Revert "Revert "use RWlock when accessing os::env #81850""

This reverts commit acdca316c3.

library/std/src/sys_common/rwlock.rs

@@ -86,3 +86,62 @@ impl RWLock {
         self.0.destroy()
     }
 }
+
+// the cfg annotations only exist due to dead code warnings. the code itself is portable
+#[cfg(unix)]
+pub struct StaticRWLock(RWLock);
+
+#[cfg(unix)]
+impl StaticRWLock {
+    pub const fn new() -> StaticRWLock {
+        StaticRWLock(RWLock::new())
+    }
+
+    /// Acquires shared access to the underlying lock, blocking the current
+    /// thread to do so.
+    ///
+    /// The lock is automatically unlocked when the returned guard is dropped.
+    #[inline]
+    pub fn read_with_guard(&'static self) -> RWLockReadGuard {
+        // SAFETY: All methods require static references, therefore self
+        // cannot be moved between invocations.
+        unsafe {
+            self.0.read();
+        }
+        RWLockReadGuard(&self.0)
+    }
+
+    /// Acquires write access to the underlying lock, blocking the current thread
+    /// to do so.
+    ///
+    /// The lock is automatically unlocked when the returned guard is dropped.
+    #[inline]
+    pub fn write_with_guard(&'static self) -> RWLockWriteGuard {
+        // SAFETY: All methods require static references, therefore self
+        // cannot be moved between invocations.
+        unsafe {
+            self.0.write();
+        }
+        RWLockWriteGuard(&self.0)
+    }
+}
+
+#[cfg(unix)]
+pub struct RWLockReadGuard(&'static RWLock);
+
+#[cfg(unix)]
+impl Drop for RWLockReadGuard {
+    fn drop(&mut self) {
+        unsafe { self.0.read_unlock() }
+    }
+}
+
+#[cfg(unix)]
+pub struct RWLockWriteGuard(&'static RWLock);
+
+#[cfg(unix)]
+impl Drop for RWLockWriteGuard {
+    fn drop(&mut self) {
+        unsafe { self.0.write_unlock() }
+    }
+}