Leave the responsibility to create Fresh lifetimes to lowering.

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

@@ -731,7 +731,17 @@ 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 {:?}",
@@ -1814,8 +1824,9 @@ 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) {

compiler/rustc_hir/src/def.rs

@@ -732,7 +732,9 @@ 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_resolve/src/late.rs

@@ -20,7 +20,6 @@ use rustc_errors::DiagnosticId;
 use rustc_hir::def::Namespace::{self, *};
 use rustc_hir::def::{self, CtorKind, DefKind, LifetimeRes, PartialRes, PerNS};
 use rustc_hir::def_id::{DefId, LocalDefId, CRATE_DEF_ID};
-use rustc_hir::definitions::DefPathData;
 use rustc_hir::{PrimTy, TraitCandidate};
 use rustc_index::vec::Idx;
 use rustc_middle::ty::DefIdTree;
@@ -1418,31 +1417,20 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
     }
 
     #[tracing::instrument(level = "debug", skip(self))]
-    fn create_fresh_lifetime(
-        &mut self,
-        id: NodeId,
-        ident: Ident,
-        item_node_id: NodeId,
-    ) -> LifetimeRes {
+    fn create_fresh_lifetime(&mut self, id: NodeId, ident: Ident, binder: NodeId) -> LifetimeRes {
         debug_assert_eq!(ident.name, kw::UnderscoreLifetime);
         debug!(?ident.span);
-        let item_def_id = self.r.local_def_id(item_node_id);
-        let def_node_id = self.r.next_node_id();
-        let def_id = self.r.create_def(
-            item_def_id,
-            def_node_id,
-            DefPathData::LifetimeNs(kw::UnderscoreLifetime),
-            self.parent_scope.expansion.to_expn_id(),
-            ident.span,
-        );
-        debug!(?def_id);
 
-        let res = LifetimeRes::Fresh { param: def_id, binder: item_node_id };
-        self.r.extra_lifetime_params_map.entry(item_node_id).or_insert_with(Vec::new).push((
-            ident,
-            def_node_id,
-            res,
-        ));
+        // Leave the responsibility to create the `LocalDefId` to lowering.
+        let param = self.r.next_node_id();
+        let res = LifetimeRes::Fresh { param, binder };
+
+        // Record the created lifetime parameter so lowering can pick it up and add it to HIR.
+        self.r
+            .extra_lifetime_params_map
+            .entry(binder)
+            .or_insert_with(Vec::new)
+            .push((ident, param, res));
         res
     }