Rollup merge of #61969 - MikailBag:master, r=Centril

Add #[repr(transparent)] for several types

In some functions, types mentioned in this PR are transmuted into their inner value.
Example for `PathBuf`: https://github.com/rust-lang/rust/blob/master/src/libstd/path.rs#L1132.
This PR adds `#[repr(transparent)]` to those types, so their correct behavior doesn't depend on compiler details. (As far as I understand, currently that line, converting `PathBuf` to `Vec<u8>`, is UB).

2 files changed, 12 insertions, 0 deletions

diff --git a/src/libstd/ffi/c_str.rs b/src/libstd/ffi/c_str.rs
index 6e111f24236..512839a12c0 100644
--- a/src/libstd/ffi/c_str.rs
+++ b/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
diff --git a/src/libstd/ffi/os_str.rs b/src/libstd/ffi/os_str.rs
index 54e1c876f4c..1f384cbada3 100644
--- a/src/libstd/ffi/os_str.rs
+++ b/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
 }