add comment about maintaining OsStr encoding

src/libstd/ffi/os_str.rs

@@ -534,6 +534,8 @@ impl OsStr {
     fn from_inner_mut(inner: &mut Slice) -> &mut OsStr {
         // Safety: OsStr is just a wrapper of Slice,
         // therefore converting &mut Slice to &mut OsStr is safe.
+        // Any method that mutates OsStr must be careful not to
+        // break platform-specific encoding, in particular Wtf8 on Windows.
         unsafe { &mut *(inner as *mut Slice as *mut OsStr) }
     }
 

src/libstd/sys/windows/os_str.rs

@@ -87,6 +87,8 @@ impl Buf {
         // Safety: Slice is just a wrapper for Wtf8,
         // and self.inner.as_mut_slice() returns &mut Wtf8.
         // Therefore, transmuting &mut Wtf8 to &mut Slice is safe.
+        // Additionally, care should be taken to ensure the slice
+        // is always valid Wtf8.
         unsafe { mem::transmute(self.inner.as_mut_slice()) }
     }