Rollup merge of #118401 - nnethercote:rustc_ast_lowering, r=compiler-errors

`rustc_ast_lowering` cleanups

Just some cleanups I found while looking through this code.

r? `@spastorino`

compiler/rustc_ast_lowering/src/lib.rs

@@ -35,7 +35,6 @@
 #![doc(rust_logo)]
 #![feature(box_patterns)]
 #![feature(let_chains)]
-#![feature(never_type)]
 #![recursion_limit = "256"]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
@@ -132,8 +131,8 @@ struct LoweringContext<'a, 'hir> {
     /// NodeIds that are lowered inside the current HIR owner.
     node_id_to_local_id: FxHashMap<NodeId, hir::ItemLocalId>,
 
-    allow_try_trait: Option<Lrc<[Symbol]>>,
-    allow_gen_future: Option<Lrc<[Symbol]>>,
+    allow_try_trait: Lrc<[Symbol]>,
+    allow_gen_future: Lrc<[Symbol]>,
 
     /// Mapping from generics `def_id`s to TAIT generics `def_id`s.
     /// For each captured lifetime (e.g., 'a), we create a new lifetime parameter that is a generic
@@ -144,6 +143,46 @@ struct LoweringContext<'a, 'hir> {
     host_param_id: Option<LocalDefId>,
 }
 
+impl<'a, 'hir> LoweringContext<'a, 'hir> {
+    fn new(tcx: TyCtxt<'hir>, resolver: &'a mut ResolverAstLowering) -> Self {
+        Self {
+            // Pseudo-globals.
+            tcx,
+            resolver: resolver,
+            arena: tcx.hir_arena,
+
+            // HirId handling.
+            bodies: Vec::new(),
+            attrs: SortedMap::default(),
+            children: Vec::default(),
+            current_hir_id_owner: hir::CRATE_OWNER_ID,
+            item_local_id_counter: hir::ItemLocalId::new(0),
+            node_id_to_local_id: Default::default(),
+            trait_map: Default::default(),
+
+            // Lowering state.
+            catch_scope: None,
+            loop_scope: None,
+            is_in_loop_condition: false,
+            is_in_trait_impl: false,
+            is_in_dyn_type: false,
+            coroutine_kind: None,
+            task_context: None,
+            current_item: None,
+            impl_trait_defs: Vec::new(),
+            impl_trait_bounds: Vec::new(),
+            allow_try_trait: [sym::try_trait_v2, sym::yeet_desugar_details].into(),
+            allow_gen_future: if tcx.features().async_fn_track_caller {
+                [sym::gen_future, sym::closure_track_caller].into()
+            } else {
+                [sym::gen_future].into()
+            },
+            generics_def_id_map: Default::default(),
+            host_param_id: None,
+        }
+    }
+}
+
 trait ResolverAstLoweringExt {
     fn legacy_const_generic_args(&self, expr: &Expr) -> Option<Vec<usize>>;
     fn get_partial_res(&self, id: NodeId) -> Option<PartialRes>;