Manually initialize GcBox contents post-allocation to reduce memory copying

src/gc-arena-derive/src/lib.rs

@@ -162,7 +162,8 @@ fn collect_derive(mut s: synstructure::Structure) -> TokenStream {
                 let ty = &b.ast().ty;
                 quote_spanned!(b.ast().span()=>
                     || <#ty as gc_arena::Collect>::needs_trace()
-                ).to_tokens(&mut needs_trace_body);
+                )
+                .to_tokens(&mut needs_trace_body);
             }
         }
         // Likewise, this will skip any fields that have `#[collect(require_static)]`

src/gc-arena/src/collect_impl.rs

@@ -1,14 +1,14 @@
 use alloc::boxed::Box;
+use alloc::collections::VecDeque;
 use alloc::collections::{BTreeMap, BTreeSet};
 use alloc::rc::Rc;
 use alloc::string::String;
 use alloc::sync::Arc;
 use alloc::vec::Vec;
-use alloc::collections::VecDeque;
 use core::cell::{Cell, RefCell};
-use core::marker::PhantomData;
 #[cfg(feature = "std")]
 use core::hash::{BuildHasher, Hash};
+use core::marker::PhantomData;
 #[cfg(feature = "std")]
 use std::collections::{HashMap, HashSet};
 
@@ -271,8 +271,7 @@ where
 }
 
 // SAFETY: `PhantomData` is a ZST, and therefore doesn't store anything
-unsafe impl<T> Collect for PhantomData<T>
-{
+unsafe impl<T> Collect for PhantomData<T> {
     #[inline]
     fn needs_trace() -> bool {
         false

src/gc-arena/src/context.rs

@@ -264,13 +264,20 @@ impl Context {
             );
         }
 
-        let gc_box = GcBox {
-            flags: GcFlags::new(),
-            next: Cell::new(self.all.get()),
-            value: UnsafeCell::new(t),
-        };
-        gc_box.flags.set_needs_trace(T::needs_trace());
-        let ptr = NonNull::new_unchecked(Box::into_raw(Box::new(gc_box)));
+        let flags = GcFlags::new();
+        flags.set_needs_trace(T::needs_trace());
+
+        let mut uninitialized = Box::new(mem::MaybeUninit::<GcBox<T>>::uninit());
+
+        core::ptr::write(&mut (*uninitialized.as_mut_ptr()).flags, flags);
+        core::ptr::write(
+            &mut (*uninitialized.as_mut_ptr()).next,
+            Cell::new(self.all.get()),
+        );
+        core::ptr::write(&mut (*uninitialized.as_mut_ptr()).value, UnsafeCell::new(t));
+
+        let ptr = NonNull::new_unchecked(Box::into_raw(uninitialized) as *mut GcBox<T>);
+
         self.all.set(Some(static_gc_box(ptr)));
         if self.phase.get() == Phase::Sweep && self.sweep_prev.get().is_none() {
             self.sweep_prev.set(self.all.get());
@@ -317,6 +324,7 @@ enum Phase {
     Sleep,
 }
 
+#[inline]
 unsafe fn static_gc_box<'gc>(
     ptr: NonNull<GcBox<dyn Collect + 'gc>>,
 ) -> NonNull<GcBox<dyn Collect>> {

src/gc-arena/src/types.rs

@@ -20,10 +20,12 @@ pub(crate) struct GcBox<T: Collect + ?Sized> {
 pub(crate) struct GcFlags(Cell<u8>);
 
 impl GcFlags {
+    #[inline]
     pub(crate) fn new() -> GcFlags {
         GcFlags(Cell::new(0))
     }
 
+    #[inline]
     pub(crate) fn color(&self) -> GcColor {
         match self.0.get() & 0x3 {
             0x0 => GcColor::White,
@@ -33,6 +35,7 @@ impl GcFlags {
         }
     }
 
+    #[inline]
     pub(crate) fn set_color(&self, color: GcColor) {
         self.0.set(
             (self.0.get() & !0x3)
@@ -44,10 +47,12 @@ impl GcFlags {
         )
     }
 
+    #[inline]
     pub(crate) fn needs_trace(&self) -> bool {
         self.0.get() & 0x4 != 0x0
     }
 
+    #[inline]
     pub(crate) fn set_needs_trace(&self, needs_trace: bool) {
         self.0
             .set((self.0.get() & !0x4) | if needs_trace { 0x4 } else { 0x0 });