Change Waiter::wait to consume self

This fixes a bug where the compiler complained about lacking `mut`
annotations when `groth16` was enabled but `multicore` was disabled.
The type of `Waiter::wait` changed based on whether or not `multicore`
was enabled; feature flags should only add APIs, not change them.

CHANGELOG.md

@@ -6,6 +6,14 @@ and this project adheres to Rust's notion of
 [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Fixed
+- Compiling with `--no-default-features --features groth16` (i.e. disabling the
+  `multicore` feature flag) works again.
+
+### Changed
+- `bellman::multicore::Waiter::wait` now consumes `self` (better reflecting the
+  fact that you can't wait on the same result twice), instead of taking `&self`
+  with `multicore` enabled and `&mut self` with multicore disabled.
 
 ## [0.11.0] - 2021-09-08
 ### Added

src/multicore.rs

@@ -96,8 +96,8 @@ mod implementation {
     }
 
     impl<T> Waiter<T> {
-        /// Wait for the result.
-        pub fn wait(&self) -> T {
+        /// Consumes this waiter and blocks until the result is ready.
+        pub fn wait(self) -> T {
             // This will be Some if this thread is in the global thread pool.
             if rayon::current_thread_index().is_some() {
                 let msg = "wait() cannot be called from within a thread pool since that would lead to deadlocks";
@@ -177,8 +177,8 @@ mod implementation {
     }
 
     impl<T> Waiter<T> {
-        /// Wait for the result.
-        pub fn wait(&mut self) -> T {
+        /// Consumes this waiter and blocks until the result is ready.
+        pub fn wait(mut self) -> T {
             self.val.take().expect("unmet data dependency")
         }