Add into_inner function for ConstraintSystemRef<F> (#347)

Co-authored-by: Weikeng Chen <[email protected]>
Co-authored-by: Pratyush Mishra <[email protected]>

CHANGELOG.md

@@ -1,3 +1,15 @@
+## Pending
+
+### Breaking changes
+
+### Features
+- [\#347](https://github.com/arkworks-rs/snark/pull/347) Add `into_inner` function for `ConstraintSystemRef<F>`.
+
+### Improvements
+
+### Bug fixes
+
+
 ## v0.2.0
 
 ### Breaking changes

relations/src/r1cs/constraint_system.rs

@@ -767,6 +767,16 @@ impl<F: Field> ConstraintSystemRef<F> {
         }
     }
 
+    /// Consumes self to return the inner `ConstraintSystem<F>`. Returns
+    /// `None` if `Self::CS` is `None` or if any other references to
+    /// `Self::CS` exist.  
+    pub fn into_inner(self) -> Option<ConstraintSystem<F>> {
+        match self {
+            Self::CS(a) => Rc::try_unwrap(a).ok().map(|s| s.into_inner()),
+            Self::None => None,
+        }
+    }
+
     /// Obtain an immutable reference to the underlying `ConstraintSystem`.
     ///
     /// # Panics