tools: add Tied value

Tie two dependent values together, such as a nix::Dir with
its nix::dir::Iter<'a> where 'a is tied to the Dir's
lifetime, making it otherwise impossible to return them or
store them in a struct.

Alternatively we could try the 'rental' crate.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

src/tools.rs

@@ -24,6 +24,7 @@ pub mod wrapped_reader_stream;
 #[macro_use]
 pub mod common_regex;
 pub mod ticket;
+pub mod borrow;
 
 /// The `BufferedReader` trait provides a single function
 /// `buffered_read`. It returns a reference to an internal buffer. The

src/tools/borrow.rs

@@ -0,0 +1,45 @@
+/// This ties two values T and U together, such that T does not move and cannot be used as long as
+/// there's an U. This essentially replaces the borrow checker's job for dependent values which
+/// need to be stored together in a struct {}, and is similar to what the 'rental' crate produces.
+pub struct Tied<T, U: ?Sized>(Option<Box<T>>, Option<Box<U>>);
+
+impl<T, U: ?Sized> Drop for Tied<T, U> {
+    fn drop(&mut self) {
+        // let's be explicit about order here!
+        std::mem::drop(self.1.take());
+    }
+}
+
+impl<T, U: ?Sized> Tied<T, U> {
+    /// Takes an owner and a function producing the depending value. The owner will be inaccessible
+    /// until the tied value is resolved. The dependent value is only accessible by reference.
+    pub fn new<F>(owner: T, producer: F) -> Self
+    where
+        F: FnOnce(*mut T) -> Box<U>,
+    {
+        let mut owner = Box::new(owner);
+        let dep = producer(&mut *owner);
+        Tied(Some(owner), Some(dep))
+    }
+
+    pub fn into_boxed_inner(mut self) -> Box<T> {
+        self.1 = None;
+        self.0.take().unwrap()
+    }
+
+    pub fn into_inner(self) -> T {
+        *self.into_boxed_inner()
+    }
+}
+
+impl<T, U: ?Sized> AsRef<U> for Tied<T, U> {
+    fn as_ref(&self) -> &U {
+        self.1.as_ref().unwrap()
+    }
+}
+
+impl<T, U: ?Sized> AsMut<U> for Tied<T, U> {
+    fn as_mut(&mut self) -> &mut U {
+        self.1.as_mut().unwrap()
+    }
+}