New RFC: improve CString construction methods

text/0000-c-string-construction.md

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+- Feature Name: c_string_from_iter
+- Start Date: 2015-02-26
+- RFC PR:
+- Rust Issue:
+
+# Summary
+
+Amend the methods available to construct a `CString` to improve composability
+and follow the conventions emerging elsewhere in the standard library.
+
+# Motivation
+
+The implementation of RFCs [#592][rfc 592] and [#840][rfc 840] has resolved
+most of the issues with the design of `std::ffi::CString`, but some new APIs
+have been created at the same time, needing stabilization. This proposal
+aims at addressing the following issues:
+
+1. The `IntoBytes` trait does not seem wholly justified: it falls short of
+   supporting `IntoIterator`, and has become yet another special-interest trait
+   to care about when designing APIs producing string-like data. 
+
+2. The exposure of `Vec` as an intermediate type for all conversions
+   precludes small-string optimizations, such as an in-place variant
+   implemented in [c_string](https://github.com/mzabaluev/rust-c-str).
+
+3. Stylistic: `Result` as return value type of `new` feels a bit too 'loaded'.
+   It's used in some other places, but the general expectation on `new` is to
+   be the most straightforward way to obtain a value of the type, while more
+   involved failure modes tend to be more typical on `from_*` constructors
+   and the like.
+
+[rfc 592]: https://github.com/rust-lang/rfcs/pull/592
+[rfc 840]: https://github.com/rust-lang/rfcs/pull/840
+
+# Detailed design
+
+Replace the constructor accepting `IntoBytes` with one accepting
+`IntoIterator`, following the `from_iter` pattern in collection types:
+
+```rust
+impl CString {
+    pub fn from_iter<I>(iterable: I) -> Result<CString, NulError>
+        where I: IntoIterator<Item=u8>
+    { ... }
+}
+```
+
+`CString::from_vec` should be reinstated as an optimized special case.
+`CString::from_cow_string` can be added later on.
+
+# Proof of concept
+
+As usual for my RFCs concerning `CString`, the proposed changes are
+implemented on its workalike `CStrBuf` in crate
+[c_string](https://github.com/mzabaluev/rust-c-str).
+
+# Drawbacks
+
+`IntoIterator` is slightly less convenient than `IntoBytes` for converting
+from standard Rust strings and byte slices.
+This can be bridged over by providing an auxiliary trait as described in
+the [Unresolved questions](#unresolved-questions) section below.
+
+# Alternatives
+
+None put forward so far. Living with `IntoBytes` is tolerable.
+
+# Unresolved questions
+
+An auxiliary trait could be provided to take over the convenience and
+optimization aspects of `IntoBytes`, but changed to return a
+`Result<CString, NulError>` directly.