Take no exit

Cargo.toml

@@ -1,9 +1,12 @@
 [package]
 name = "take_mut"
-version = "0.1.3"
+version = "0.1.4"
 authors = ["Sgeo <sgeoster@gmail.com>"]
 license = "MIT"
 homepage = "https://github.com/Sgeo/take_mut"
 repository = "https://github.com/Sgeo/take_mut"
 description = "Take a T from a &mut T temporarily"
-documentation = "https://crates.fyi/crates/take_mut/0.1.3/"
+documentation = "https://crates.fyi/crates/take_mut/0.1.3/"
+
+[dependencies]
+unreachable="0.1.1"

src/lib.rs

@@ -1,10 +1,15 @@
-//! This crate provides (at this time) a single function, `take()`.
+//! This crate provides function, `take()`.
 //!
 //! `take()` allows for taking `T` out of a `&mut T`, doing anything with it including consuming it, and producing another `T` to put back in the `&mut T`.
 //!
 //! During `take()`, if a panic occurs, the entire process will be exited, as there's no valid `T` to put back into the `&mut T`.
 //!
 //! Contrast with `std::mem::replace()`, which allows for putting a different `T` into a `&mut T`, but requiring the new `T` to be available before being able to consume the old `T`.
+//!
+//! The crate also provides `take_no_exit()` function, which behaves similarly but instead of exiting
+//! the program on panic, it leaves a sentinel value there.
+
+extern crate unreachable;
 
 mod exit_on_panic;
 
@@ -39,6 +44,43 @@ pub fn take<T, F>(mut_ref: &mut T, closure: F)
     });
 }
 
+/// Represents an invalid value that is safe to drop
+pub trait Sentinel: Sized {
+    /// Creates the sentinel.
+    fn new_sentinel() -> Self;
+
+    /// Releases the sentinel. Calling this indicates that nothing unexpected happened.
+    /// The caller must make sure that the value this function is called with is the exact same
+    /// value the `new_sentinel()` funtion returned.
+    unsafe fn release_sentinel(self) {
+    }
+}
+
+impl<T> Sentinel for Option<T> {
+    fn new_sentinel() -> Self {
+        None
+    }
+
+    unsafe fn release_sentinel(self) {
+        // This avoids unnecessary check for None
+        use unreachable::UncheckedOptionExt;
+        self.unchecked_unwrap_none();
+    }
+}
+
+/// This function is similar to `take()` but instead of exiting, it will leave sentinel value in
+/// place of the original in case of panic.
+pub fn take_no_exit<T, F>(mut_ref: &mut T, closure: F)
+  where T: Sentinel,
+        F: FnOnce(T) -> T {
+    use std::mem::replace;
+    unsafe {
+        let old_t = replace(mut_ref, Sentinel::new_sentinel());
+        let new_t = closure(old_t);
+        replace(mut_ref, new_t).release_sentinel();
+    }
+}
+
 
 #[test]
 fn it_works() {
@@ -58,4 +100,4 @@ fn it_works() {
        Foo::B
     });
     assert_eq!(&foo, &Foo::B);
-}
+}