Make Errno::clear safe

All it does is assign a value to a thread-local int.  There's nothing
unsafe about that.

Author: asomers

CHANGELOG.md

@@ -48,6 +48,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   `::nix::sys::reboot` now return `Result<Infallible>` instead of `Result<Void>` (#[1239](https://github.com/nix-rust/nix/pull/1239))
 - `sys::socket::sockaddr_storage_to_addr` is no longer `unsafe`.  So is
   `offset_of!`.
+- `sys::socket::sockaddr_storage_to_addr`, `offset_of!`, and `Errno::clear` are
+  no longer `unsafe`.
   (#[1244](https://github.com/nix-rust/nix/pull/1244))
 - Several `Inotify` methods now take `self` by value instead of by reference
   (#[1244](https://github.com/nix-rust/nix/pull/1244))

src/errno.rs

@@ -46,8 +46,11 @@ cfg_if! {
 }
 
 /// Sets the platform-specific errno to no-error
-unsafe fn clear() {
-    *errno_location() = 0;
+fn clear() {
+    // Safe because errno is a thread-local variable
+    unsafe {
+        *errno_location() = 0;
+    }
 }
 
 /// Returns the platform-specific value of errno
@@ -70,7 +73,7 @@ impl Errno {
         from_i32(err)
     }
 
-    pub unsafe fn clear() {
+    pub fn clear() {
         clear()
     }