Less Mutex poisoning

If a test case panics while holding a static method's internal Mutex
locked, for example by violating a times() constraint, then clear the
Mutex's poison during Drop.  We're going to delete all of its
Expectations anyway, so there's no data to be poisoned.

Requires nightly.

Fixes #515

CHANGELOG.md

@@ -20,6 +20,10 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Changed
 
+- No longer poison a Context object's internal `Mutex` when panicing.  This
+  requires the "nightly" feature.
+  ([#527](https://github.com/asomers/mockall/pull/527))
+
 - Raised predicates, which is reexported, to 3.0.0.  This may affect backwards
   compatibility for users who make use of predicates's "color" feature.
   ([#467](https://github.com/asomers/mockall/pull/467))

mockall/tests/clear_expectations_on_panic.rs

@@ -34,7 +34,7 @@ fn too_few_calls() {
 
 // We shouldn't panic during drop in this case.  Regression test for
 // https://github.com/asomers/mockall/issues/491
-#[should_panic(expected = "called `Result::unwrap()` on an `Err` value: PoisonError { .. }")]
+#[cfg_attr(not(feature = "nightly"), ignore)]
 #[test]
 fn too_many_calls() {
     panic::catch_unwind(|| {

mockall_derive/src/mock_function.rs

@@ -1547,6 +1547,11 @@ impl<'a> ToTokens for Context<'a> {
         #[cfg(feature = "nightly_derive")]
         let must_use = quote!();
 
+        #[cfg(not(feature = "nightly_derive"))]
+        let clear_poison = quote!();
+        #[cfg(feature = "nightly_derive")]
+        let clear_poison = quote!(EXPECTATIONS.clear_poison(););
+
         quote!(
             /// Manages the context for expectations of static methods.
             ///
@@ -1594,6 +1599,9 @@ impl<'a> ToTokens for Context<'a> {
             impl #ty_ig Drop for Context #ty_tg #ty_wc {
                 fn drop(&mut self) {
                     if ::std::thread::panicking() {
+                        // Clear poison since we're about to clear the Mutex's
+                        // contents anyway.
+                        #clear_poison
                         // Drain all expectations so other tests can run with a
                         // blank slate.  But ignore errors so we don't
                         // double-panic.