Rollup merge of #88445 - inquisitivecrystal:ast-lowering, r=cjgillot

Clean up the lowering of AST items

This PR simplifies and improves `rustc_ast_lowering::item` in various minor ways. The reasons for the changes should mostly be self evident, though I'm happy to specifically explain anything if needed.

These changes used to be part of #88019, but I removed them after it was pointed out that some of my other changes to `rustc_ast_lowering` were unnecessary. It felt like a bad idea to clean up code which I didn't even need to touch anymore.

r? `@cjgillot`

compiler/rustc_ast_lowering/src/item.rs

@@ -26,34 +26,34 @@ pub(super) struct ItemLowerer<'a, 'lowering, 'hir> {
 }
 
 impl ItemLowerer<'_, '_, '_> {
-    fn with_trait_impl_ref(&mut self, impl_ref: &Option<TraitRef>, f: impl FnOnce(&mut Self)) {
+    fn with_trait_impl_ref<T>(
+        &mut self,
+        impl_ref: &Option<TraitRef>,
+        f: impl FnOnce(&mut Self) -> T,
+    ) -> T {
         let old = self.lctx.is_in_trait_impl;
         self.lctx.is_in_trait_impl = impl_ref.is_some();
-        f(self);
+        let ret = f(self);
         self.lctx.is_in_trait_impl = old;
+        ret
     }
 }
 
 impl<'a> Visitor<'a> for ItemLowerer<'a, '_, '_> {
     fn visit_item(&mut self, item: &'a Item) {
-        let mut item_hir_id = None;
+        let hir_id = self.lctx.with_hir_id_owner(item.id, |lctx| {
-        self.lctx.with_hir_id_owner(item.id, |lctx| {
             lctx.without_in_scope_lifetime_defs(|lctx| {
-                if let Some(hir_item) = lctx.lower_item(item) {
+                let hir_item = lctx.lower_item(item);
-                    let id = lctx.insert_item(hir_item);
+                lctx.insert_item(hir_item)
-                    item_hir_id = Some(id);
-                }
             })
         });
 
-        if let Some(hir_id) = item_hir_id {
         self.lctx.with_parent_item_lifetime_defs(hir_id, |this| {
             let this = &mut ItemLowerer { lctx: this };
             match item.kind {
                 ItemKind::Mod(..) => {
                     let def_id = this.lctx.lower_node_id(item.id).expect_owner();
-                        let old_current_module =
+                    let old_current_module = mem::replace(&mut this.lctx.current_module, def_id);
-                            mem::replace(&mut this.lctx.current_module, def_id);
                     visit::walk_item(this, item);
                     this.lctx.current_module = old_current_module;
                 }
@@ -64,7 +64,6 @@ impl<'a> Visitor<'a> for ItemLowerer<'a, '_, '_> {
             }
         });
     }
-    }
 
     fn visit_fn(&mut self, fk: FnKind<'a>, sp: Span, _: NodeId) {
         match fk {
@@ -113,7 +112,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     fn with_parent_item_lifetime_defs<T>(
         &mut self,
         parent_hir_id: hir::ItemId,
-        f: impl FnOnce(&mut LoweringContext<'_, '_>) -> T,
+        f: impl FnOnce(&mut Self) -> T,
     ) -> T {
         let old_len = self.in_scope_lifetimes.len();
 
@@ -137,10 +136,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     // Clears (and restores) the `in_scope_lifetimes` field. Used when
     // visiting nested items, which never inherit in-scope lifetimes
     // from their surrounding environment.
-    fn without_in_scope_lifetime_defs<T>(
+    fn without_in_scope_lifetime_defs<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
-        &mut self,
-        f: impl FnOnce(&mut LoweringContext<'_, '_>) -> T,
-    ) -> T {
         let old_in_scope_lifetimes = mem::replace(&mut self.in_scope_lifetimes, vec![]);
 
         // this vector is only used when walking over impl headers,
@@ -208,19 +204,19 @@ impl<'hir> LoweringContext<'_, 'hir> {
         }
     }
 
-    pub fn lower_item(&mut self, i: &Item) -> Option<hir::Item<'hir>> {
+    pub fn lower_item(&mut self, i: &Item) -> hir::Item<'hir> {
         let mut ident = i.ident;
         let mut vis = self.lower_visibility(&i.vis, None);
         let hir_id = self.lower_node_id(i.id);
         let attrs = self.lower_attrs(hir_id, &i.attrs);
         let kind = self.lower_item_kind(i.span, i.id, hir_id, &mut ident, attrs, &mut vis, &i.kind);
-        Some(hir::Item {
+        hir::Item {
             def_id: hir_id.expect_owner(),
             ident: self.lower_ident(ident),
             kind,
             vis,
             span: self.lower_span(i.span),
-        })
+        }
     }
 
     fn lower_item_kind(