A type representing an owned C-compatible wide string

text/0000-cwstring.md

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+- Feature Name: A type representing an owned C-compatible wide string
+- Start Date: 2016-10-20
+- RFC PR:
+- Rust Issue:
+
+# Summary
+
+Add CWideString/CWideStr for more simple interaction with not well-formed UTF-16
+external API (for example: with Windows API).
+
+# Motivation
+
+This RFC born from issue: [rust-lang/rust#34303](https://github.com/rust-lang/rust/issues/34303)
+
+Many of Windows API use not well-formed UTF-16 strings. Some of this API use
+null-terminated strings.
+
+Rust lack simple null-terminated UTF-16 string conversions and now working with this
+API need too many boilerplate copy-paste code like:
+```
+unsafe fn from_wide_null(ptr: *const u16) -> OsString {
+    let mut len = 0;
+    while *ptr.offset(len) != 0 {
+        len += 1;
+    }
+    OsStringExt::from_wide(slice::from_raw_parts(ptr, len as usize))
+}
+
+fn to_wide_null(s: &str) -> Vec<u16> {
+    self.encode_utf16().chain(Some(0)).collect()
+}
+```
+
+OsString also can't be used with Windows API directly, because it stores string internally
+in WTF-8 encoding.
+
+So, this RFC try to add simple and effective way to work with UTF-16 string.
+
+Also it can be usefull:
+
+ * Inside Rust OsString implementatino on Windows platform;
+ * With Java FFI (Java internally use UTF-16 strings).
+
+# Detailed design
+
+Copy CStr/CString as CWideStr/CWideString with using u16 instead of u8.
+
+A preliminary implementation can be found at: https://github.com/bozaro/rust-cwstring
+
+# Drawbacks
+
+This classes is not generally platform specific, but it mostly useful on Windows.
+
+Also CWideString name can be confused, because it use differ element size then
+`std::wstring` in C++ world (`std::wstring` type defined as `std::basic_string<wchar_t>`,
+but `sizeof(wchar_t)` is platform depended: 4 bytes on Linux, 2 bytes on Windows).
+
+# Alternatives
+
+Keep all as is.
+
+# Unresolved questions
+
+I try implement CWideStr/CWideString and got some issues/questions:
+
+ * NulError is copied as WideNulError, but it breaking currect code or name convensions.
+ * into_string method is removed, because unlike CString, he does not give the performance
+   profit. Also IntoStringError is not copied from c_str.
+ * I don't find good name for `u16` method like `to_bytes`.
+ * memchr and strlen replaced by wmemchr and wstrlen failback implementation.
+ * May be need better implementation fmt::Debug for CWideStr.
+