Rollup merge of rust-lang#63013 - nivkner:ffi-safe-slice, r=sfackler

add `repr(transparent)` to `IoSliceMut` where missing

tried using `IoSliceMut` in FFI, got `improper_ctypes` warning.

according to the docs: `IoSliceMut` is  "guaranteed to be ABI compatible with the `iovec` type" so it should be usable in FFI.
`IoSlice` is also `repr(transparent)` for every platform where these types contain `iovec`-like types.
vxworks also has `IoSliceMut` as transparent so its not even consistently one or the other.

no comment about this next to the types or in the PR that introduced the types, so assuming this was just missed.

r? @sfackler

src/libstd/sys/unix/io.rs

@@ -29,6 +29,7 @@ impl<'a> IoSlice<'a> {
     }
 }
 
+#[repr(transparent)]
 pub struct IoSliceMut<'a> {
     vec: iovec,
     _p: PhantomData<&'a mut [u8]>,

src/libstd/sys/wasi/io.rs

@@ -29,6 +29,7 @@ impl<'a> IoSlice<'a> {
     }
 }
 
+#[repr(transparent)]
 pub struct IoSliceMut<'a> {
     vec: __wasi_iovec_t,
     _p: PhantomData<&'a mut [u8]>,

src/libstd/sys/windows/io.rs

@@ -29,6 +29,7 @@ impl<'a> IoSlice<'a> {
     }
 }
 
+#[repr(transparent)]
 pub struct IoSliceMut<'a> {
     vec: c::WSABUF,
     _p: PhantomData<&'a mut [u8]>,