Rollup merge of rust-lang#79222 - yoshuawuyts:slice-fill-with, r=m-ou-se

Add `core::slice::fill_with`

Tracking issue rust-lang#79221.

As suggested by `@m-ou-se` in rust-lang#70758 (comment) this implements `slice::fill_with` as a counterpart to `slice::fill`. This mirrors `Vec::resize` and `Vec::resize_with`. Thanks!

r? `@m-ou-se`

library/core/src/slice/mod.rs

@@ -2599,6 +2599,34 @@ impl<T> [T] {
         }
     }
 
+    /// Fills `self` with elements returned by calling a closure repeatedly.
+    ///
+    /// This method uses a closure to create new values. If you'd rather
+    /// [`Clone`] a given value, use [`fill`]. If you want to use the [`Default`]
+    /// trait to generate values, you can pass [`Default::default`] as the
+    /// argument.
+    ///
+    /// [`fill`]: #method.fill
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(slice_fill_with)]
+    ///
+    /// let mut buf = vec![1; 10];
+    /// buf.fill_with(Default::default);
+    /// assert_eq!(buf, vec![0; 10]);
+    /// ```
+    #[unstable(feature = "slice_fill_with", issue = "79221")]
+    pub fn fill_with<F>(&mut self, mut f: F)
+    where
+        F: FnMut() -> T,
+    {
+        for el in self {
+            *el = f();
+        }
+    }
+
     /// Copies the elements from `src` into `self`.
     ///
     /// The length of `src` must be the same as `self`.