_id,
-            // Any `Binders` are handled elsewhere
+                lifetimes,
-            intravisit::FnKind::ItemFn(..) | intravisit::FnKind::Method(..) => {
+                s: self.scope,
-                intravisit::walk_fn(self, fk, fd, b, s, hir_id)
+                next_early_index,
-            }
+                opaque_type_parent: false,
-            intravisit::FnKind::Closure => {
+                scope_type: BinderScopeType::Normal,
-                self.map.late_bound_vars.insert(hir_id, vec![]);
+                allow_late_bound: true,
-                let scope = Scope::Binder {
+                where_bound_origin: None,
-                    hir_id,
+            };
-                    lifetimes: FxIndexMap::default(),
+            self.with(scope, |this| {
-                    next_early_index: self.next_early_index(),
+                // a closure has no bounds, so everything
-                    s: self.scope,
+                // contained within is scoped within its binder.
-                    opaque_type_parent: false,
+                intravisit::walk_expr(this, e)
-                    scope_type: BinderScopeType::Normal,
+            });
-                    allow_late_bound: true,
+        } else {
-                    where_bound_origin: None,
+            intravisit::walk_expr(self, e)
-                };
-                self.with(scope, move |this| intravisit::walk_fn(this, fk, fd, b, s, hir_id));
-            }
         }
     }
 
@@ -755,7 +750,9 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                 let next_early_index = self.next_early_index();
                 let lifetime_span: Option<Span> =
                     c.generic_params.iter().rev().find_map(|param| match param.kind {
-                        GenericParamKind::Lifetime { .. } => Some(param.span),
+                        GenericParamKind::Lifetime { kind: hir::LifetimeParamKind::Explicit } => {
+                            Some(param.span)
+                        }
                         _ => None,
                     });
                 let (span, span_type) = if let Some(span) = lifetime_span {

compiler/rustc_typeck/src/astconv/mod.rs

@@ -2920,13 +2920,15 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     ) {
         for br in referenced_regions.difference(&constrained_regions) {
             let br_name = match *br {
+                ty::BrNamed(_, kw::UnderscoreLifetime) | ty::BrAnon(_) | ty::BrEnv => {
+                    "an anonymous lifetime".to_string()
+                }
                 ty::BrNamed(_, name) => format!("lifetime `{}`", name),
-                ty::BrAnon(_) | ty::BrEnv => "an anonymous lifetime".to_string(),
             };
 
             let mut err = generate_err(&br_name);
 
-            if let ty::BrAnon(_) = *br {
+            if let ty::BrNamed(_, kw::UnderscoreLifetime) | ty::BrAnon(_) = *br {
                 // The only way for an anonymous lifetime to wind up
                 // in the return type but **also** be unconstrained is
                 // if it only appears in "associated types" in the

compiler/rustc_typeck/src/check/compare_method.rs

@@ -628,24 +628,40 @@ fn compare_number_of_generics<'tcx>(
     let mut err_occurred = None;
     for (kind, trait_count, impl_count) in matchings {
         if impl_count != trait_count {
+            let arg_spans = |kind: ty::AssocKind, generics: &hir::Generics<'_>| {
+                let mut spans = generics
+                    .params
+                    .iter()
+                    .filter(|p| match p.kind {
+                        hir::GenericParamKind::Lifetime {
+                            kind: hir::LifetimeParamKind::Elided,
+                        } => {
+                            // A fn can have an arbitrary number of extra elided lifetimes for the
+                            // same signature.
+                            !matches!(kind, ty::AssocKind::Fn)
+                        }
+                        _ => true,
+                    })
+                    .map(|p| p.span)
+                    .collect::<Vec<Span>>();
+                if spans.is_empty() {
+                    spans = vec![generics.span]
+                }
+                spans
+            };
             let (trait_spans, impl_trait_spans) = if let Some(def_id) = trait_.def_id.as_local() {
                 let trait_item = tcx.hir().expect_trait_item(def_id);
-                if trait_item.generics.params.is_empty() {
+                let arg_spans: Vec<Span> = arg_spans(trait_.kind, trait_item.generics);
-                    (Some(vec![trait_item.generics.span]), vec![])
+                let impl_trait_spans: Vec<Span> = trait_item
-                } else {
+                    .generics
-                    let arg_spans: Vec<Span> =
+                    .params
-                        trait_item.generics.params.iter().map(|p| p.span).collect();
+                    .iter()
-                    let impl_trait_spans: Vec<Span> = trait_item
+                    .filter_map(|p| match p.kind {
-                        .generics
+                        GenericParamKind::Type { synthetic: true, .. } => Some(p.span),
-                        .params
+                        _ => None,
-                        .iter()
+                    })
-                        .filter_map(|p| match p.kind {
+                    .collect();
-                            GenericParamKind::Type { synthetic: true, .. } => Some(p.span),
+                (Some(arg_spans), impl_trait_spans)
-                            _ => None,
-                        })
-                        .collect();
-                    (Some(arg_spans), impl_trait_spans)
-                }
             } else {
                 (trait_span.map(|s| vec![s]), vec![])
             };
@@ -660,23 +676,7 @@ fn compare_number_of_generics<'tcx>(
                     _ => None,
                 })
                 .collect();
-            let spans = if impl_item.generics.params.is_empty() {
+            let spans = arg_spans(impl_.kind, impl_item.generics);
-                vec![impl_item.generics.span]
-            } else {
-                impl_item
-                    .generics
-                    .params
-                    .iter()
-                    .filter(|p| {
-                        matches!(
-                            p.kind,
-                            hir::GenericParamKind::Type { .. }
-                                | hir::GenericParamKind::Const { .. }
-                        )
-                    })
-                    .map(|p| p.span)
-                    .collect::<Vec<Span>>()
-            };
             let span = spans.first().copied();
 
             let mut err = tcx.sess.struct_span_err_with_code(

src/librustdoc/clean/mod.rs

@@ -222,10 +222,15 @@ impl<'tcx> Clean<'tcx, Option<Lifetime>> for ty::Region<'tcx> {
         match **self {
             ty::ReStatic => Some(Lifetime::statik()),
             ty::ReLateBound(_, ty::BoundRegion { kind: ty::BrNamed(_, name), .. }) => {
-                Some(Lifetime(name))
+                if name != kw::UnderscoreLifetime { Some(Lifetime(name)) } else { None }
+            }
+            ty::ReEarlyBound(ref data) => {
+                if data.name != kw::UnderscoreLifetime {
+                    Some(Lifetime(data.name))
+                } else {
+                    None
+                }
             }
-            ty::ReEarlyBound(ref data) => Some(Lifetime(data.name)),
-
             ty::ReLateBound(..)
             | ty::ReFree(..)
             | ty::ReVar(..)
@@ -530,29 +535,25 @@ fn clean_generic_param<'tcx>(
     GenericParamDef { name, kind }
 }
 
+/// Synthetic type-parameters are inserted after normal ones.
+/// In order for normal parameters to be able to refer to synthetic ones,
+/// scans them first.
+fn is_impl_trait(param: &hir::GenericParam<'_>) -> bool {
+    match param.kind {
+        hir::GenericParamKind::Type { synthetic, .. } => synthetic,
+        _ => false,
+    }
+}
+
+/// This can happen for `async fn`, e.g. `async fn f<'_>(&'_ self)`.
+///
+/// See `lifetime_to_generic_param` in `rustc_ast_lowering` for more information.
+fn is_elided_lifetime(param: &hir::GenericParam<'_>) -> bool {
+    matches!(param.kind, hir::GenericParamKind::Lifetime { kind: hir::LifetimeParamKind::Elided })
+}
+
 impl<'tcx> Clean<'tcx, Generics> for hir::Generics<'tcx> {
     fn clean(&self, cx: &mut DocContext<'tcx>) -> Generics {
-        // Synthetic type-parameters are inserted after normal ones.
-        // In order for normal parameters to be able to refer to synthetic ones,
-        // scans them first.
-        fn is_impl_trait(param: &hir::GenericParam<'_>) -> bool {
-            match param.kind {
-                hir::GenericParamKind::Type { synthetic, .. } => synthetic,
-                _ => false,
-            }
-        }
-        /// This can happen for `async fn`, e.g. `async fn f<'_>(&'_ self)`.
-        ///
-        /// See [`lifetime_to_generic_param`] in [`rustc_ast_lowering`] for more information.
-        ///
-        /// [`lifetime_to_generic_param`]: rustc_ast_lowering::LoweringContext::lifetime_to_generic_param
-        fn is_elided_lifetime(param: &hir::GenericParam<'_>) -> bool {
-            matches!(
-                param.kind,
-                hir::GenericParamKind::Lifetime { kind: hir::LifetimeParamKind::Elided }
-            )
-        }
-
         let impl_trait_params = self
             .params
             .iter()
@@ -991,6 +992,7 @@ impl<'tcx> Clean<'tcx, PolyTrait> for hir::PolyTraitRef<'tcx> {
             generic_params: self
                 .bound_generic_params
                 .iter()
+                .filter(|p| !is_elided_lifetime(p))
                 .map(|x| clean_generic_param(cx, None, x))
                 .collect(),
         }
@@ -1865,8 +1867,12 @@ impl<'tcx> Clean<'tcx, BareFunctionDecl> for hir::BareFnTy<'tcx> {
     fn clean(&self, cx: &mut DocContext<'tcx>) -> BareFunctionDecl {
         let (generic_params, decl) = enter_impl_trait(cx, |cx| {
             // NOTE: generics must be cleaned before args
-            let generic_params =
+            let generic_params = self
-                self.generic_params.iter().map(|x| clean_generic_param(cx, None, x)).collect();
+                .generic_params
+                .iter()
+                .filter(|p| !is_elided_lifetime(p))
+                .map(|x| clean_generic_param(cx, None, x))
+                .collect();
             let args = clean_args_from_types_and_names(cx, self.decl.inputs, self.param_names);
             let decl = clean_fn_decl_with_args(cx, self.decl, args);
             (generic_params, decl)

src/librustdoc/clean/utils.rs

@@ -84,12 +84,9 @@ pub(crate) fn substs_to_args<'tcx>(
     let mut ret_val =
         Vec::with_capacity(substs.len().saturating_sub(if skip_first { 1 } else { 0 }));
     ret_val.extend(substs.iter().filter_map(|kind| match kind.unpack() {
-        GenericArgKind::Lifetime(lt) => match *lt {
+        GenericArgKind::Lifetime(lt) => {
-            ty::ReLateBound(_, ty::BoundRegion { kind: ty::BrAnon(_), .. }) => {
+            Some(GenericArg::Lifetime(lt.clean(cx).unwrap_or(Lifetime::elided())))
-                Some(GenericArg::Lifetime(Lifetime::elided()))
+        }
-            }
-            _ => lt.clean(cx).map(GenericArg::Lifetime),
-        },
         GenericArgKind::Type(_) if skip_first => {
             skip_first = false;
             None

src/test/ui/generic-associated-types/issue-70304.rs

@@ -45,7 +45,7 @@ where
 }
 
 fn create_doc() -> impl Document<Cursor<'_> = DocCursorImpl<'_>> {
-    //~^ ERROR: missing lifetime specifier
+    //~^ ERROR `'_` cannot be used here [E0637]
     //~| ERROR: missing lifetime specifier
     DocumentImpl {}
 }

src/test/ui/generic-associated-types/issue-70304.stderr

@@ -1,14 +1,8 @@
-error[E0106]: missing lifetime specifier
+error[E0637]: `'_` cannot be used here
   --> $DIR/issue-70304.rs:47:41
    |
 LL | fn create_doc() -> impl Document<Cursor<'_> = DocCursorImpl<'_>> {
-   |                                         ^^ expected named lifetime parameter
+   |                                         ^^ `'_` is a reserved lifetime name
-   |
-   = 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
-   |
-LL | fn create_doc() -> impl Document<Cursor<'static> = DocCursorImpl<'_>> {
-   |                                         ~~~~~~~
 
 error[E0106]: missing lifetime specifier
   --> $DIR/issue-70304.rs:47:61
@@ -24,4 +18,5 @@ LL | fn create_doc() -> impl Document<Cursor<'_> = DocCursorImpl<'static>> {
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0106`.
+Some errors have detailed explanations: E0106, E0637.
+For more information about an error, try `rustc --explain E0106`.

src/test/ui/generic-associated-types/issue-95305.rs

@@ -9,7 +9,7 @@ trait Foo {
 }
 
 fn foo(x: &impl Foo<Item<'_> = u32>) { }
-                       //~^ ERROR missing lifetime specifier
+                       //~^ ERROR `'_` cannot be used here [E0637]
 
 fn bar(x: &impl for<'a> Foo<Item<'a> = &'_ u32>) { }
                                       //~^ ERROR missing lifetime specifier

src/test/ui/generic-associated-types/issue-95305.stderr

@@ -1,13 +1,8 @@
-error[E0106]: missing lifetime specifier
+error[E0637]: `'_` cannot be used here
   --> $DIR/issue-95305.rs:11:26
    |
 LL | fn foo(x: &impl Foo<Item<'_> = u32>) { }
-   |                          ^^ expected named lifetime parameter
+   |                          ^^ `'_` is a reserved lifetime name
-   |
-help: consider introducing a named lifetime parameter
-   |
-LL | fn foo<'a>(x: &impl Foo<Item<'a> = u32>) { }
-   |       ++++                   ~~
 
 error[E0106]: missing lifetime specifier
   --> $DIR/issue-95305.rs:14:41
@@ -22,4 +17,5 @@ LL | fn bar(x: &impl for<'a> Foo<Item<'a> = &'a u32>) { }
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0106`.
+Some errors have detailed explanations: E0106, E0637.
+For more information about an error, try `rustc --explain E0106`.

src/test/ui/generic-associated-types/parameter_number_and_kind_impl.stderr

@@ -8,7 +8,7 @@ LL |     type A = u32;
    |           ^ lifetimes do not match type in trait
 
 error[E0049]: type `B` has 1 type parameter but its trait declaration has 0 type parameters
-  --> $DIR/parameter_number_and_kind_impl.rs:17:16
+  --> $DIR/parameter_number_and_kind_impl.rs:17:12
    |
 LL |     type B<'a, 'b>;
    |            --  --
@@ -16,7 +16,9 @@ LL |     type B<'a, 'b>;
    |            expected 0 type parameters
 ...
 LL |     type B<'a, T> = Vec<T>;
-   |                ^ found 1 type parameter
+   |            ^^  ^
+   |            |
+   |            found 1 type parameter
 
 error[E0195]: lifetime parameters or bounds on type `C` do not match the trait declaration
   --> $DIR/parameter_number_and_kind_impl.rs:19:11

src/test/ui/higher-rank-trait-bounds/normalize-under-binder/issue-71955.stderr

@@ -23,7 +23,7 @@ error[E0308]: mismatched types
 LL |     foo(bar, "string", |s| s.len() == 5);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
    |
-   = note: expected trait `FnOnce<(&&str,)>`
+   = note: expected trait `for<'r, 's> FnOnce<(&'r &'s str,)>`
               found trait `for<'r> FnOnce<(&'r &str,)>`
 note: this closure does not fulfill the lifetime requirements
   --> $DIR/issue-71955.rs:45:24
@@ -61,7 +61,7 @@ error[E0308]: mismatched types
 LL |     foo(baz, "string", |s| s.0.len() == 5);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
    |
-   = note: expected trait `FnOnce<(&Wrapper<'_>,)>`
+   = note: expected trait `for<'r, 's> FnOnce<(&'r Wrapper<'s>,)>`
               found trait `for<'r> FnOnce<(&'r Wrapper<'_>,)>`
 note: this closure does not fulfill the lifetime requirements
   --> $DIR/issue-71955.rs:48:24

src/test/ui/higher-rank-trait-bounds/normalize-under-binder/issue-85455.rs

@@ -6,8 +6,8 @@ trait SomeTrait<'a> {
 
 fn give_me_ice<T>() {
     callee::<fn(&()) -> <T as SomeTrait<'_>>::Associated>();
-    //~^ ERROR the trait bound `T: SomeTrait<'_>` is not satisfied [E0277]
+    //~^ ERROR the trait bound `for<'r> T: SomeTrait<'r>` is not satisfied [E0277]
-    //~| ERROR the trait bound `T: SomeTrait<'_>` is not satisfied [E0277]
+    //~| ERROR the trait bound `for<'r> T: SomeTrait<'r>` is not satisfied [E0277]
 }
 
 fn callee<T: Fn<(&'static (),)>>() {

src/test/ui/higher-rank-trait-bounds/normalize-under-binder/issue-85455.stderr

@@ -1,24 +1,24 @@
-error[E0277]: the trait bound `T: SomeTrait<'_>` is not satisfied
+error[E0277]: the trait bound `for<'r> T: SomeTrait<'r>` is not satisfied
   --> $DIR/issue-85455.rs:8:5
    |
 LL |     callee::<fn(&()) -> <T as SomeTrait<'_>>::Associated>();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `SomeTrait<'_>` is not implemented for `T`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `for<'r> SomeTrait<'r>` is not implemented for `T`
    |
 help: consider restricting type parameter `T`
    |
-LL | fn give_me_ice<T: SomeTrait<'_>>() {
+LL | fn give_me_ice<T: for<'r> SomeTrait<'r>>() {
-   |                 +++++++++++++++
+   |                 +++++++++++++++++++++++
 
-error[E0277]: the trait bound `T: SomeTrait<'_>` is not satisfied
+error[E0277]: the trait bound `for<'r> T: SomeTrait<'r>` is not satisfied
   --> $DIR/issue-85455.rs:8:14
    |
 LL |     callee::<fn(&()) -> <T as SomeTrait<'_>>::Associated>();
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `SomeTrait<'_>` is not implemented for `T`
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `for<'r> SomeTrait<'r>` is not implemented for `T`
    |
 help: consider restricting type parameter `T`
    |
-LL | fn give_me_ice<T: SomeTrait<'_>>() {
+LL | fn give_me_ice<T: for<'r> SomeTrait<'r>>() {
-   |                 +++++++++++++++
+   |                 +++++++++++++++++++++++
 
 error: aborting due to 2 previous errors
 

src/test/ui/impl-header-lifetime-elision/dyn-trait.rs

@@ -13,6 +13,10 @@ impl StaticTrait for Box<dyn Debug> { }
 trait NotStaticTrait { }
 impl NotStaticTrait for Box<dyn Debug + '_> { }
 
+// Check that we don't err when the trait has a lifetime parameter.
+trait TraitWithLifetime<'a> { }
+impl NotStaticTrait for &dyn TraitWithLifetime<'_> { }
+
 fn static_val<T: StaticTrait>(_: T) {
 }
 

src/test/ui/impl-header-lifetime-elision/dyn-trait.stderr

@@ -1,5 +1,5 @@
 error[E0521]: borrowed data escapes outside of function
-  --> $DIR/dyn-trait.rs:20:5
+  --> $DIR/dyn-trait.rs:24:5
    |
 LL | fn with_dyn_debug_static<'a>(x: Box<dyn Debug + 'a>) {
    |                          --  - `x` is a reference that is only valid in the function body

src/test/ui/lifetimes/elided-lifetime-in-path-in-pat.rs

@@ -0,0 +1,13 @@
+// check-pass
+
+struct Foo<'a> {
+    x: &'a (),
+}
+
+// The lifetime in pattern-position `Foo` is elided.
+// Verify that lowering does not create an independent lifetime parameter for it.
+fn foo<'a>(Foo { x }: Foo<'a>) {
+    *x
+}
+
+fn main() {}

src/test/ui/lifetimes/lifetime-errors/ex3-both-anon-regions-using-trait-objects.stderr

@@ -10,8 +10,8 @@ LL |   y.push(z);
    |
 help: consider introducing a named lifetime parameter
    |
-LL | fn foo<'a>(x:Box<dyn Fn(&'a u8, &'a u8)> , y: Vec<&u8>, z: &u8) {
+LL | fn foo<'a>(x:Box<dyn Fn(&u8, &u8)> , y: Vec<&'a u8>, z: &'a u8) {
-   |       ++++               ++      ++
+   |       ++++                                   ++          ++
 
 error[E0596]: cannot borrow `y` as mutable, as it is not declared as mutable
   --> $DIR/ex3-both-anon-regions-using-trait-objects.rs:2:3