Auto merge of #28977 - arcnmx:cstring-into, r=alexcrichton

`OsString` has these sorts of conversions, while `CString` has been missing them. I'm iffy on `into_string` simply because the return type would be better off as `FromUtf8Error<CString>`, which of course isn't generic 😢

Also should a different/new feature gate be used?

Author: bors

src/libstd/ffi/c_str.rs

@@ -23,7 +23,7 @@ use ops::Deref;
 use option::Option::{self, Some, None};
 use result::Result::{self, Ok, Err};
 use slice;
-use str;
+use str::{self, Utf8Error};
 use string::String;
 use vec::Vec;
 
@@ -151,6 +151,15 @@ pub struct CStr {
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct NulError(usize, Vec<u8>);
 
+/// An error returned from `CString::into_string` to indicate that a UTF-8 error
+/// was encountered during the conversion.
+#[derive(Clone, PartialEq, Debug)]
+#[unstable(feature = "cstring_into", reason = "recently added", issue = "29157")]
+pub struct IntoStringError {
+    inner: CString,
+    error: Utf8Error,
+}
+
 impl CString {
     /// Creates a new C-compatible string from a container of bytes.
     ///
@@ -255,6 +264,38 @@ impl CString {
         Box::into_raw(self.inner) as *mut libc::c_char
     }
 
+    /// Converts the `CString` into a `String` if it contains valid Unicode data.
+    ///
+    /// On failure, ownership of the original `CString` is returned.
+    #[unstable(feature = "cstring_into", reason = "recently added", issue = "29157")]
+    pub fn into_string(self) -> Result<String, IntoStringError> {
+        String::from_utf8(self.into_bytes())
+            .map_err(|e| IntoStringError {
+                error: e.utf8_error(),
+                inner: unsafe { CString::from_vec_unchecked(e.into_bytes()) },
+            })
+    }
+
+    /// Returns the underlying byte buffer.
+    ///
+    /// The returned buffer does **not** contain the trailing nul separator and
+    /// it is guaranteed to not have any interior nul bytes.
+    #[unstable(feature = "cstring_into", reason = "recently added", issue = "29157")]
+    pub fn into_bytes(self) -> Vec<u8> {
+        // FIXME: Once this method becomes stable, add an `impl Into<Vec<u8>> for CString`
+        let mut vec = self.inner.into_vec();
+        let _nul = vec.pop();
+        debug_assert_eq!(_nul, Some(0u8));
+        vec
+    }
+
+    /// Equivalent to the `into_bytes` function except that the returned vector
+    /// includes the trailing nul byte.
+    #[unstable(feature = "cstring_into", reason = "recently added", issue = "29157")]
+    pub fn into_bytes_with_nul(self) -> Vec<u8> {
+        self.inner.into_vec()
+    }
+
     /// Returns the contents of this `CString` as a slice of bytes.
     ///
     /// The returned slice does **not** contain the trailing nul separator and
@@ -336,6 +377,35 @@ impl From<NulError> for io::Error {
     }
 }
 
+impl IntoStringError {
+    /// Consumes this error, returning original `CString` which generated the
+    /// error.
+    #[unstable(feature = "cstring_into", reason = "recently added", issue = "29157")]
+    pub fn into_cstring(self) -> CString {
+        self.inner
+    }
+
+    /// Access the underlying UTF-8 error that was the cause of this error.
+    #[unstable(feature = "cstring_into", reason = "recently added", issue = "29157")]
+    pub fn utf8_error(&self) -> Utf8Error {
+        self.error
+    }
+}
+
+#[unstable(feature = "cstring_into", reason = "recently added", issue = "29157")]
+impl Error for IntoStringError {
+    fn description(&self) -> &str {
+        Error::description(&self.error)
+    }
+}
+
+#[unstable(feature = "cstring_into", reason = "recently added", issue = "29157")]
+impl fmt::Display for IntoStringError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Display::fmt(&self.error, f)
+    }
+}
+
 impl CStr {
     /// Casts a raw C string to a safe C string wrapper.
     ///

src/libstd/ffi/mod.rs

@@ -13,7 +13,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 #[stable(feature = "rust1", since = "1.0.0")]
-pub use self::c_str::{CString, CStr, NulError};
+pub use self::c_str::{CString, CStr, NulError, IntoStringError};
 
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::os_str::{OsString, OsStr};