Add FIXME-s that some types should be transparent

src/libstd/ffi/c_str.rs

@@ -195,6 +195,12 @@ pub struct CString {
 /// [`from_ptr`]: #method.from_ptr
 #[derive(Hash)]
 #[stable(feature = "rust1", since = "1.0.0")]
+// FIXME:
+// `fn from` in `impl From<&CStr> for Box<CStr>` current implementation relies
+// on `CStr` being layout-compatible with `[u8]`.
+// When attribute privacy is implemented, `CStr` should be annotated as `#[repr(transparent)]`.
+// Anyway, `CStr` representation and layout are considered implementation detail, are
+// not documented and must not be relied upon.
 pub struct CStr {
     // FIXME: this should not be represented with a DST slice but rather with
     //        just a raw `c_char` along with some form of marker to make

src/libstd/ffi/os_str.rs

@@ -97,6 +97,12 @@ pub struct OsString {
 /// [`String`]: ../string/struct.String.html
 /// [conversions]: index.html#conversions
 #[stable(feature = "rust1", since = "1.0.0")]
+// FIXME:
+// `OsStr::from_inner` current implementation relies
+// on `OsStr` being layout-compatible with `Slice`.
+// When attribute privacy is implemented, `OsStr` should be annotated as `#[repr(transparent)]`.
+// Anyway, `OsStr` representation and layout are considered implementation detail, are
+// not documented and must not be relied upon.
 pub struct OsStr {
     inner: Slice
 }

src/libstd/path.rs

@@ -1123,6 +1123,12 @@ impl FusedIterator for Ancestors<'_> {}
 /// Which method works best depends on what kind of situation you're in.
 #[derive(Clone)]
 #[stable(feature = "rust1", since = "1.0.0")]
+// FIXME:
+// `PathBuf::as_mut_vec` current implementation relies
+// on `PathBuf` being layout-compatible with `Vec<u8>`.
+// When attribute privacy is implemented, `PathBuf` should be annotated as `#[repr(transparent)]`.
+// Anyway, `PathBuf` representation and layout are considered implementation detail, are
+// not documented and must not be relied upon.
 pub struct PathBuf {
     inner: OsString,
 }
@@ -1745,6 +1751,12 @@ impl AsRef<OsStr> for PathBuf {
 /// assert_eq!(extension, Some(OsStr::new("txt")));
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
+// FIXME:
+// `Path::new` current implementation relies
+// on `Path` being layout-compatible with `OsStr`.
+// When attribute privacy is implemented, `Path` should be annotated as `#[repr(transparent)]`.
+// Anyway, `Path` representation and layout are considered implementation detail, are
+// not documented and must not be relied upon.
 pub struct Path {
     inner: OsStr,
 }

src/libstd/sys_common/os_str_bytes.rs

@@ -18,6 +18,12 @@ pub(crate) struct Buf {
     pub inner: Vec<u8>
 }
 
+// FIXME:
+// `Buf::as_slice` current implementation relies
+// on `Slice` being layout-compatible with `[u8]`.
+// When attribute privacy is implemented, `Slice` should be annotated as `#[repr(transparent)]`.
+// Anyway, `Slice` representation and layout are considered implementation detail, are
+// not documented and must not be relied upon.
 pub(crate) struct Slice {
     pub inner: [u8]
 }