Add some additional utility methods to OsString and OsStr

text/0000-osstring-methods.md

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+- Feature Name: osstring_simple_functions
+- Start Date: 2015-10-04
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Add some additional utility methods to OsString and OsStr.
+
+# Motivation
+
+OsString and OsStr are extremely bare at the moment; some utilities would make them
+easier to work with. The given set of utilities is taken from String, and don't add
+any additional restrictions to the implementation.
+
+I don't think any of the proposed methods are controversial.
+
+# Detailed design
+
+Add the following methods to OsString:
+
+```rust
+/// Creates a new `OsString` with the given capacity. The string will be able
+/// to hold exactly `capacity` bytes without reallocating. If `capacity` is 0,
+/// the string will not allocate.
+///
+/// See main `OsString` documentation information about encoding.
+fn with_capacity(capacity: usize) -> OsString;
+
+/// Truncates `self` to zero length.
+fn clear(&mut self);
+
+/// Returns the number of bytes this `OsString` can hold without reallocating.
+///
+/// See `OsString` introduction for information about encoding.
+fn capacity(&self) -> usize;
+
+/// Reserves capacity for at least `additional` more bytes to be inserted in the
+/// given `OsString`. The collection may reserve more space to avoid frequent
+/// reallocations.
+fn reserve(&mut self, additional: usize);
+
+/// Reserves the minimum capacity for exactly `additional` more bytes to be
+/// inserted in the given `OsString`. Does nothing if the capacity is already
+/// sufficient.
+///
+/// Note that the allocator may give the collection more space than it
+/// requests. Therefore capacity can not be relied upon to be precisely
+/// minimal. Prefer reserve if future insertions are expected.
+fn reserve_exact(&mut self, additional: usize);
+```
+
+Add the following methods to OsStr:
+
+```rust
+/// Checks whether `self` is empty.
+fn is_empty(&self) -> bool;
+
+/// Returns the number of bytes in this string.
+///
+/// See `OsStr` introduction for information about encoding.
+fn len(&self) -> usize;
+```
+
+# Drawbacks
+
+The meaning of `len()` might be a bit confusing because it's the size of
+the internal representation on Windows, which isn't otherwise visible to the
+user.
+
+# Alternatives
+
+None.
+
+# Unresolved questions
+
+None.