Rollup merge of rust-lang#125171 - scottmcm:rename-flatten, r=jhpratt

Rename `flatten(_mut)` → `as_flattened(_mut)`

As requested by libs-api in rust-lang#95629 (comment)

(This is just the rename, not the stabilization, so can land without waiting on the FCP in that other issue.)

Author: matthiaskrgr

Diff for: core/src/slice/mod.rs

@@ -4533,21 +4533,21 @@ impl<T, const N: usize> [[T; N]] {
     /// ```
     /// #![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]].as_flattened(), &[1, 2, 3, 4, 5, 6]);
     ///
     /// assert_eq!(
-    ///     [[1, 2, 3], [4, 5, 6]].flatten(),
-    ///     [[1, 2], [3, 4], [5, 6]].flatten(),
+    ///     [[1, 2, 3], [4, 5, 6]].as_flattened(),
+    ///     [[1, 2], [3, 4], [5, 6]].as_flattened(),
     /// );
     ///
     /// let slice_of_empty_arrays: &[[i32; 0]] = &[[], [], [], [], []];
-    /// assert!(slice_of_empty_arrays.flatten().is_empty());
+    /// assert!(slice_of_empty_arrays.as_flattened().is_empty());
     ///
     /// let empty_slice_of_arrays: &[[u32; 10]] = &[];
-    /// assert!(empty_slice_of_arrays.flatten().is_empty());
+    /// assert!(empty_slice_of_arrays.as_flattened().is_empty());
     /// ```
     #[unstable(feature = "slice_flatten", issue = "95629")]
-    pub const fn flatten(&self) -> &[T] {
+    pub const fn as_flattened(&self) -> &[T] {
         let len = if T::IS_ZST {
             self.len().checked_mul(N).expect("slice len overflow")
         } else {
@@ -4581,11 +4581,11 @@ impl<T, const N: usize> [[T; N]] {
     /// }
     ///
     /// let mut array = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];
-    /// add_5_to_all(array.flatten_mut());
+    /// add_5_to_all(array.as_flattened_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] {
+    pub fn as_flattened_mut(&mut self) -> &mut [T] {
         let len = if T::IS_ZST {
             self.len().checked_mul(N).expect("slice len overflow")
         } else {

Diff for: core/tests/slice.rs

@@ -2609,14 +2609,14 @@ fn test_slice_from_ptr_range() {
 #[should_panic = "slice len overflow"]
 fn test_flatten_size_overflow() {
     let x = &[[(); usize::MAX]; 2][..];
-    let _ = x.flatten();
+    let _ = x.as_flattened();
 }
 
 #[test]
 #[should_panic = "slice len overflow"]
 fn test_flatten_mut_size_overflow() {
     let x = &mut [[(); usize::MAX]; 2][..];
-    let _ = x.flatten_mut();
+    let _ = x.as_flattened_mut();
 }
 
 #[test]