Add some of [T]’s methods to strings and vice versa

text/0000-slice-string-symmetry.md

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+- Feature Name: `slice_string_symmetry`
+- Start Date: 2015-06-06
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Add some methods that already exist on slices to strings and vice versa.
+Specifically, the following methods should be added:
+
+- `str::chunks`
+- `str::windows`
+- `str::into_string`
+- `String::into_boxed_slice`
+- `<[T]>::subslice_offset`
+
+# Motivation
+
+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 both strings and slices to even out these two types’
+available methods.
+
+# Detailed design
+
+Add the following methods to `str`, presumably as inherent methods:
+
+- `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`.
+
+- `windows(&self, n: usize) -> Windows`: Returns an iterator over all contiguous
+  windows of character length `n`. Iterator element type: `&str`.
+
+- `into_string(self: Box<str>) -> String`: Returns `self` as a `String`. This is
+  equivalent to `[T]`’s `into_vec`.
+
+`split_at(&self, mid: usize) -> (&str, &str)` would also be on this list, but
+there is [an existing RFC](https://github.com/rust-lang/rfcs/pull/1123) for it.
+
+Add the following method to `String` as an inherent method:
+
+- `into_boxed_slice(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>`.
+
+Add the following method to `[T]` (for all `T`), presumably as an inherent
+method:
+
+- `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`.
+
+# Drawbacks
+
+- `str::subslice_offset` is already unstable, so creating a similar method on
+  `[T]` is perhaps not such a good idea.
+
+# Alternatives
+
+- Do a subset of the proposal. For example, the `Box<str>`-related methods could
+  be removed.
+
+# Unresolved questions
+
+None.