Refactor Rc::default

Author: EFanZh

library/alloc/src/raw_rc.rs

@@ -964,6 +964,37 @@ impl<T, A> RawRc<T, A> {
         unsafe { Self::from_weak_with_value(RawWeak::new_uninit_in::<1>(alloc), value) }
     }
 
+    #[cfg(not(no_global_oom_handling))]
+    pub fn new_with<F>(f: F) -> Self
+    where
+        A: Allocator + Default,
+        F: FnOnce() -> T,
+    {
+        struct Guard<'a, T, A>
+        where
+            A: Allocator,
+        {
+            weak: &'a mut RawWeak<T, A>,
+        }
+
+        impl<'a, T, A> Drop for Guard<'a, T, A>
+        where
+            A: Allocator,
+        {
+            fn drop(&mut self) {
+                unsafe { self.weak.deallocate() };
+            }
+        }
+
+        let mut weak = RawWeak::new_uninit::<1>();
+        let guard = Guard { weak: &mut weak };
+        let value = f();
+
+        mem::forget(guard);
+
+        unsafe { Self::from_weak_with_value(weak, value) }
+    }
+
     #[cfg(not(no_global_oom_handling))]
     pub unsafe fn new_cyclic<F, R>(data_fn: F) -> Self
     where
@@ -1283,212 +1314,14 @@ where
     }
 }
 
-#[cfg(not(no_global_oom_handling))]
-trait SpecRcDefault {
-    fn spec_default() -> Self;
-}
-
-#[cfg(not(no_global_oom_handling))]
-impl<T, A> SpecRcDefault for RawRc<T, A>
-where
-    T: Default,
-    A: Allocator + Default,
-{
-    default fn spec_default() -> Self {
-        Self::new(T::default())
-    }
-}
-
-#[cfg(not(no_global_oom_handling))]
-impl<T> SpecRcDefault for RawRc<T, Global>
-where
-    T: Default,
-{
-    fn spec_default() -> Self {
-        fn spec_default_impl<T, const N: usize>() -> RawRc<T, Global>
-        where
-            T: Default,
-        {
-            #[repr(C)]
-            struct RcAllocation<T, const N: usize> {
-                _padding: MaybeUninit<[RefCounts; N]>,
-                _ref_counts: RefCounts,
-                _value: T,
-            }
-
-            impl<T, const N: usize> Default for RcAllocation<T, N>
-            where
-                T: Default,
-            {
-                fn default() -> Self {
-                    Self {
-                        _padding: MaybeUninit::uninit(),
-                        _ref_counts: RefCounts::new(1),
-                        _value: T::default(),
-                    }
-                }
-            }
-
-            const {
-                assert!(
-                    RefCounts::LAYOUT.size().checked_mul(N).unwrap()
-                        == RefCounts::LAYOUT.padding_needed_for(T::LAYOUT.align())
-                );
-            };
-
-            let (allocation_ptr, alloc) =
-                Box::into_non_null_with_allocator(Box::<RcAllocation<T, N>>::default());
-
-            unsafe {
-                RawRc::from_raw_parts(
-                    allocation_ptr.byte_add(T::RC_LAYOUT.allocation_offset_bytes).cast(),
-                    alloc,
-                )
-            }
-        }
-
-        macro_rules! select_impl {
-            ($($value:literal,)*) => {
-                match RefCounts::LAYOUT.padding_needed_for(T::LAYOUT.align()) / RefCounts::LAYOUT.size() {
-                    $($value => spec_default_impl::<T, $value>,)*
-                    _ => panic!("invalid padding"),
-                }
-            };
-        }
-
-        #[cfg(target_pointer_width = "16")]
-        let selected_impl: fn() -> Self = const {
-            select_impl![
-                0x0000, // (1 << 0) - 1
-                0x0001, // (1 << 1) - 1
-                0x0003, // (1 << 2) - 1
-                0x0007, // (1 << 3) - 1
-                0x000f, // (1 << 4) - 1
-                0x001f, // (1 << 5) - 1
-                0x003f, // (1 << 6) - 1
-                0x007f, // (1 << 7) - 1
-                0x00ff, // (1 << 8) - 1
-                0x01ff, // (1 << 9) - 1
-                0x03ff, // (1 << 10) - 1
-                0x07ff, // (1 << 11) - 1
-                0x0fff, // (1 << 12) - 1
-                0x1fff, // (1 << 13) - 1
-            ]
-        };
-
-        #[cfg(target_pointer_width = "32")]
-        let selected_impl: fn() -> Self = const {
-            select_impl![
-                0x00000000, // (1 << 0) - 1
-                0x00000001, // (1 << 1) - 1
-                0x00000003, // (1 << 2) - 1
-                0x00000007, // (1 << 3) - 1
-                0x0000000f, // (1 << 4) - 1
-                0x0000001f, // (1 << 5) - 1
-                0x0000003f, // (1 << 6) - 1
-                0x0000007f, // (1 << 7) - 1
-                0x000000ff, // (1 << 8) - 1
-                0x000001ff, // (1 << 9) - 1
-                0x000003ff, // (1 << 10) - 1
-                0x000007ff, // (1 << 11) - 1
-                0x00000fff, // (1 << 12) - 1
-                0x00001fff, // (1 << 13) - 1
-                0x00003fff, // (1 << 14) - 1
-                0x00007fff, // (1 << 15) - 1
-                0x0000ffff, // (1 << 16) - 1
-                0x0001ffff, // (1 << 17) - 1
-                0x0003ffff, // (1 << 18) - 1
-                0x0007ffff, // (1 << 19) - 1
-                0x000fffff, // (1 << 20) - 1
-                0x001fffff, // (1 << 21) - 1
-                0x003fffff, // (1 << 22) - 1
-                0x007fffff, // (1 << 23) - 1
-                0x00ffffff, // (1 << 24) - 1
-                0x01ffffff, // (1 << 25) - 1
-                0x03ffffff, // (1 << 26) - 1
-                0x07ffffff, // (1 << 27) - 1
-                0x0fffffff, // (1 << 28) - 1
-            ]
-        };
-
-        #[cfg(target_pointer_width = "64")]
-        let selected_impl: fn() -> Self = const {
-            select_impl![
-                0x0000000000000000, // (1 << 0) - 1
-                0x0000000000000001, // (1 << 1) - 1
-                0x0000000000000003, // (1 << 2) - 1
-                0x0000000000000007, // (1 << 3) - 1
-                0x000000000000000f, // (1 << 4) - 1
-                0x000000000000001f, // (1 << 5) - 1
-                0x000000000000003f, // (1 << 6) - 1
-                0x000000000000007f, // (1 << 7) - 1
-                0x00000000000000ff, // (1 << 8) - 1
-                0x00000000000001ff, // (1 << 9) - 1
-                0x00000000000003ff, // (1 << 10) - 1
-                0x00000000000007ff, // (1 << 11) - 1
-                0x0000000000000fff, // (1 << 12) - 1
-                0x0000000000001fff, // (1 << 13) - 1
-                0x0000000000003fff, // (1 << 14) - 1
-                0x0000000000007fff, // (1 << 15) - 1
-                0x000000000000ffff, // (1 << 16) - 1
-                0x000000000001ffff, // (1 << 17) - 1
-                0x000000000003ffff, // (1 << 18) - 1
-                0x000000000007ffff, // (1 << 19) - 1
-                0x00000000000fffff, // (1 << 20) - 1
-                0x00000000001fffff, // (1 << 21) - 1
-                0x00000000003fffff, // (1 << 22) - 1
-                0x00000000007fffff, // (1 << 23) - 1
-                0x0000000000ffffff, // (1 << 24) - 1
-                0x0000000001ffffff, // (1 << 25) - 1
-                0x0000000003ffffff, // (1 << 26) - 1
-                0x0000000007ffffff, // (1 << 27) - 1
-                0x000000000fffffff, // (1 << 28) - 1
-                0x000000001fffffff, // (1 << 29) - 1
-                0x000000003fffffff, // (1 << 30) - 1
-                0x000000007fffffff, // (1 << 31) - 1
-                0x00000000ffffffff, // (1 << 32) - 1
-                0x00000001ffffffff, // (1 << 33) - 1
-                0x00000003ffffffff, // (1 << 34) - 1
-                0x00000007ffffffff, // (1 << 35) - 1
-                0x0000000fffffffff, // (1 << 36) - 1
-                0x0000001fffffffff, // (1 << 37) - 1
-                0x0000003fffffffff, // (1 << 38) - 1
-                0x0000007fffffffff, // (1 << 39) - 1
-                0x000000ffffffffff, // (1 << 40) - 1
-                0x000001ffffffffff, // (1 << 41) - 1
-                0x000003ffffffffff, // (1 << 42) - 1
-                0x000007ffffffffff, // (1 << 43) - 1
-                0x00000fffffffffff, // (1 << 44) - 1
-                0x00001fffffffffff, // (1 << 45) - 1
-                0x00003fffffffffff, // (1 << 46) - 1
-                0x00007fffffffffff, // (1 << 47) - 1
-                0x0000ffffffffffff, // (1 << 48) - 1
-                0x0001ffffffffffff, // (1 << 49) - 1
-                0x0003ffffffffffff, // (1 << 50) - 1
-                0x0007ffffffffffff, // (1 << 51) - 1
-                0x000fffffffffffff, // (1 << 52) - 1
-                0x001fffffffffffff, // (1 << 53) - 1
-                0x003fffffffffffff, // (1 << 54) - 1
-                0x007fffffffffffff, // (1 << 55) - 1
-                0x00ffffffffffffff, // (1 << 56) - 1
-                0x01ffffffffffffff, // (1 << 57) - 1
-                0x03ffffffffffffff, // (1 << 58) - 1
-                0x07ffffffffffffff, // (1 << 59) - 1
-            ]
-        };
-
-        selected_impl()
-    }
-}
-
 #[cfg(not(no_global_oom_handling))]
 impl<T, A> Default for RawRc<T, A>
 where
     T: Default,
     A: Allocator + Default,
 {
     fn default() -> Self {
-        Self::spec_default()
+        Self::new_with(T::default)
     }
 }