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. Specifically, the
+following methods should be added:
+
+- `str::into_string`
+- `String::into_boxed_str`
+
+# 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 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`.
+
+
+# Drawbacks
+
+None, yet.
+
+# Alternatives
+
+- 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
+
+None.