diff --git a/compiler/rustc_arena/src/lib.rs b/compiler/rustc_arena/src/lib.rs
index e54fcaf6fc1b7..6f9ecb9cd21e0 100644
--- a/compiler/rustc_arena/src/lib.rs
+++ b/compiler/rustc_arena/src/lib.rs
@@ -111,7 +111,7 @@ impl<T> Default for TypedArena<T> {
             // alloc() will trigger a grow().
             ptr: Cell::new(ptr::null_mut()),
             end: Cell::new(ptr::null_mut()),
-            chunks: RefCell::new(vec![]),
+            chunks: Default::default(),
             _own: PhantomData,
         }
     }
@@ -325,13 +325,17 @@ unsafe impl<#[may_dangle] T> Drop for TypedArena<T> {
 
 unsafe impl<T: Send> Send for TypedArena<T> {}
 
+/// An arena that can hold objects of multiple different types that impl `Copy`
+/// and/or satisfy `!mem::needs_drop`.
 pub struct DroplessArena {
     /// A pointer to the start of the free space.
     start: Cell<*mut u8>,
 
     /// A pointer to the end of free space.
     ///
-    /// The allocation proceeds from the end of the chunk towards the start.
+    /// The allocation proceeds downwards from the end of the chunk towards the
+    /// start. (This is slightly simpler and faster than allocating upwards,
+    /// see <https://fitzgeraldnick.com/2019/11/01/always-bump-downwards.html>.)
     /// When this pointer crosses the start pointer, a new chunk is allocated.
     end: Cell<*mut u8>,
 
@@ -516,10 +520,14 @@ impl DroplessArena {
     }
 }
 
+// Declare an `Arena` containing one dropless arena and many typed arenas (the
+// types of the typed arenas are specified by the arguments). The dropless
+// arena will be used for any types that impl `Copy`, and also for any of the
+// specified types that satisfy `!mem::needs_drop`.
 #[rustc_macro_transparency = "semitransparent"]
-pub macro declare_arena([$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) {
+pub macro declare_arena([$($a:tt $name:ident: $ty:ty,)*]) {
     #[derive(Default)]
-    pub struct Arena<$tcx> {
+    pub struct Arena<'tcx> {
         pub dropless: $crate::DroplessArena,
         $($name: $crate::TypedArena<$ty>,)*
     }
@@ -532,6 +540,7 @@ pub macro declare_arena([$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) {
         ) -> &'a mut [Self];
     }
 
+    // Any type that impls `Copy` can be arena-allocated in the `DroplessArena`.
     impl<'tcx, T: Copy> ArenaAllocatable<'tcx, ()> for T {
         #[inline]
         fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self {
@@ -544,12 +553,11 @@ pub macro declare_arena([$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) {
         ) -> &'a mut [Self] {
             arena.dropless.alloc_from_iter(iter)
         }
-
     }
     $(
-        impl<$tcx> ArenaAllocatable<$tcx, $ty> for $ty {
+        impl<'tcx> ArenaAllocatable<'tcx, $ty> for $ty {
             #[inline]
-            fn allocate_on<'a>(self, arena: &'a Arena<$tcx>) -> &'a mut Self {
+            fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self {
                 if !::std::mem::needs_drop::<Self>() {
                     arena.dropless.alloc(self)
                 } else {
@@ -559,7 +567,7 @@ pub macro declare_arena([$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) {
 
             #[inline]
             fn allocate_from_iter<'a>(
-                arena: &'a Arena<$tcx>,
+                arena: &'a Arena<'tcx>,
                 iter: impl ::std::iter::IntoIterator<Item = Self>,
             ) -> &'a mut [Self] {
                 if !::std::mem::needs_drop::<Self>() {
@@ -577,6 +585,7 @@ pub macro declare_arena([$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) {
             value.allocate_on(self)
         }
 
+        // Any type that impls `Copy` can have slices be arena-allocated in the `DroplessArena`.
         #[inline]
         pub fn alloc_slice<T: ::std::marker::Copy>(&self, value: &[T]) -> &mut [T] {
             if value.is_empty() {
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 79464a7517217..cd468bcf4474e 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -84,7 +84,7 @@ mod item;
 mod pat;
 mod path;
 
-rustc_hir::arena_types!(rustc_arena::declare_arena, 'tcx);
+rustc_hir::arena_types!(rustc_arena::declare_arena);
 
 struct LoweringContext<'a, 'hir: 'a> {
     /// Used to assign IDs to HIR nodes that do not directly correspond to AST nodes.
diff --git a/compiler/rustc_hir/src/arena.rs b/compiler/rustc_hir/src/arena.rs
index 5091a7bccc55b..f19ca497d8bf2 100644
--- a/compiler/rustc_hir/src/arena.rs
+++ b/compiler/rustc_hir/src/arena.rs
@@ -1,52 +1,52 @@
-/// This declares a list of types which can be allocated by `Arena`.
+/// This higher-order macro declares a list of types which can be allocated by `Arena`.
 ///
 /// Specifying the `decode` modifier will add decode impls for `&T` and `&[T]`,
 /// where `T` is the type listed. These impls will appear in the implement_ty_decoder! macro.
 #[macro_export]
 macro_rules! arena_types {
-    ($macro:path, $tcx:lifetime) => (
+    ($macro:path) => (
         $macro!([
             // HIR types
-            [] hir_krate: rustc_hir::Crate<$tcx>,
-            [] arm: rustc_hir::Arm<$tcx>,
-            [] asm_operand: (rustc_hir::InlineAsmOperand<$tcx>, Span),
+            [] hir_krate: rustc_hir::Crate<'tcx>,
+            [] arm: rustc_hir::Arm<'tcx>,
+            [] asm_operand: (rustc_hir::InlineAsmOperand<'tcx>, Span),
             [] asm_template: rustc_ast::InlineAsmTemplatePiece,
             [] attribute: rustc_ast::Attribute,
-            [] block: rustc_hir::Block<$tcx>,
-            [] bare_fn_ty: rustc_hir::BareFnTy<$tcx>,
-            [] body: rustc_hir::Body<$tcx>,
-            [] generic_arg: rustc_hir::GenericArg<$tcx>,
-            [] generic_args: rustc_hir::GenericArgs<$tcx>,
-            [] generic_bound: rustc_hir::GenericBound<$tcx>,
-            [] generic_param: rustc_hir::GenericParam<$tcx>,
-            [] expr: rustc_hir::Expr<$tcx>,
-            [] expr_field: rustc_hir::ExprField<$tcx>,
-            [] pat_field: rustc_hir::PatField<$tcx>,
-            [] fn_decl: rustc_hir::FnDecl<$tcx>,
-            [] foreign_item: rustc_hir::ForeignItem<$tcx>,
+            [] block: rustc_hir::Block<'tcx>,
+            [] bare_fn_ty: rustc_hir::BareFnTy<'tcx>,
+            [] body: rustc_hir::Body<'tcx>,
+            [] generic_arg: rustc_hir::GenericArg<'tcx>,
+            [] generic_args: rustc_hir::GenericArgs<'tcx>,
+            [] generic_bound: rustc_hir::GenericBound<'tcx>,
+            [] generic_param: rustc_hir::GenericParam<'tcx>,
+            [] expr: rustc_hir::Expr<'tcx>,
+            [] expr_field: rustc_hir::ExprField<'tcx>,
+            [] pat_field: rustc_hir::PatField<'tcx>,
+            [] fn_decl: rustc_hir::FnDecl<'tcx>,
+            [] foreign_item: rustc_hir::ForeignItem<'tcx>,
             [] foreign_item_ref: rustc_hir::ForeignItemRef,
-            [] impl_item: rustc_hir::ImplItem<$tcx>,
+            [] impl_item: rustc_hir::ImplItem<'tcx>,
             [] impl_item_ref: rustc_hir::ImplItemRef,
-            [] item: rustc_hir::Item<$tcx>,
-            [] inline_asm: rustc_hir::InlineAsm<$tcx>,
-            [] llvm_inline_asm: rustc_hir::LlvmInlineAsm<$tcx>,
-            [] local: rustc_hir::Local<$tcx>,
-            [] mod_: rustc_hir::Mod<$tcx>,
-            [] owner_info: rustc_hir::OwnerInfo<$tcx>,
-            [] param: rustc_hir::Param<$tcx>,
-            [] pat: rustc_hir::Pat<$tcx>,
-            [] path: rustc_hir::Path<$tcx>,
-            [] path_segment: rustc_hir::PathSegment<$tcx>,
-            [] poly_trait_ref: rustc_hir::PolyTraitRef<$tcx>,
-            [] qpath: rustc_hir::QPath<$tcx>,
-            [] stmt: rustc_hir::Stmt<$tcx>,
-            [] field_def: rustc_hir::FieldDef<$tcx>,
-            [] trait_item: rustc_hir::TraitItem<$tcx>,
+            [] item: rustc_hir::Item<'tcx>,
+            [] inline_asm: rustc_hir::InlineAsm<'tcx>,
+            [] llvm_inline_asm: rustc_hir::LlvmInlineAsm<'tcx>,
+            [] local: rustc_hir::Local<'tcx>,
+            [] mod_: rustc_hir::Mod<'tcx>,
+            [] owner_info: rustc_hir::OwnerInfo<'tcx>,
+            [] param: rustc_hir::Param<'tcx>,
+            [] pat: rustc_hir::Pat<'tcx>,
+            [] path: rustc_hir::Path<'tcx>,
+            [] path_segment: rustc_hir::PathSegment<'tcx>,
+            [] poly_trait_ref: rustc_hir::PolyTraitRef<'tcx>,
+            [] qpath: rustc_hir::QPath<'tcx>,
+            [] stmt: rustc_hir::Stmt<'tcx>,
+            [] field_def: rustc_hir::FieldDef<'tcx>,
+            [] trait_item: rustc_hir::TraitItem<'tcx>,
             [] trait_item_ref: rustc_hir::TraitItemRef,
-            [] ty: rustc_hir::Ty<$tcx>,
-            [] type_binding: rustc_hir::TypeBinding<$tcx>,
-            [] variant: rustc_hir::Variant<$tcx>,
-            [] where_predicate: rustc_hir::WherePredicate<$tcx>,
-        ], $tcx);
+            [] ty: rustc_hir::Ty<'tcx>,
+            [] type_binding: rustc_hir::TypeBinding<'tcx>,
+            [] variant: rustc_hir::Variant<'tcx>,
+            [] where_predicate: rustc_hir::WherePredicate<'tcx>,
+        ]);
     )
 }
diff --git a/compiler/rustc_middle/src/arena.rs b/compiler/rustc_middle/src/arena.rs
index 2bcc2a4f7cff7..ee2e190e7cd44 100644
--- a/compiler/rustc_middle/src/arena.rs
+++ b/compiler/rustc_middle/src/arena.rs
@@ -1,33 +1,33 @@
-/// This declares a list of types which can be allocated by `Arena`.
+/// This higher-order macro declares a list of types which can be allocated by `Arena`.
 ///
 /// Specifying the `decode` modifier will add decode impls for `&T` and `&[T]` where `T` is the type
 /// listed. These impls will appear in the implement_ty_decoder! macro.
 #[macro_export]
 macro_rules! arena_types {
-    ($macro:path, $tcx:lifetime) => (
+    ($macro:path) => (
         $macro!([
             [] layout: rustc_target::abi::Layout,
-            [] fn_abi: rustc_target::abi::call::FnAbi<$tcx, rustc_middle::ty::Ty<$tcx>>,
+            [] fn_abi: rustc_target::abi::call::FnAbi<'tcx, rustc_middle::ty::Ty<'tcx>>,
             // AdtDef are interned and compared by address
             [] adt_def: rustc_middle::ty::AdtDef,
-            [] steal_thir: rustc_data_structures::steal::Steal<rustc_middle::thir::Thir<$tcx>>,
-            [] steal_mir: rustc_data_structures::steal::Steal<rustc_middle::mir::Body<$tcx>>,
-            [decode] mir: rustc_middle::mir::Body<$tcx>,
+            [] steal_thir: rustc_data_structures::steal::Steal<rustc_middle::thir::Thir<'tcx>>,
+            [] steal_mir: rustc_data_structures::steal::Steal<rustc_middle::mir::Body<'tcx>>,
+            [decode] mir: rustc_middle::mir::Body<'tcx>,
             [] steal_promoted:
                 rustc_data_structures::steal::Steal<
                     rustc_index::vec::IndexVec<
                         rustc_middle::mir::Promoted,
-                        rustc_middle::mir::Body<$tcx>
+                        rustc_middle::mir::Body<'tcx>
                     >
                 >,
             [decode] promoted:
                 rustc_index::vec::IndexVec<
                     rustc_middle::mir::Promoted,
-                    rustc_middle::mir::Body<$tcx>
+                    rustc_middle::mir::Body<'tcx>
                 >,
-            [decode] typeck_results: rustc_middle::ty::TypeckResults<$tcx>,
+            [decode] typeck_results: rustc_middle::ty::TypeckResults<'tcx>,
             [decode] borrowck_result:
-                rustc_middle::mir::BorrowCheckResult<$tcx>,
+                rustc_middle::mir::BorrowCheckResult<'tcx>,
             [decode] unsafety_check_result: rustc_middle::mir::UnsafetyCheckResult,
             [decode] code_region: rustc_middle::mir::coverage::CodeRegion,
             [] const_allocs: rustc_middle::mir::interpret::Allocation,
@@ -78,14 +78,14 @@ macro_rules! arena_types {
             [] foreign_modules: Vec<rustc_session::cstore::ForeignModule>,
             [] upvars_mentioned: rustc_data_structures::fx::FxIndexMap<rustc_hir::HirId, rustc_hir::Upvar>,
             [] object_safety_violations: rustc_middle::traits::ObjectSafetyViolation,
-            [] codegen_unit: rustc_middle::mir::mono::CodegenUnit<$tcx>,
+            [] codegen_unit: rustc_middle::mir::mono::CodegenUnit<'tcx>,
             [] attribute: rustc_ast::Attribute,
             [] name_set: rustc_data_structures::fx::FxHashSet<rustc_span::symbol::Symbol>,
             [] hir_id_set: rustc_hir::HirIdSet,
 
             // Interned types
-            [] tys: rustc_middle::ty::TyS<$tcx>,
-            [] predicates: rustc_middle::ty::PredicateInner<$tcx>,
+            [] tys: rustc_middle::ty::TyS<'tcx>,
+            [] predicates: rustc_middle::ty::PredicateInner<'tcx>,
 
             // Note that this deliberately duplicates items in the `rustc_hir::arena`,
             // since we need to allocate this type on both the `rustc_hir` arena
@@ -97,8 +97,8 @@ macro_rules! arena_types {
             [decode] used_trait_imports: rustc_data_structures::fx::FxHashSet<rustc_hir::def_id::LocalDefId>,
 
             [] dep_kind: rustc_middle::dep_graph::DepKindStruct,
-        ], $tcx);
+        ]);
     )
 }
 
-arena_types!(rustc_arena::declare_arena, 'tcx);
+arena_types!(rustc_arena::declare_arena);
diff --git a/compiler/rustc_middle/src/ty/codec.rs b/compiler/rustc_middle/src/ty/codec.rs
index 434008ecb1f4f..3f2b987b1e630 100644
--- a/compiler/rustc_middle/src/ty/codec.rs
+++ b/compiler/rustc_middle/src/ty/codec.rs
@@ -417,17 +417,17 @@ macro_rules! __impl_decoder_methods {
 macro_rules! impl_arena_allocatable_decoder {
     ([]$args:tt) => {};
     ([decode $(, $attrs:ident)*]
-     [[$name:ident: $ty:ty], $tcx:lifetime]) => {
-        impl<$tcx, D: TyDecoder<$tcx>> RefDecodable<$tcx, D> for $ty {
+     [$name:ident: $ty:ty]) => {
+        impl<'tcx, D: TyDecoder<'tcx>> RefDecodable<'tcx, D> for $ty {
             #[inline]
-            fn decode(decoder: &mut D) -> Result<&$tcx Self, D::Error> {
+            fn decode(decoder: &mut D) -> Result<&'tcx Self, D::Error> {
                 decode_arena_allocable(decoder)
             }
         }
 
-        impl<$tcx, D: TyDecoder<$tcx>> RefDecodable<$tcx, D> for [$ty] {
+        impl<'tcx, D: TyDecoder<'tcx>> RefDecodable<'tcx, D> for [$ty] {
             #[inline]
-            fn decode(decoder: &mut D) -> Result<&$tcx Self, D::Error> {
+            fn decode(decoder: &mut D) -> Result<&'tcx Self, D::Error> {
                 decode_arena_allocable_slice(decoder)
             }
         }
@@ -438,15 +438,15 @@ macro_rules! impl_arena_allocatable_decoder {
 }
 
 macro_rules! impl_arena_allocatable_decoders {
-    ([$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) => {
+    ([$($a:tt $name:ident: $ty:ty,)*]) => {
         $(
-            impl_arena_allocatable_decoder!($a [[$name: $ty], $tcx]);
+            impl_arena_allocatable_decoder!($a [$name: $ty]);
         )*
     }
 }
 
-rustc_hir::arena_types!(impl_arena_allocatable_decoders, 'tcx);
-arena_types!(impl_arena_allocatable_decoders, 'tcx);
+rustc_hir::arena_types!(impl_arena_allocatable_decoders);
+arena_types!(impl_arena_allocatable_decoders);
 
 #[macro_export]
 macro_rules! implement_ty_decoder {