Rollup merge of #159955 - Zalathar:arena, r=mejrs

Stop using higher-order macros to declare arenas

The arenas used by `rustc_hir` and `rustc_middle` are declared via macros. But instead of invoking `rustc_arena::declare_arena!` directly, those crates declare a higher-order macro containing a list of types, and then invoke that macro with `declare_arena!` as an argument.

From what I can tell, the only reason for this arrangement is so that the list of types can also contain `[decode]` modifiers that produce a corresponding implementation of `RefDecodable` that decodes into the arena. But the number of types involved is small enough that it's easy to list them separately, and the advantage of doing so is that we can remove a layer of macro indirection, making it easier to navigate to the real code.

There should be no change to compiler behaviour.

Author: JonathanBrouwer

compiler/rustc_arena/src/lib.rs

@@ -598,21 +598,32 @@ impl DroplessArena {
     }
 }
 
-/// Declare an `Arena` containing one dropless arena and many typed arenas (the
-/// types of the typed arenas are specified by the arguments).
+/// Declares an `Arena` that can allocate values of a variety of `Copy`, `needs_drop` and
+/// `!needs_drop` types.
 ///
-/// There are three cases of interest.
-/// - Types that are `Copy`: these need not be specified in the arguments. They
-///   will use the `DroplessArena`.
-/// - Types that are `!Copy` and `!Drop`: these must be specified in the
-///   arguments. An empty `TypedArena` will be created for each one, but the
-///   `DroplessArena` will always be used and the `TypedArena` will stay empty.
-///   This is odd but harmless, because an empty arena allocates no memory.
-/// - Types that are `!Copy` and `Drop`: these must be specified in the
-///   arguments. The `TypedArena` will be used for them.
+/// The declared arena actually contains a single [`DroplessArena`], plus a separate
+/// [`TypedArena`] for each of the types listed in the body of the macro invocation.
 ///
+/// Any type that is `Copy` can be allocated in the arena without needing to be listed
+/// explicitly. Those values will be stored in the [`DroplessArena`].
+///
+/// Types that are `!Copy` can only be allocated if they are listed in the macro invocation.
+/// For types that are `!Copy + needs_drop`, values will be stored in the corresponding
+/// [`TypedArena`] and will be dropped when the arena is dropped.
+///
+/// As an optimization, types that are `!Copy + !needs_drop` will actually be stored in the
+/// [`DroplessArena`], and the corresponding [`TypedArena`] will remain empty. This makes
+/// better use of the dropless arena's storage blocks, while the overhead of having a few
+/// unused typed-arenas is negligible.
 #[rustc_macro_transparency = "semiopaque"]
-pub macro declare_arena([$($a:tt $name:ident: $ty:ty,)*]) {
+pub macro declare_arena(
+    // Each of these entries becomes a `$name: TypedArena<$ty>` field in the arena.
+    // This allows values of non-copy type $ty to be allocated in the arena.
+    // The field names must be distinct, but have no further significance.
+    $(
+        $name:ident: $ty:ty,
+    )*
+) {
     #[derive(Default)]
     pub struct Arena<'tcx> {
         pub dropless: $crate::DroplessArena,

compiler/rustc_hir/src/arena.rs

@@ -1,15 +1,11 @@
-/// This higher-order macro declares a list of types which can be allocated by `Arena`.
-/// Note that all `Copy` types can be allocated by default and need not be specified here.
-#[macro_export]
-macro_rules! arena_types {
-    ($macro:path) => (
-        $macro!([
-            // HIR types
-            [] asm_template: rustc_ast::InlineAsmTemplatePiece,
-            [] attribute: crate::Attribute,
-            [] owner_info: crate::OwnerInfo<'tcx>,
-            [] macro_def: rustc_ast::MacroDef,
-            [] delegation_info: crate::DelegationInfo,
-        ]);
-    )
+//! Declares an arena that can allocate values of any `Copy` type, and of
+//! any `!Copy` type listed below.
+
+rustc_arena::declare_arena! {
+    // HIR types
+    asm_template: rustc_ast::InlineAsmTemplatePiece,
+    attribute: crate::Attribute,
+    owner_info: crate::OwnerInfo<'tcx>,
+    macro_def: rustc_ast::MacroDef,
+    delegation_info: crate::DelegationInfo,
 }

compiler/rustc_hir/src/lib.rs

@@ -42,4 +42,4 @@ pub use rustc_span::def_id;
 pub use stability::*;
 pub use target::{MethodKind, Target};
 
-arena_types!(rustc_arena::declare_arena);
+pub use crate::arena::Arena;