add <[[T; N]]>::flatten, <[[T; N]]>::flatten_mut, and Vec::<[T; N]>::into_flattened

library/alloc/src/lib.rs

@@ -131,6 +131,7 @@
 #![feature(trusted_len)]
 #![feature(trusted_random_access)]
 #![feature(try_trait_v2)]
+#![feature(unchecked_math)]
 #![feature(unicode_internals)]
 #![feature(unsize)]
 //

library/alloc/src/vec/mod.rs

@@ -2274,6 +2274,51 @@ pub fn extend_from_within<R>(&mut self, src: R)
     }
 }
 
+impl<T, A: Allocator, const N: usize> Vec<[T; N], A> {
+    /// Takes a `Vec<[T; N]>` and flattens it into a `Vec<T>`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the length of the resulting vector would overflow a `usize`.
+    ///
+    /// This is only possible when flattening a vector of arrays of zero-sized
+    /// types, and thus tends to be irrelevant in practice. If
+    /// `size_of::<T>() > 0`, this will never panic.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(slice_flatten)]
+    ///
+    /// let mut vec = vec![[1, 2, 3], [4, 5, 6], [7, 8, 9]];
+    /// assert_eq!(vec.pop(), Some([7, 8, 9]));
+    ///
+    /// let mut flattened = vec.into_flattened();
+    /// assert_eq!(flattened.pop(), Some(6));
+    /// ```
+    #[unstable(feature = "slice_flatten", issue = "95629")]
+    pub fn into_flattened(self) -> Vec<T, A> {
+        let (ptr, len, cap, alloc) = self.into_raw_parts_with_alloc();
+        let (new_len, new_cap) = if mem::size_of::<T>() == 0 {
+            (len.checked_mul(N).expect("vec len overflow"), usize::MAX)
+        } else {
+            // SAFETY:
+            // - `cap * N` cannot overflow because the allocation is already in
+            // the address space.
+            // - Each `[T; N]` has `N` valid elements, so there are `len * N`
+            // valid elements in the allocation.
+            unsafe { (len.unchecked_mul(N), cap.unchecked_mul(N)) }
+        };
+        // SAFETY:
+        // - `ptr` was allocated by `self`
+        // - `ptr` is well-aligned because `[T; N]` has the same alignment as `T`.
+        // - `new_cap` refers to the same sized allocation as `cap` because
+        // `new_cap * size_of::<T>()` == `cap * size_of::<[T; N]>()`
+        // - `len` <= `cap`, so `len * N` <= `cap * N`.
+        unsafe { Vec::<T, A>::from_raw_parts_in(ptr.cast(), new_len, new_cap, alloc) }
+    }
+}
+
 // This code generalizes `extend_with_{element,default}`.
 trait ExtendWith<T> {
     fn next(&mut self) -> T;

library/alloc/tests/lib.rs

@@ -38,6 +38,7 @@
 #![feature(const_str_from_utf8)]
 #![feature(nonnull_slice_from_raw_parts)]
 #![feature(panic_update_hook)]
+#![feature(slice_flatten)]
 
 use std::collections::hash_map::DefaultHasher;
 use std::hash::{Hash, Hasher};

library/alloc/tests/vec.rs

@@ -2408,3 +2408,10 @@ fn drop(&mut self) {
 
     assert_eq!(count.load(Ordering::SeqCst), 4);
 }
+
+#[test]
+#[should_panic = "vec len overflow"]
+fn test_into_flattened_size_overflow() {
+    let v = vec![[(); usize::MAX]; 2];
+    let _ = v.into_flattened();
+}

library/core/src/slice/mod.rs

@@ -3994,6 +3994,88 @@ pub fn take_last_mut<'a>(self: &mut &'a mut Self) -> Option<&'a mut T> {
     }
 }
 
+#[cfg(not(bootstrap))]
+impl<T, const N: usize> [[T; N]] {
+    /// Takes a `&[[T; N]]`, and flattens it to a `&[T]`.
+    ///
+    /// # Panics
+    ///
+    /// This panics if the length of the resulting slice would overflow a `usize`.
+    ///
+    /// This is only possible when flattening a slice of arrays of zero-sized
+    /// types, and thus tends to be irrelevant in practice. If
+    /// `size_of::<T>() > 0`, this will never panic.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(slice_flatten)]
+    ///
+    /// assert_eq!([[1, 2, 3], [4, 5, 6]].flatten(), &[1, 2, 3, 4, 5, 6]);
+    ///
+    /// assert_eq!(
+    ///     [[1, 2, 3], [4, 5, 6]].flatten(),
+    ///     [[1, 2], [3, 4], [5, 6]].flatten(),
+    /// );
+    ///
+    /// let slice_of_empty_arrays: &[[i32; 0]] = &[[], [], [], [], []];
+    /// assert!(slice_of_empty_arrays.flatten().is_empty());
+    ///
+    /// let empty_slice_of_arrays: &[[u32; 10]] = &[];
+    /// assert!(empty_slice_of_arrays.flatten().is_empty());
+    /// ```
+    #[unstable(feature = "slice_flatten", issue = "95629")]
+    pub fn flatten(&self) -> &[T] {
+        let len = if crate::mem::size_of::<T>() == 0 {
+            self.len().checked_mul(N).expect("slice len overflow")
+        } else {
+            // SAFETY: `self.len() * N` cannot overflow because `self` is
+            // already in the address space.
+            unsafe { self.len().unchecked_mul(N) }
+        };
+        // SAFETY: `[T]` is layout-identical to `[T; N]`
+        unsafe { from_raw_parts(self.as_ptr().cast(), len) }
+    }
+
+    /// Takes a `&mut [[T; N]]`, and flattens it to a `&mut [T]`.
+    ///
+    /// # Panics
+    ///
+    /// This panics if the length of the resulting slice would overflow a `usize`.
+    ///
+    /// This is only possible when flattening a slice of arrays of zero-sized
+    /// types, and thus tends to be irrelevant in practice. If
+    /// `size_of::<T>() > 0`, this will never panic.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(slice_flatten)]
+    ///
+    /// fn add_5_to_all(slice: &mut [i32]) {
+    ///     for i in slice {
+    ///         *i += 5;
+    ///     }
+    /// }
+    ///
+    /// let mut array = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];
+    /// add_5_to_all(array.flatten_mut());
+    /// assert_eq!(array, [[6, 7, 8], [9, 10, 11], [12, 13, 14]]);
+    /// ```
+    #[unstable(feature = "slice_flatten", issue = "95629")]
+    pub fn flatten_mut(&mut self) -> &mut [T] {
+        let len = if crate::mem::size_of::<T>() == 0 {
+            self.len().checked_mul(N).expect("slice len overflow")
+        } else {
+            // SAFETY: `self.len() * N` cannot overflow because `self` is
+            // already in the address space.
+            unsafe { self.len().unchecked_mul(N) }
+        };
+        // SAFETY: `[T]` is layout-identical to `[T; N]`
+        unsafe { from_raw_parts_mut(self.as_mut_ptr().cast(), len) }
+    }
+}
+
 trait CloneFromSpec<T> {
     fn spec_clone_from(&mut self, src: &[T]);
 }

library/core/tests/lib.rs

@@ -93,6 +93,7 @@
 #![feature(const_array_from_ref)]
 #![feature(const_slice_from_ref)]
 #![feature(waker_getters)]
+#![feature(slice_flatten)]
 #![deny(unsafe_op_in_unsafe_fn)]
 
 extern crate test;

library/core/tests/slice.rs

@@ -2504,3 +2504,19 @@ fn test_slice_from_ptr_range() {
         assert_eq!(slice::from_ptr_range(range), &arr);
     }
 }
+
+#[test]
+#[cfg(not(bootstrap))]
+#[should_panic = "slice len overflow"]
+fn test_flatten_size_overflow() {
+    let x = &[[(); usize::MAX]; 2][..];
+    let _ = x.flatten();
+}
+
+#[test]
+#[cfg(not(bootstrap))]
+#[should_panic = "slice len overflow"]
+fn test_flatten_mut_size_overflow() {
+    let x = &mut [[(); usize::MAX]; 2][..];
+    let _ = x.flatten_mut();
+}