(almost) get rid of the unsound #[rustc_unsafe_specialization_marker] on Copy, introduce TrivialClone

Author: joboet

library/alloc/src/boxed/convert.rs

@@ -1,4 +1,6 @@
 use core::any::Any;
+#[cfg(not(no_global_oom_handling))]
+use core::clone::TrivialClone;
 use core::error::Error;
 use core::mem;
 use core::pin::Pin;
@@ -75,11 +77,13 @@ impl<T: Clone> BoxFromSlice<T> for Box<[T]> {
 }
 
 #[cfg(not(no_global_oom_handling))]
-impl<T: Copy> BoxFromSlice<T> for Box<[T]> {
+impl<T: TrivialClone> BoxFromSlice<T> for Box<[T]> {
     #[inline]
     fn from_slice(slice: &[T]) -> Self {
         let len = slice.len();
         let buf = RawVec::with_capacity(len);
+        // SAFETY: since `T` implements `TrivialClone`, this is sound and
+        // equivalent to the above.
         unsafe {
             ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
             buf.into_box(slice.len()).assume_init()

library/alloc/src/collections/vec_deque/mod.rs

@@ -7,6 +7,8 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+#[cfg(not(no_global_oom_handling))]
+use core::clone::TrivialClone;
 use core::cmp::{self, Ordering};
 use core::hash::{Hash, Hasher};
 use core::iter::{ByRefSized, repeat_n, repeat_with};
@@ -3419,7 +3421,7 @@ impl<T: Clone, A: Allocator> SpecExtendFromWithin for VecDeque<T, A> {
 }
 
 #[cfg(not(no_global_oom_handling))]
-impl<T: Copy, A: Allocator> SpecExtendFromWithin for VecDeque<T, A> {
+impl<T: TrivialClone, A: Allocator> SpecExtendFromWithin for VecDeque<T, A> {
     unsafe fn spec_extend_from_within(&mut self, src: Range<usize>) {
         let dst = self.len();
         let count = src.end - src.start;

library/alloc/src/lib.rs

@@ -146,6 +146,7 @@
 #![feature(std_internals)]
 #![feature(str_internals)]
 #![feature(temporary_niche_types)]
+#![feature(trivial_clone)]
 #![feature(trusted_fused)]
 #![feature(trusted_len)]
 #![feature(trusted_random_access)]

library/alloc/src/rc.rs

@@ -243,9 +243,9 @@
 
 use core::any::Any;
 use core::cell::{Cell, CloneFromCell};
-#[cfg(not(no_global_oom_handling))]
-use core::clone::CloneToUninit;
 use core::clone::UseCloned;
+#[cfg(not(no_global_oom_handling))]
+use core::clone::{CloneToUninit, TrivialClone};
 use core::cmp::Ordering;
 use core::hash::{Hash, Hasher};
 use core::intrinsics::abort;
@@ -2224,7 +2224,8 @@ impl<T> Rc<[T]> {
 
     /// Copy elements from slice into newly allocated `Rc<[T]>`
     ///
-    /// Unsafe because the caller must either take ownership or bind `T: Copy`
+    /// Unsafe because the caller must either take ownership, bind `T: Copy` or
+    /// bind `T: TrivialClone`.
     #[cfg(not(no_global_oom_handling))]
     unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
         unsafe {
@@ -2314,9 +2315,11 @@ impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
 }
 
 #[cfg(not(no_global_oom_handling))]
-impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
+impl<T: TrivialClone> RcFromSlice<T> for Rc<[T]> {
     #[inline]
     fn from_slice(v: &[T]) -> Self {
+        // SAFETY: `T` implements `TrivialClone`, so this is sound and equivalent
+        // to the above.
         unsafe { Rc::copy_from_slice(v) }
     }
 }

library/alloc/src/slice.rs

@@ -11,6 +11,8 @@
 
 use core::borrow::{Borrow, BorrowMut};
 #[cfg(not(no_global_oom_handling))]
+use core::clone::TrivialClone;
+#[cfg(not(no_global_oom_handling))]
 use core::cmp::Ordering::{self, Less};
 #[cfg(not(no_global_oom_handling))]
 use core::mem::MaybeUninit;
@@ -439,7 +441,7 @@ impl<T> [T] {
             }
         }
 
-        impl<T: Copy> ConvertVec for T {
+        impl<T: TrivialClone> ConvertVec for T {
             #[inline]
             fn to_vec<A: Allocator>(s: &[Self], alloc: A) -> Vec<Self, A> {
                 let mut v = Vec::with_capacity_in(s.len(), alloc);
@@ -822,7 +824,7 @@ impl<T: Clone, A: Allocator> SpecCloneIntoVec<T, A> for [T] {
 }
 
 #[cfg(not(no_global_oom_handling))]
-impl<T: Copy, A: Allocator> SpecCloneIntoVec<T, A> for [T] {
+impl<T: TrivialClone, A: Allocator> SpecCloneIntoVec<T, A> for [T] {
     fn clone_into(&self, target: &mut Vec<T, A>) {
         target.clear();
         target.extend_from_slice(self);

library/alloc/src/sync.rs

@@ -12,6 +12,8 @@ use core::any::Any;
 use core::cell::CloneFromCell;
 #[cfg(not(no_global_oom_handling))]
 use core::clone::CloneToUninit;
+#[cfg(not(no_global_oom_handling))]
+use core::clone::TrivialClone;
 use core::clone::UseCloned;
 use core::cmp::Ordering;
 use core::hash::{Hash, Hasher};
@@ -2156,7 +2158,8 @@ impl<T> Arc<[T]> {
 
     /// Copy elements from slice into newly allocated `Arc<[T]>`
     ///
-    /// Unsafe because the caller must either take ownership or bind `T: Copy`.
+    /// Unsafe because the caller must either take ownership, bind `T: Copy` or
+    /// bind `T: TrivialClone`.
     #[cfg(not(no_global_oom_handling))]
     unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
         unsafe {
@@ -2248,9 +2251,11 @@ impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
 }
 
 #[cfg(not(no_global_oom_handling))]
-impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
+impl<T: TrivialClone> ArcFromSlice<T> for Arc<[T]> {
     #[inline]
     fn from_slice(v: &[T]) -> Self {
+        // SAFETY: `T` implements `TrivialClone`, so this is sound and equivalent
+        // to the above.
         unsafe { Arc::copy_from_slice(v) }
     }
 }

library/alloc/src/vec/mod.rs

@@ -73,6 +73,8 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+#[cfg(not(no_global_oom_handling))]
+use core::clone::TrivialClone;
 #[cfg(not(no_global_oom_handling))]
 use core::cmp;
 use core::cmp::Ordering;
@@ -3494,7 +3496,7 @@ impl<T: Clone, A: Allocator> ExtendFromWithinSpec for Vec<T, A> {
 }
 
 #[cfg(not(no_global_oom_handling))]
-impl<T: Copy, A: Allocator> ExtendFromWithinSpec for Vec<T, A> {
+impl<T: TrivialClone, A: Allocator> ExtendFromWithinSpec for Vec<T, A> {
     unsafe fn spec_extend_from_within(&mut self, src: Range<usize>) {
         let count = src.len();
         {
@@ -3507,8 +3509,8 @@ impl<T: Copy, A: Allocator> ExtendFromWithinSpec for Vec<T, A> {
             // SAFETY:
             // - Both pointers are created from unique slice references (`&mut [_]`)
             //   so they are valid and do not overlap.
-            // - Elements are :Copy so it's OK to copy them, without doing
-            //   anything with the original values
+            // - Elements implement `TrivialClone` so this is equivalent to calling
+            //   `clone` on every one of them.
             // - `count` is equal to the len of `source`, so source is valid for
             //   `count` reads
             // - `.reserve(count)` guarantees that `spare.len() >= count` so spare

library/alloc/src/vec/spec_extend.rs

@@ -1,3 +1,4 @@
+use core::clone::TrivialClone;
 use core::iter::TrustedLen;
 use core::slice::{self};
 
@@ -48,7 +49,7 @@ where
 
 impl<'a, T: 'a, A: Allocator> SpecExtend<&'a T, slice::Iter<'a, T>> for Vec<T, A>
 where
-    T: Copy,
+    T: TrivialClone,
 {
     fn spec_extend(&mut self, iterator: slice::Iter<'a, T>) {
         let slice = iterator.as_slice();

library/alloctests/lib.rs

@@ -40,6 +40,7 @@
 #![feature(slice_range)]
 #![feature(std_internals)]
 #![feature(temporary_niche_types)]
+#![feature(trivial_clone)]
 #![feature(trusted_fused)]
 #![feature(trusted_len)]
 #![feature(trusted_random_access)]

library/core/src/array/mod.rs

@@ -5,10 +5,10 @@
 #![stable(feature = "core_array", since = "1.35.0")]
 
 use crate::borrow::{Borrow, BorrowMut};
+use crate::clone::TrivialClone;
 use crate::cmp::Ordering;
 use crate::convert::Infallible;
 use crate::error::Error;
-use crate::fmt;
 use crate::hash::{self, Hash};
 use crate::intrinsics::transmute_unchecked;
 use crate::iter::{UncheckedIterator, repeat_n};
@@ -18,6 +18,7 @@ use crate::ops::{
 };
 use crate::ptr::{null, null_mut};
 use crate::slice::{Iter, IterMut};
+use crate::{fmt, ptr};
 
 mod ascii;
 mod drain;
@@ -451,6 +452,9 @@ impl<T: Clone, const N: usize> Clone for [T; N] {
     }
 }
 
+#[unstable(feature = "trivial_clone", issue = "none")]
+unsafe impl<T: TrivialClone, const N: usize> TrivialClone for [T; N] {}
+
 trait SpecArrayClone: Clone {
     fn clone<const N: usize>(array: &[Self; N]) -> [Self; N];
 }
@@ -462,10 +466,12 @@ impl<T: Clone> SpecArrayClone for T {
     }
 }
 
-impl<T: Copy> SpecArrayClone for T {
+impl<T: TrivialClone> SpecArrayClone for T {
     #[inline]
     fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
-        *array
+        // SAFETY: `TrivialClone` implies that this is equivalent to calling
+        // `Clone` on every element.
+        unsafe { ptr::read(array) }
     }
 }