Auto merge of #124810 - lincot:speed-up-string-push-and-string-insert, r=tgross35

speed up `String::push` and `String::insert`

Addresses the concerns described in #116235.

The performance gain comes mainly from avoiding temporary buffers.

Complex pattern matching in `encode_utf8` (introduced in #67569) has been simplified to a comparison and an exhaustive `match` in the `encode_utf8_raw_unchecked` helper function. It takes a slice of `MaybeUninit<u8>` because otherwise we'd have to construct a normal slice to uninitialized data, which is not desirable, I guess.

Several functions still have that [unneeded zeroing](https://rust.godbolt.org/z/5oKfMPo7j), but a single instruction is not that important, I guess.

`@rustbot` label T-libs C-optimization A-str

1 files changed, 47 insertions, 18 deletions

diff --git a/library/alloc/src/string.rs b/library/alloc/src/string.rs
index 9236f5cb8d1..5193a0b2d4b 100644
--- a/library/alloc/src/string.rs
+++ b/library/alloc/src/string.rs
@@ -1401,11 +1401,14 @@ impl String {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn push(&mut self, ch: char) {
-        match ch.len_utf8() {
-            1 => self.vec.push(ch as u8),
-            _ => {
-                self.vec.extend_from_slice(ch.encode_utf8(&mut [0; char::MAX_LEN_UTF8]).as_bytes())
-            }
+        let len = self.len();
+        let ch_len = ch.len_utf8();
+        self.reserve(ch_len);
+
+        // SAFETY: Just reserved capacity for at least the length needed to encode `ch`.
+        unsafe {
+            core::char::encode_utf8_raw_unchecked(ch as u32, self.vec.as_mut_ptr().add(self.len()));
+            self.vec.set_len(len + ch_len);
         }
     }
 
@@ -1702,24 +1705,31 @@ impl String {
     #[rustc_confusables("set")]
     pub fn insert(&mut self, idx: usize, ch: char) {
         assert!(self.is_char_boundary(idx));
-        let mut bits = [0; char::MAX_LEN_UTF8];
-        let bits = ch.encode_utf8(&mut bits).as_bytes();
 
+        let len = self.len();
+        let ch_len = ch.len_utf8();
+        self.reserve(ch_len);
+
+        // SAFETY: Move the bytes starting from `idx` to their new location `ch_len`
+        // bytes ahead. This is safe because sufficient capacity was reserved, and `idx`
+        // is a char boundary.
         unsafe {
-            self.insert_bytes(idx, bits);
+            ptr::copy(
+                self.vec.as_ptr().add(idx),
+                self.vec.as_mut_ptr().add(idx + ch_len),
+                len - idx,
+            );
         }
-    }
 
-    #[cfg(not(no_global_oom_handling))]
-    unsafe fn insert_bytes(&mut self, idx: usize, bytes: &[u8]) {
-        let len = self.len();
-        let amt = bytes.len();
-        self.vec.reserve(amt);
+        // SAFETY: Encode the character into the vacated region if `idx != len`,
+        // or into the uninitialized spare capacity otherwise.
+        unsafe {
+            core::char::encode_utf8_raw_unchecked(ch as u32, self.vec.as_mut_ptr().add(idx));
+        }
 
+        // SAFETY: Update the length to include the newly added bytes.
         unsafe {
-            ptr::copy(self.vec.as_ptr().add(idx), self.vec.as_mut_ptr().add(idx + amt), len - idx);
-            ptr::copy_nonoverlapping(bytes.as_ptr(), self.vec.as_mut_ptr().add(idx), amt);
-            self.vec.set_len(len + amt);
+            self.vec.set_len(len + ch_len);
         }
     }
 
@@ -1749,8 +1759,27 @@ impl String {
     pub fn insert_str(&mut self, idx: usize, string: &str) {
         assert!(self.is_char_boundary(idx));
 
+        let len = self.len();
+        let amt = string.len();
+        self.reserve(amt);
+
+        // SAFETY: Move the bytes starting from `idx` to their new location `amt` bytes
+        // ahead. This is safe because sufficient capacity was just reserved, and `idx`
+        // is a char boundary.
+        unsafe {
+            ptr::copy(self.vec.as_ptr().add(idx), self.vec.as_mut_ptr().add(idx + amt), len - idx);
+        }
+
+        // SAFETY: Copy the new string slice into the vacated region if `idx != len`,
+        // or into the uninitialized spare capacity otherwise. The borrow checker
+        // ensures that the source and destination do not overlap.
+        unsafe {
+            ptr::copy_nonoverlapping(string.as_ptr(), self.vec.as_mut_ptr().add(idx), amt);
+        }
+
+        // SAFETY: Update the length to include the newly added bytes.
         unsafe {
-            self.insert_bytes(idx, string.as_bytes());
+            self.vec.set_len(len + amt);
         }
     }