Add some methods that already exist on slices to strings. Specifically, the following methods should be added:

  • str::into_string
  • String::into_boxed_str


Conceptually, strings and slices are similar types. Many methods are already shared between the two types due to their similarity. However, not all methods are shared between the types, even though many could be. This is a little unexpected and inconsistent. Because of that, this RFC proposes to remedy this by adding a few methods to strings to even out these two types’ available methods.

Specifically, it is currently very difficult to construct a Box<str>, while it is fairly simple to make a Box<[T]> by using Vec::into_boxed_slice. This RFC proposes a means of creating a Box<str> by converting a String.

Detailed design

Add the following method to str, presumably as an inherent method:

  • into_string(self: Box<str>) -> String: Returns self as a String. This is equivalent to [T]’s into_vec.

Add the following method to String as an inherent method:

  • into_boxed_str(self) -> Box<str>: Returns self as a Box<str>, reallocating to cut off any excess capacity if needed. This is required to provide a safe means of creating Box<str>. This is equivalent to Vec<T>’s into_boxed_slice.


None, yet.


  • The original version of this RFC had a few extra methods:
    • str::chunks(&self, n: usize) -> Chunks: Returns an iterator that yields the characters (not bytes) of the string in groups of n at a time. Iterator element type: &str.

    • str::windows(&self, n: usize) -> Windows: Returns an iterator over all contiguous windows of character length n. Iterator element type: &str.

      This and str::chunks aren’t really useful without proper treatment of graphemes, so they were removed from the RFC.

    • <[T]>::subslice_offset(&self, inner: &[T]) -> usize: Returns the offset (in elements) of an inner slice relative to an outer slice. Panics of inner is not contained within self.

      str::subslice_offset isn’t yet stable and its usefulness is dubious, so this method was removed from the RFC.

Unresolved questions