SigSet: Add the repr(transparent) attribute

This commit adds the `repr(transparent)` attribute to the `SigSet`
struct, to make sure that its representation is exactly like the
`sigset_t` struct from C, in all cases.

Signed-off-by: German Maglione <gmaglione@redhat.com>

CHANGELOG.md

@@ -34,6 +34,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
   (#[1713](https://github.com/nix-rust/nix/pull/1713))
 - `nix::poll::ppoll`: `sigmask` parameter is now optional.
   (#[1739](https://github.com/nix-rust/nix/pull/1739))
+- `signal:SigSet` it's now marked as `repr(trasparent)`.
+  (#[1741](https://github.com/nix-rust/nix/pull/1741))
 
 ### Fixed
 

src/sys/signal.rs

@@ -523,6 +523,9 @@ use std::iter::FromIterator;
 use std::iter::IntoIterator;
 
 /// Specifies a set of [`Signal`]s that may be blocked, waited for, etc.
+// We are using `transparent` here to be super sure that `SigSet`
+// is represented exactly like the `sigset_t` struct from C.
+#[repr(transparent)]
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
 pub struct SigSet {
     sigset: libc::sigset_t