Rollup merge of #70201 - cuviper:clone_into, r=dtolnay

Small tweaks in ToOwned::clone_into

- `<[T]>::clone_into` is slightly more optimized.
- `CStr::clone_into` is new, letting it reuse its allocation.
- `OsStr::clone_into` now forwards to the underlying slice/`Vec`.

src/liballoc/slice.rs

@@ -733,14 +733,14 @@ fn to_owned(&self) -> Vec<T> {
     fn clone_into(&self, target: &mut Vec<T>) {
         // drop anything in target that will not be overwritten
         target.truncate(self.len());
-        let len = target.len();
-
-        // reuse the contained values' allocations/resources.
-        target.clone_from_slice(&self[..len]);
 
         // target.len <= self.len due to the truncate above, so the
-        // slice here is always in-bounds.
-        target.extend_from_slice(&self[len..]);
+        // slices here are always in-bounds.
+        let (init, tail) = self.split_at(target.len());
+
+        // reuse the contained values' allocations/resources.
+        target.clone_from_slice(init);
+        target.extend_from_slice(tail);
     }
 }
 

src/libstd/ffi/c_str.rs

@@ -1329,6 +1329,12 @@ impl ToOwned for CStr {
     fn to_owned(&self) -> CString {
         CString { inner: self.to_bytes_with_nul().into() }
     }
+
+    fn clone_into(&self, target: &mut CString) {
+        let mut b = Vec::from(mem::take(&mut target.inner));
+        self.to_bytes_with_nul().clone_into(&mut b);
+        target.inner = b.into_boxed_slice();
+    }
 }
 
 #[stable(feature = "cstring_asref", since = "1.7.0")]
@@ -1510,6 +1516,17 @@ fn boxed_default() {
         assert_eq!(boxed.to_bytes_with_nul(), &[0]);
     }
 
+    #[test]
+    fn test_c_str_clone_into() {
+        let mut c_string = CString::new("lorem").unwrap();
+        let c_ptr = c_string.as_ptr();
+        let c_str = CStr::from_bytes_with_nul(b"ipsum\0").unwrap();
+        c_str.clone_into(&mut c_string);
+        assert_eq!(c_str, c_string.as_c_str());
+        // The exact same size shouldn't have needed to move its allocation
+        assert_eq!(c_ptr, c_string.as_ptr());
+    }
+
     #[test]
     fn into_rc() {
         let orig: &[u8] = b"Hello, world!\0";

src/libstd/ffi/os_str.rs

@@ -1120,8 +1120,7 @@ fn to_owned(&self) -> OsString {
         self.to_os_string()
     }
     fn clone_into(&self, target: &mut OsString) {
-        target.clear();
-        target.push(self);
+        self.inner.clone_into(&mut target.inner)
     }
 }
 

src/libstd/sys/windows/os_str.rs

@@ -159,6 +159,10 @@ pub fn to_owned(&self) -> Buf {
         Buf { inner: buf }
     }
 
+    pub fn clone_into(&self, buf: &mut Buf) {
+        self.inner.clone_into(&mut buf.inner)
+    }
+
     #[inline]
     pub fn into_box(&self) -> Box<Slice> {
         unsafe { mem::transmute(self.inner.into_box()) }

src/libstd/sys_common/os_str_bytes.rs

@@ -173,6 +173,10 @@ pub fn to_owned(&self) -> Buf {
         Buf { inner: self.inner.to_vec() }
     }
 
+    pub fn clone_into(&self, buf: &mut Buf) {
+        self.inner.clone_into(&mut buf.inner)
+    }
+
     #[inline]
     pub fn into_box(&self) -> Box<Slice> {
         let boxed: Box<[u8]> = self.inner.into();

src/libstd/sys_common/wtf8.rs

@@ -613,6 +613,10 @@ fn initial_trail_surrogate(&self) -> Option<u16> {
         }
     }
 
+    pub fn clone_into(&self, buf: &mut Wtf8Buf) {
+        self.bytes.clone_into(&mut buf.bytes)
+    }
+
     /// Boxes this `Wtf8`.
     #[inline]
     pub fn into_box(&self) -> Box<Wtf8> {