cstring: avoid excessive growth just to 0-terminate
Based on following what happens in CString::new("string literal"):
1. Using `Into<Vec<u8>>`, a Vec is allocated with capacity exactly equal
to the string's input length.
2. By `v.push(0)`, the Vec is grown to twice capacity, since it was full.
3. By `v.into_boxed_slice()`, the Vec capacity is shrunk to fit the length again.
If we use `.reserve_exact(1)` just before the push, then we avoid the
capacity doubling that we're going to have to shrink anyway.
Growing by just 1 byte means that the step (2) is less likely to have to
move the memory to a larger allocation chunk, and that the step (3) does
not have to reallocate.
Addresses part of #35838
/// This method is equivalent to `new` except that no runtime assertion
/// is made that `v` contains no 0 bytes, and it requires an actual
/// byte vector, not anything that can be converted to one with Into.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use std::ffi::CString;
+ ///
+ /// let raw = b"foo".to_vec();
+ /// unsafe {
+ /// let c_string = CString::from_vec_unchecked(raw);
+ /// }
+ /// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub unsafe fn from_vec_unchecked(mut v: Vec<u8>) -> CString {
+ v.reserve_exact(1);
v.push(0);
CString { inner: v.into_boxed_slice() }
}