Auto merge of #48999 - GuillaumeGomez:add-repeat-on-slice, r=Kimundi

Add repeat method on slice

Fixes #48784.

src/liballoc/str.rs

@@ -436,59 +436,7 @@ impl str {
     /// ```
     #[stable(feature = "repeat_str", since = "1.16.0")]
     pub fn repeat(&self, n: usize) -> String {
-        if n == 0 {
-            return String::new();
-        }
-
-        // If `n` is larger than zero, it can be split as
-        // `n = 2^expn + rem (2^expn > rem, expn >= 0, rem >= 0)`.
-        // `2^expn` is the number represented by the leftmost '1' bit of `n`,
-        // and `rem` is the remaining part of `n`.
-
-        // Using `Vec` to access `set_len()`.
-        let mut buf = Vec::with_capacity(self.len() * n);
-
-        // `2^expn` repetition is done by doubling `buf` `expn`-times.
-        buf.extend(self.as_bytes());
-        {
-            let mut m = n >> 1;
-            // If `m > 0`, there are remaining bits up to the leftmost '1'.
-            while m > 0 {
-                // `buf.extend(buf)`:
-                unsafe {
-                    ptr::copy_nonoverlapping(
-                        buf.as_ptr(),
-                        (buf.as_mut_ptr() as *mut u8).add(buf.len()),
-                        buf.len(),
-                    );
-                    // `buf` has capacity of `self.len() * n`.
-                    let buf_len = buf.len();
-                    buf.set_len(buf_len * 2);
-                }
-
-                m >>= 1;
-            }
-        }
-
-        // `rem` (`= n - 2^expn`) repetition is done by copying
-        // first `rem` repetitions from `buf` itself.
-        let rem_len = self.len() * n - buf.len(); // `self.len() * rem`
-        if rem_len > 0 {
-            // `buf.extend(buf[0 .. rem_len])`:
-            unsafe {
-                // This is non-overlapping since `2^expn > rem`.
-                ptr::copy_nonoverlapping(
-                    buf.as_ptr(),
-                    (buf.as_mut_ptr() as *mut u8).add(buf.len()),
-                    rem_len,
-                );
-                // `buf.len() + rem_len` equals to `buf.capacity()` (`= self.len() * n`).
-                let buf_cap = buf.capacity();
-                buf.set_len(buf_cap);
-            }
-        }
-
-        unsafe { String::from_utf8_unchecked(buf) }
+        unsafe { String::from_utf8_unchecked(self.as_bytes().repeat(n)) }
     }
 
     /// Returns a copy of this string where each character is mapped to its