Rollup merge of rust-lang#73139 - poliorcetics:cstring-from-vec-with-nul, r=dtolnay

Add methods to go from a nul-terminated Vec<u8> to a CString

Fixes rust-lang#73100.

Doc tests have been written and the documentation on the error type
updated too.

I used `#[stable(feature = "cstring_from_vec_with_nul", since = "1.46.0")]` but I don't know if the version is correct.

Author: RalfJung

src/libstd/ffi/c_str.rs

@@ -234,15 +234,14 @@ pub struct NulError(usize, Vec<u8>);
 
 /// An error indicating that a nul byte was not in the expected position.
 ///
-/// The slice used to create a [`CStr`] must have one and only one nul
-/// byte at the end of the slice.
+/// The slice used to create a [`CStr`] must have one and only one nul byte,
+/// positioned at the end.
 ///
-/// This error is created by the
-/// [`from_bytes_with_nul`][`CStr::from_bytes_with_nul`] method on
-/// [`CStr`]. See its documentation for more.
+/// This error is created by the [`from_bytes_with_nul`] method on [`CStr`].
+/// See its documentation for more.
 ///
 /// [`CStr`]: struct.CStr.html
-/// [`CStr::from_bytes_with_nul`]: struct.CStr.html#method.from_bytes_with_nul
+/// [`from_bytes_with_nul`]: struct.CStr.html#method.from_bytes_with_nul
 ///
 /// # Examples
 ///
@@ -257,6 +256,32 @@ pub struct FromBytesWithNulError {
     kind: FromBytesWithNulErrorKind,
 }
 
+/// An error indicating that a nul byte was not in the expected position.
+///
+/// The vector used to create a [`CString`] must have one and only one nul byte,
+/// positioned at the end.
+///
+/// This error is created by the [`from_vec_with_nul`] method on [`CString`].
+/// See its documentation for more.
+///
+/// [`CString`]: struct.CString.html
+/// [`from_vec_with_nul`]: struct.CString.html#method.from_vec_with_nul
+///
+/// # Examples
+///
+/// ```
+/// #![feature(cstring_from_vec_with_nul)]
+/// use std::ffi::{CString, FromVecWithNulError};
+///
+/// let _: FromVecWithNulError = CString::from_vec_with_nul(b"f\0oo".to_vec()).unwrap_err();
+/// ```
+#[derive(Clone, PartialEq, Eq, Debug)]
+#[unstable(feature = "cstring_from_vec_with_nul", issue = "73179")]
+pub struct FromVecWithNulError {
+    error_kind: FromBytesWithNulErrorKind,
+    bytes: Vec<u8>,
+}
+
 #[derive(Clone, PartialEq, Eq, Debug)]
 enum FromBytesWithNulErrorKind {
     InteriorNul(usize),
@@ -272,6 +297,59 @@ impl FromBytesWithNulError {
     }
 }
 
+#[unstable(feature = "cstring_from_vec_with_nul", issue = "73179")]
+impl FromVecWithNulError {
+    /// Returns a slice of [`u8`]s bytes that were attempted to convert to a [`CString`].
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(cstring_from_vec_with_nul)]
+    /// use std::ffi::CString;
+    ///
+    /// // Some invalid bytes in a vector
+    /// let bytes = b"f\0oo".to_vec();
+    ///
+    /// let value = CString::from_vec_with_nul(bytes.clone());
+    ///
+    /// assert_eq!(&bytes[..], value.unwrap_err().as_bytes());
+    /// ```
+    ///
+    /// [`CString`]: struct.CString.html
+    pub fn as_bytes(&self) -> &[u8] {
+        &self.bytes[..]
+    }
+
+    /// Returns the bytes that were attempted to convert to a [`CString`].
+    ///
+    /// This method is carefully constructed to avoid allocation. It will
+    /// consume the error, moving out the bytes, so that a copy of the bytes
+    /// does not need to be made.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(cstring_from_vec_with_nul)]
+    /// use std::ffi::CString;
+    ///
+    /// // Some invalid bytes in a vector
+    /// let bytes = b"f\0oo".to_vec();
+    ///
+    /// let value = CString::from_vec_with_nul(bytes.clone());
+    ///
+    /// assert_eq!(bytes, value.unwrap_err().into_bytes());
+    /// ```
+    ///
+    /// [`CString`]: struct.CString.html
+    pub fn into_bytes(self) -> Vec<u8> {
+        self.bytes
+    }
+}
+
 /// An error indicating invalid UTF-8 when converting a [`CString`] into a [`String`].
 ///
 /// `CString` is just a wrapper over a buffer of bytes with a nul
@@ -643,6 +721,86 @@ impl CString {
         let this = mem::ManuallyDrop::new(self);
         unsafe { ptr::read(&this.inner) }
     }
+
+    /// Converts a `Vec` of `u8` to a `CString` without checking the invariants
+    /// on the given `Vec`.
+    ///
+    /// # Safety
+    ///
+    /// The given `Vec` **must** have one nul byte as its last element.
+    /// This means it cannot be empty nor have any other nul byte anywhere else.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// #![feature(cstring_from_vec_with_nul)]
+    /// use std::ffi::CString;
+    /// assert_eq!(
+    ///     unsafe { CString::from_vec_with_nul_unchecked(b"abc\0".to_vec()) },
+    ///     unsafe { CString::from_vec_unchecked(b"abc".to_vec()) }
+    /// );
+    /// ```
+    #[unstable(feature = "cstring_from_vec_with_nul", issue = "73179")]
+    pub unsafe fn from_vec_with_nul_unchecked(v: Vec<u8>) -> Self {
+        Self { inner: v.into_boxed_slice() }
+    }
+
+    /// Attempts to converts a `Vec` of `u8` to a `CString`.
+    ///
+    /// Runtime checks are present to ensure there is only one nul byte in the
+    /// `Vec`, its last element.
+    ///
+    /// # Errors
+    ///
+    /// If a nul byte is present and not the last element or no nul bytes
+    /// is present, an error will be returned.
+    ///
+    /// # Examples
+    ///
+    /// A successful conversion will produce the same result as [`new`] when
+    /// called without the ending nul byte.
+    ///
+    /// ```
+    /// #![feature(cstring_from_vec_with_nul)]
+    /// use std::ffi::CString;
+    /// assert_eq!(
+    ///     CString::from_vec_with_nul(b"abc\0".to_vec())
+    ///         .expect("CString::from_vec_with_nul failed"),
+    ///     CString::new(b"abc".to_vec()).expect("CString::new failed")
+    /// );
+    /// ```
+    ///
+    /// A incorrectly formatted vector will produce an error.
+    ///
+    /// ```
+    /// #![feature(cstring_from_vec_with_nul)]
+    /// use std::ffi::{CString, FromVecWithNulError};
+    /// // Interior nul byte
+    /// let _: FromVecWithNulError = CString::from_vec_with_nul(b"a\0bc".to_vec()).unwrap_err();
+    /// // No nul byte
+    /// let _: FromVecWithNulError = CString::from_vec_with_nul(b"abc".to_vec()).unwrap_err();
+    /// ```
+    ///
+    /// [`new`]: #method.new
+    #[unstable(feature = "cstring_from_vec_with_nul", issue = "73179")]
+    pub fn from_vec_with_nul(v: Vec<u8>) -> Result<Self, FromVecWithNulError> {
+        let nul_pos = memchr::memchr(0, &v);
+        match nul_pos {
+            Some(nul_pos) if nul_pos + 1 == v.len() => {
+                // SAFETY: We know there is only one nul byte, at the end
+                // of the vec.
+                Ok(unsafe { Self::from_vec_with_nul_unchecked(v) })
+            }
+            Some(nul_pos) => Err(FromVecWithNulError {
+                error_kind: FromBytesWithNulErrorKind::InteriorNul(nul_pos),
+                bytes: v,
+            }),
+            None => Err(FromVecWithNulError {
+                error_kind: FromBytesWithNulErrorKind::NotNulTerminated,
+                bytes: v,
+            }),
+        }
+    }
 }
 
 // Turns this `CString` into an empty string to prevent
@@ -976,6 +1134,23 @@ impl fmt::Display for FromBytesWithNulError {
     }
 }
 
+#[unstable(feature = "cstring_from_vec_with_nul", issue = "73179")]
+impl Error for FromVecWithNulError {}
+
+#[unstable(feature = "cstring_from_vec_with_nul", issue = "73179")]
+impl fmt::Display for FromVecWithNulError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.error_kind {
+            FromBytesWithNulErrorKind::InteriorNul(pos) => {
+                write!(f, "data provided contains an interior nul byte at pos {}", pos)
+            }
+            FromBytesWithNulErrorKind::NotNulTerminated => {
+                write!(f, "data provided is not nul terminated")
+            }
+        }
+    }
+}
+
 impl IntoStringError {
     /// Consumes this error, returning original [`CString`] which generated the
     /// error.

src/libstd/ffi/mod.rs

@@ -157,6 +157,8 @@
 
 #[stable(feature = "cstr_from_bytes", since = "1.10.0")]
 pub use self::c_str::FromBytesWithNulError;
+#[unstable(feature = "cstring_from_vec_with_nul", issue = "73179")]
+pub use self::c_str::FromVecWithNulError;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::c_str::{CStr, CString, IntoStringError, NulError};