diff options
| -rw-r--r-- | compiler/rustc_arena/src/lib.rs | 28 |
1 files changed, 21 insertions, 7 deletions
diff --git a/compiler/rustc_arena/src/lib.rs b/compiler/rustc_arena/src/lib.rs index fb93fa152ac..d36e357f58f 100644 --- a/compiler/rustc_arena/src/lib.rs +++ b/compiler/rustc_arena/src/lib.rs @@ -125,6 +125,11 @@ impl<I, T> IterExt<T> for I where I: IntoIterator<Item = T>, { + // This default collects into a `SmallVec` and then allocates by copying + // from it. The specializations below for types like `Vec` are more + // efficient, copying directly without the intermediate collecting step. + // This default could be made more efficient, like + // `DroplessArena::alloc_from_iter`, but it's not hot enough to bother. #[inline] default fn alloc_from_iter(self, arena: &TypedArena<T>) -> &mut [T] { let vec: SmallVec<[_; 8]> = self.into_iter().collect(); @@ -139,7 +144,7 @@ impl<T, const N: usize> IterExt<T> for std::array::IntoIter<T, N> { if len == 0 { return &mut []; } - // Move the content to the arena by copying and then forgetting it + // Move the content to the arena by copying and then forgetting it. unsafe { let start_ptr = arena.alloc_raw_slice(len); self.as_slice().as_ptr().copy_to_nonoverlapping(start_ptr, len); @@ -156,7 +161,7 @@ impl<T> IterExt<T> for Vec<T> { if len == 0 { return &mut []; } - // Move the content to the arena by copying and then forgetting it + // Move the content to the arena by copying and then forgetting it. unsafe { let start_ptr = arena.alloc_raw_slice(len); self.as_ptr().copy_to_nonoverlapping(start_ptr, len); @@ -173,7 +178,7 @@ impl<A: smallvec::Array> IterExt<A::Item> for SmallVec<A> { if len == 0 { return &mut []; } - // Move the content to the arena by copying and then forgetting it + // Move the content to the arena by copying and then forgetting it. unsafe { let start_ptr = arena.alloc_raw_slice(len); self.as_ptr().copy_to_nonoverlapping(start_ptr, len); @@ -520,10 +525,19 @@ 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`. +/// Declare an `Arena` containing one dropless arena and many typed arenas (the +/// types of the typed arenas are specified by the arguments). +/// +/// 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. +/// #[rustc_macro_transparency = "semitransparent"] pub macro declare_arena([$($a:tt $name:ident: $ty:ty,)*]) { #[derive(Default)] |