Rollup merge of rust-lang#65705 - shepmaster:vec-into-raw, r=SimonSapin

Add {String,Vec}::into_raw_parts

Aspects to address:

- [x] Create a tracking issue
  - rust-lang#65816

Author: JohnTitor

src/liballoc/string.rs

@@ -196,20 +196,21 @@ use crate::vec::Vec;
 ///
 /// let story = String::from("Once upon a time...");
 ///
-/// let ptr = story.as_ptr();
+// FIXME Update this when vec_into_raw_parts is stabilized
+/// // Prevent automatically dropping the String's data
+/// let mut story = mem::ManuallyDrop::new(story);
+///
+/// let ptr = story.as_mut_ptr();
 /// let len = story.len();
 /// let capacity = story.capacity();
 ///
 /// // story has nineteen bytes
 /// assert_eq!(19, len);
 ///
-/// // Now that we have our parts, we throw the story away.
-/// mem::forget(story);
-///
 /// // We can re-build a String out of ptr, len, and capacity. This is all
 /// // unsafe because we are responsible for making sure the components are
 /// // valid:
-/// let s = unsafe { String::from_raw_parts(ptr as *mut _, len, capacity) } ;
+/// let s = unsafe { String::from_raw_parts(ptr, len, capacity) } ;
 ///
 /// assert_eq!(String::from("Once upon a time..."), s);
 /// ```
@@ -647,6 +648,37 @@ impl String {
         decode_utf16(v.iter().cloned()).map(|r| r.unwrap_or(REPLACEMENT_CHARACTER)).collect()
     }
 
+    /// Decomposes a `String` into its raw components.
+    ///
+    /// Returns the raw pointer to the underlying data, the length of
+    /// the string (in bytes), and the allocated capacity of the data
+    /// (in bytes). These are the same arguments in the same order as
+    /// the arguments to [`from_raw_parts`].
+    ///
+    /// After calling this function, the caller is responsible for the
+    /// memory previously managed by the `String`. The only way to do
+    /// this is to convert the raw pointer, length, and capacity back
+    /// into a `String` with the [`from_raw_parts`] function, allowing
+    /// the destructor to perform the cleanup.
+    ///
+    /// [`from_raw_parts`]: #method.from_raw_parts
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(vec_into_raw_parts)]
+    /// let s = String::from("hello");
+    ///
+    /// let (ptr, len, cap) = s.into_raw_parts();
+    ///
+    /// let rebuilt = unsafe { String::from_raw_parts(ptr, len, cap) };
+    /// assert_eq!(rebuilt, "hello");
+    /// ```
+    #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
+    pub fn into_raw_parts(self) -> (*mut u8, usize, usize) {
+        self.vec.into_raw_parts()
+    }
+
     /// Creates a new `String` from a length, capacity, and pointer.
     ///
     /// # Safety
@@ -677,13 +709,16 @@ impl String {
     ///
     /// unsafe {
     ///     let s = String::from("hello");
-    ///     let ptr = s.as_ptr();
+    ///
+    // FIXME Update this when vec_into_raw_parts is stabilized
+    ///     // Prevent automatically dropping the String's data
+    ///     let mut s = mem::ManuallyDrop::new(s);
+    ///
+    ///     let ptr = s.as_mut_ptr();
     ///     let len = s.len();
     ///     let capacity = s.capacity();
     ///
-    ///     mem::forget(s);
-    ///
-    ///     let s = String::from_raw_parts(ptr as *mut _, len, capacity);
+    ///     let s = String::from_raw_parts(ptr, len, capacity);
     ///
     ///     assert_eq!(String::from("hello"), s);
     /// }

src/liballoc/vec.rs

@@ -358,6 +358,44 @@ impl<T> Vec<T> {
         }
     }
 
+    /// Decomposes a `Vec<T>` into its raw components.
+    ///
+    /// Returns the raw pointer to the underlying data, the length of
+    /// the vector (in elements), and the allocated capacity of the
+    /// data (in elements). These are the same arguments in the same
+    /// order as the arguments to [`from_raw_parts`].
+    ///
+    /// After calling this function, the caller is responsible for the
+    /// memory previously managed by the `Vec`. The only way to do
+    /// this is to convert the raw pointer, length, and capacity back
+    /// into a `Vec` with the [`from_raw_parts`] function, allowing
+    /// the destructor to perform the cleanup.
+    ///
+    /// [`from_raw_parts`]: #method.from_raw_parts
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(vec_into_raw_parts)]
+    /// let v: Vec<i32> = vec![-1, 0, 1];
+    ///
+    /// let (ptr, len, cap) = v.into_raw_parts();
+    ///
+    /// let rebuilt = unsafe {
+    ///     // We can now make changes to the components, such as
+    ///     // transmuting the raw pointer to a compatible type.
+    ///     let ptr = ptr as *mut u32;
+    ///
+    ///     Vec::from_raw_parts(ptr, len, cap)
+    /// };
+    /// assert_eq!(rebuilt, [4294967295, 0, 1]);
+    /// ```
+    #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
+    pub fn into_raw_parts(self) -> (*mut T, usize, usize) {
+        let mut me = mem::ManuallyDrop::new(self);
+        (me.as_mut_ptr(), me.len(), me.capacity())
+    }
+
     /// Creates a `Vec<T>` directly from the raw components of another vector.
     ///
     /// # Safety
@@ -389,18 +427,19 @@ impl<T> Vec<T> {
     /// use std::ptr;
     /// use std::mem;
     ///
-    /// let mut v = vec![1, 2, 3];
+    /// let v = vec![1, 2, 3];
+    ///
+    // FIXME Update this when vec_into_raw_parts is stabilized
+    /// // Prevent running `v`'s destructor so we are in complete control
+    /// // of the allocation.
+    /// let mut v = mem::ManuallyDrop::new(v);
     ///
     /// // Pull out the various important pieces of information about `v`
     /// let p = v.as_mut_ptr();
     /// let len = v.len();
     /// let cap = v.capacity();
     ///
     /// unsafe {
-    ///     // Cast `v` into the void: no destructor run, so we are in
-    ///     // complete control of the allocation to which `p` points.
-    ///     mem::forget(v);
-    ///
     ///     // Overwrite memory with 4, 5, 6
     ///     for i in 0..len as isize {
     ///         ptr::write(p.offset(i), 4 + i);

src/libcore/intrinsics.rs

@@ -874,6 +874,7 @@ extern "rust-intrinsic" {
     /// // the original inner type (`&i32`) to the converted inner type
     /// // (`Option<&i32>`), so read the nomicon pages linked above.
     /// let v_from_raw = unsafe {
+    // FIXME Update this when vec_into_raw_parts is stabilized
     ///     // Ensure the original vector is not dropped.
     ///     let mut v_clone = std::mem::ManuallyDrop::new(v_clone);
     ///     Vec::from_raw_parts(v_clone.as_mut_ptr() as *mut Option<&i32>,