Rollup merge of rust-lang#52340 - cypher:document-from-trait-in-ffi, …

…r=steveklabnik

Document From trait implementations for OsStr, OsString, CString, and CStr

As part of issue rust-lang#51430 (cc @skade).

The allocation and copy claims should be double-checked.

r? @steveklabnik

src/libstd/ffi/c_str.rs

@@ -642,6 +642,12 @@ impl fmt::Debug for CString {
 
 #[stable(feature = "cstring_into", since = "1.7.0")]
 impl From<CString> for Vec<u8> {
+    /// Converts a [`CString`] into a [`Vec`]`<u8>`.
+    ///
+    /// The conversion consumes the [`CString`], and removes the terminating NUL byte.
+    ///
+    /// [`Vec`]: ../vec/struct.Vec.html
+    /// [`CString`]: ../ffi/struct.CString.html
     #[inline]
     fn from(s: CString) -> Vec<u8> {
         s.into_bytes()
@@ -700,6 +706,10 @@ impl<'a> From<&'a CStr> for Box<CStr> {
 
 #[stable(feature = "c_string_from_box", since = "1.18.0")]
 impl From<Box<CStr>> for CString {
+    /// Converts a [`Box`]`<CStr>` into a [`CString`] without copying or allocating.
+    ///
+    /// [`Box`]: ../boxed/struct.Box.html
+    /// [`CString`]: ../ffi/struct.CString.html
     #[inline]
     fn from(s: Box<CStr>) -> CString {
         s.into_c_string()
@@ -716,6 +726,10 @@ impl Clone for Box<CStr> {
 
 #[stable(feature = "box_from_c_string", since = "1.20.0")]
 impl From<CString> for Box<CStr> {
+    /// Converts a [`CString`] into a [`Box`]`<CStr>` without copying or allocating.
+    ///
+    /// [`CString`]: ../ffi/struct.CString.html
+    /// [`Box`]: ../boxed/struct.Box.html
     #[inline]
     fn from(s: CString) -> Box<CStr> {
         s.into_boxed_c_str()
@@ -748,6 +762,10 @@ impl<'a> From<&'a CString> for Cow<'a, CStr> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<CString> for Arc<CStr> {
+    /// Converts a [`CString`] into a [`Arc`]`<CStr>` without copying or allocating.
+    ///
+    /// [`CString`]: ../ffi/struct.CString.html
+    /// [`Arc`]: ../sync/struct.Arc.html
     #[inline]
     fn from(s: CString) -> Arc<CStr> {
         let arc: Arc<[u8]> = Arc::from(s.into_inner());
@@ -766,6 +784,10 @@ impl<'a> From<&'a CStr> for Arc<CStr> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<CString> for Rc<CStr> {
+    /// Converts a [`CString`] into a [`Rc`]`<CStr>` without copying or allocating.
+    ///
+    /// [`CString`]: ../ffi/struct.CString.html
+    /// [`Rc`]: ../rc/struct.Rc.html
     #[inline]
     fn from(s: CString) -> Rc<CStr> {
         let rc: Rc<[u8]> = Rc::from(s.into_inner());
@@ -839,6 +861,10 @@ impl fmt::Display for NulError {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl From<NulError> for io::Error {
+    /// Converts a [`NulError`] into a [`io::Error`].
+    ///
+    /// [`NulError`]: ../ffi/struct.NulError.html
+    /// [`io::Error`]: ../io/struct.Error.html
     fn from(_: NulError) -> io::Error {
         io::Error::new(io::ErrorKind::InvalidInput,
                        "data provided contains a nul byte")

src/libstd/ffi/os_str.rs

@@ -348,6 +348,12 @@ impl OsString {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl From<String> for OsString {
+    /// Converts a [`String`] into a [`OsString`].
+    ///
+    /// The conversion copies the data, and includes an allocation on the heap.
+    ///
+    /// [`String`]: ../string/struct.String.html
+    /// [`OsString`]: struct.OsString.html
     fn from(s: String) -> OsString {
         OsString { inner: Buf::from_string(s) }
     }
@@ -630,13 +636,21 @@ impl<'a> From<&'a OsStr> for Box<OsStr> {
 
 #[stable(feature = "os_string_from_box", since = "1.18.0")]
 impl From<Box<OsStr>> for OsString {
+    /// Converts a `Box<OsStr>` into a `OsString` without copying or allocating.
+    ///
+    /// [`Box`]: ../boxed/struct.Box.html
+    /// [`OsString`]: ../ffi/struct.OsString.html
     fn from(boxed: Box<OsStr>) -> OsString {
         boxed.into_os_string()
     }
 }
 
 #[stable(feature = "box_from_os_string", since = "1.20.0")]
 impl From<OsString> for Box<OsStr> {
+    /// Converts a [`OsString`] into a [`Box`]`<OsStr>` without copying or allocating.
+    ///
+    /// [`Box`]: ../boxed/struct.Box.html
+    /// [`OsString`]: ../ffi/struct.OsString.html
     fn from(s: OsString) -> Box<OsStr> {
         s.into_boxed_os_str()
     }
@@ -652,6 +666,10 @@ impl Clone for Box<OsStr> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<OsString> for Arc<OsStr> {
+    /// Converts a [`OsString`] into a [`Arc`]`<OsStr>` without copying or allocating.
+    ///
+    /// [`Arc`]: ../sync/struct.Arc.html
+    /// [`OsString`]: ../ffi/struct.OsString.html
     #[inline]
     fn from(s: OsString) -> Arc<OsStr> {
         let arc = s.inner.into_arc();
@@ -670,6 +688,10 @@ impl<'a> From<&'a OsStr> for Arc<OsStr> {
 
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<OsString> for Rc<OsStr> {
+    /// Converts a [`OsString`] into a [`Rc`]`<OsStr>` without copying or allocating.
+    ///
+    /// [`Rc`]: ../rc/struct.Rc.html
+    /// [`OsString`]: ../ffi/struct.OsString.html
     #[inline]
     fn from(s: OsString) -> Rc<OsStr> {
         let rc = s.inner.into_rc();