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Summary

Introduce the method split_at(&self, mid: usize) -> (&str, &str) on str, to divide a slice into two, just like we can with [T].

Motivation

Adding split_at is a measure to provide a method from [T] in a version that makes sense for str.

Once used to [T], users might even expect that split_at is present on str.

It is a simple method with an obvious implementation, but it provides convenience while working with string segmentation manually, which we already have ample tools for (for example the method find that returns the first matching byte offset).

Using split_at can lead to less repeated bounds checks, since it is easy to use cumulatively, splitting off a piece at a time.

This feature is requested in rust-lang/rust#18063

Detailed design

Introduce the method split_at(&self, mid: usize) -> (&str, &str) on str, to divide a slice into two.

mid will be a byte offset from the start of the string, and it must be on a character boundary. Both 0 and self.len() are valid splitting points.

split_at will be an inherent method on str where possible, and will be available from libcore and the layers above it.

The following is a working implementation, implemented as a trait just for illustration and to be testable as a custom extension:

trait SplitAt {
    fn split_at(&self, mid: usize) -> (&Self, &Self);
}

impl SplitAt for str {
    /// Divide one string slice into two at an index.
    ///
    /// The index `mid` is a byte offset from the start of the string
    /// that must be on a character boundary.
    ///
    /// Return slices `&self[..mid]` and `&self[mid..]`.
    ///
    /// # Panics
    ///
    /// Panics if `mid` is beyond the last character of the string,
    /// or if it is not on a character boundary.
    ///
    /// # Examples
    /// ```
    /// let s = "Löwe 老虎 Léopard";
    /// let first_space = s.find(' ').unwrap_or(s.len());
    /// let (a, b) = s.split_at(first_space);
    ///
    /// assert_eq!(a, "Löwe");
    /// assert_eq!(b, " 老虎 Léopard");
    /// ```
    fn split_at(&self, mid: usize) -> (&str, &str) {
        (&self[..mid], &self[mid..])
    }
}

split_at will use a byte offset (a.k.a byte index) to be consistent with slicing and the offset used by interrogator methods such as find or iterators such as char_indices. Byte offsets are our standard lightweight position indicators that we use to support efficient operations on string slices.

Implementing split_at_mut is not relevant for str at this time.

Drawbacks

  • split_at panics on 1) index out of bounds 2) index not on character boundary.
  • Possible name confusion with other str methods like .split()
  • According to our developing API evolution and semver guidelines this is a breaking change but a (very) minor change. Adding methods is something we expect to be able to. (See RFC PR #1105).

Alternatives

  • Recommend other splitting methods, like the split iterators.
  • Stick to writing (&foo[..mid], &foo[mid..])

Unresolved questions

  • None
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