Auto merge of #149079 - zachs18:clone_from_ref, r=Mark-Simulacrum

Add `Box::clone_from_ref` and similar under `feature(clone_from_ref)`

Tracking issue: #149075

Accepted ACP: rust-lang/libs-team#483

This PR implements `clone_from_ref` (and `try_*` and `_*in` variants), to get a `Box<T>`, `Arc<T>`, or `Rc<T>` by cloning from a `&T` where `T: CloneToUninit`.

The "Implement..." commits replace some ad-hoc conversions with `clone_from_ref` variants, which can be split out to a separate PR if desired.

This PR will conflict with #148769 due to usage of `Layout::dangling` (which that PR is renaming to `dangling_ptr`), so they should not be rolled up together, and the one which merges later will need to be amended.

Author: bors

library/alloc/src/boxed.rs

@@ -184,7 +184,6 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use core::borrow::{Borrow, BorrowMut};
-#[cfg(not(no_global_oom_handling))]
 use core::clone::CloneToUninit;
 use core::cmp::Ordering;
 use core::error::{self, Error};
@@ -733,6 +732,128 @@ impl<T, A: Allocator> Box<T, A> {
     }
 }
 
+impl<T: ?Sized + CloneToUninit> Box<T> {
+    /// Allocates memory on the heap then clones `src` into it.
+    ///
+    /// This doesn't actually allocate if `src` is zero-sized.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(clone_from_ref)]
+    ///
+    /// let hello: Box<str> = Box::clone_from_ref("hello");
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[unstable(feature = "clone_from_ref", issue = "149075")]
+    #[must_use]
+    #[inline]
+    pub fn clone_from_ref(src: &T) -> Box<T> {
+        Box::clone_from_ref_in(src, Global)
+    }
+
+    /// Allocates memory on the heap then clones `src` into it, returning an error if allocation fails.
+    ///
+    /// This doesn't actually allocate if `src` is zero-sized.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(clone_from_ref)]
+    /// #![feature(allocator_api)]
+    ///
+    /// let hello: Box<str> = Box::try_clone_from_ref("hello")?;
+    /// # Ok::<(), std::alloc::AllocError>(())
+    /// ```
+    #[unstable(feature = "clone_from_ref", issue = "149075")]
+    //#[unstable(feature = "allocator_api", issue = "32838")]
+    #[must_use]
+    #[inline]
+    pub fn try_clone_from_ref(src: &T) -> Result<Box<T>, AllocError> {
+        Box::try_clone_from_ref_in(src, Global)
+    }
+}
+
+impl<T: ?Sized + CloneToUninit, A: Allocator> Box<T, A> {
+    /// Allocates memory in the given allocator then clones `src` into it.
+    ///
+    /// This doesn't actually allocate if `src` is zero-sized.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(clone_from_ref)]
+    /// #![feature(allocator_api)]
+    ///
+    /// use std::alloc::System;
+    ///
+    /// let hello: Box<str, System> = Box::clone_from_ref_in("hello", System);
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[unstable(feature = "clone_from_ref", issue = "149075")]
+    //#[unstable(feature = "allocator_api", issue = "32838")]
+    #[must_use]
+    #[inline]
+    pub fn clone_from_ref_in(src: &T, alloc: A) -> Box<T, A> {
+        let layout = Layout::for_value::<T>(src);
+        match Box::try_clone_from_ref_in(src, alloc) {
+            Ok(bx) => bx,
+            Err(_) => handle_alloc_error(layout),
+        }
+    }
+
+    /// Allocates memory in the given allocator then clones `src` into it, returning an error if allocation fails.
+    ///
+    /// This doesn't actually allocate if `src` is zero-sized.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(clone_from_ref)]
+    /// #![feature(allocator_api)]
+    ///
+    /// use std::alloc::System;
+    ///
+    /// let hello: Box<str, System> = Box::try_clone_from_ref_in("hello", System)?;
+    /// # Ok::<(), std::alloc::AllocError>(())
+    /// ```
+    #[unstable(feature = "clone_from_ref", issue = "149075")]
+    //#[unstable(feature = "allocator_api", issue = "32838")]
+    #[must_use]
+    #[inline]
+    pub fn try_clone_from_ref_in(src: &T, alloc: A) -> Result<Box<T, A>, AllocError> {
+        struct DeallocDropGuard<'a, A: Allocator>(Layout, &'a A, NonNull<u8>);
+        impl<'a, A: Allocator> Drop for DeallocDropGuard<'a, A> {
+            fn drop(&mut self) {
+                let &mut DeallocDropGuard(layout, alloc, ptr) = self;
+                // Safety: `ptr` was allocated by `*alloc` with layout `layout`
+                unsafe {
+                    alloc.deallocate(ptr, layout);
+                }
+            }
+        }
+        let layout = Layout::for_value::<T>(src);
+        let (ptr, guard) = if layout.size() == 0 {
+            (layout.dangling(), None)
+        } else {
+            // Safety: layout is non-zero-sized
+            let ptr = alloc.allocate(layout)?.cast();
+            (ptr, Some(DeallocDropGuard(layout, &alloc, ptr)))
+        };
+        let ptr = ptr.as_ptr();
+        // Safety: `*ptr` is newly allocated, correctly aligned to `align_of_val(src)`,
+        // and is valid for writes for `size_of_val(src)`.
+        // If this panics, then `guard` will deallocate for us (if allocation occuured)
+        unsafe {
+            <T as CloneToUninit>::clone_to_uninit(src, ptr);
+        }
+        // Defuse the deallocate guard
+        core::mem::forget(guard);
+        // Safety: We just initialized `*ptr` as a clone of `src`
+        Ok(unsafe { Box::from_raw_in(ptr.with_metadata_of(src), alloc) })
+    }
+}
+
 impl<T> Box<[T]> {
     /// Constructs a new boxed slice with uninitialized contents.
     ///

library/alloc/src/boxed/convert.rs

@@ -1,21 +1,15 @@
 use core::any::Any;
-#[cfg(not(no_global_oom_handling))]
-use core::clone::TrivialClone;
 use core::error::Error;
+#[cfg(not(no_global_oom_handling))]
+use core::fmt;
 use core::mem;
 use core::pin::Pin;
-#[cfg(not(no_global_oom_handling))]
-use core::{fmt, ptr};
 
 use crate::alloc::Allocator;
 #[cfg(not(no_global_oom_handling))]
 use crate::borrow::Cow;
 use crate::boxed::Box;
 #[cfg(not(no_global_oom_handling))]
-use crate::raw_vec::RawVec;
-#[cfg(not(no_global_oom_handling))]
-use crate::str::from_boxed_utf8_unchecked;
-#[cfg(not(no_global_oom_handling))]
 use crate::string::String;
 #[cfg(not(no_global_oom_handling))]
 use crate::vec::Vec;
@@ -62,35 +56,6 @@ where
     }
 }
 
-/// Specialization trait used for `From<&[T]>`.
-#[cfg(not(no_global_oom_handling))]
-trait BoxFromSlice<T> {
-    fn from_slice(slice: &[T]) -> Self;
-}
-
-#[cfg(not(no_global_oom_handling))]
-impl<T: Clone> BoxFromSlice<T> for Box<[T]> {
-    #[inline]
-    default fn from_slice(slice: &[T]) -> Self {
-        slice.to_vec().into_boxed_slice()
-    }
-}
-
-#[cfg(not(no_global_oom_handling))]
-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()
-        }
-    }
-}
-
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "box_from_slice", since = "1.17.0")]
 impl<T: Clone> From<&[T]> for Box<[T]> {
@@ -109,7 +74,7 @@ impl<T: Clone> From<&[T]> for Box<[T]> {
     /// ```
     #[inline]
     fn from(slice: &[T]) -> Box<[T]> {
-        <Self as BoxFromSlice<T>>::from_slice(slice)
+        Box::clone_from_ref(slice)
     }
 }
 
@@ -170,7 +135,7 @@ impl From<&str> for Box<str> {
     /// ```
     #[inline]
     fn from(s: &str) -> Box<str> {
-        unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
+        Box::clone_from_ref(s)
     }
 }
 

library/alloc/src/ffi/c_str.rs

@@ -766,8 +766,7 @@ impl From<&CStr> for Box<CStr> {
     /// Converts a `&CStr` into a `Box<CStr>`,
     /// by copying the contents into a newly allocated [`Box`].
     fn from(s: &CStr) -> Box<CStr> {
-        let boxed: Box<[u8]> = Box::from(s.to_bytes_with_nul());
-        unsafe { Box::from_raw(Box::into_raw(boxed) as *mut CStr) }
+        Box::clone_from_ref(s)
     }
 }