Support returning default values for methods returning references

When the nightly feature is enabled, methods whose return values aren't
explicitly set will return a default value, if the type implements
Default.  This feature has now been extended to methods returning
references to types that implement Default.

CHANGELOG.md

@@ -3,6 +3,19 @@
 All notable changes to this project will be documented in this file.
 This project adheres to [Semantic Versioning](http://semver.org/).
 
+## [Unreleased] - ReleaseDate
+### Added
+### Changed
+### Fixed
+
+- Methods returning non-`'static` references (mutable or otherwise) will now
+  return a default value if no return value is set, if the output type
+  implements `Default` and the `nightly` feature is enabled.  This more closely
+  matches existing behavior for methods returning non-reference types.
+  ([#32](https://github.com/asomers/mockall/pull/32))
+
+### Removed
+
 ## [0.3.0] - 10 August 2019
 ### Added
 

mockall/src/lib.rs

@@ -929,7 +929,7 @@
 //!
 //! With **nightly** enabled, you can omit the return value like this:
 #![cfg_attr(feature = "nightly", doc = "```")]
-#![cfg_attr(not(feature = "nightly"), doc = "```ignore")]
+#![cfg_attr(not(feature = "nightly"), doc = "```should_panic")]
 //! # use mockall::*;
 //! #[automock]
 //! trait Foo {
@@ -1169,6 +1169,7 @@ downcast!(dyn AnyExpectations);
 
 #[doc(hidden)]
 pub trait ReturnDefault<O> {
+    fn maybe_return_default() -> Option<O>;
     fn return_default() -> O;
 }
 
@@ -1179,18 +1180,30 @@ pub struct DefaultReturner<O: 'static>(PhantomData<O>);
 ::cfg_if::cfg_if! {
     if #[cfg(feature = "nightly")] {
         impl<O> ReturnDefault<O> for DefaultReturner<O> {
+            default fn maybe_return_default() -> Option<O> {
+                None
+            }
+
             default fn return_default() -> O {
                 panic!("Can only return default values for types that impl std::Default");
             }
         }
 
         impl<O: Default> ReturnDefault<O> for DefaultReturner<O> {
+            fn maybe_return_default() -> Option<O> {
+                Some(O::default())
+            }
+
             fn return_default() -> O {
                 O::default()
             }
         }
     } else {
         impl<O> ReturnDefault<O> for DefaultReturner<O> {
+            fn maybe_return_default() -> Option<O> {
+                None
+            }
+
             fn return_default() -> O {
                 panic!("Returning default values requires the \"nightly\" feature");
             }

mockall/tests/mock_return_mutable_reference.rs

@@ -9,6 +9,16 @@ mock! {
     }
 }
 
+#[test]
+#[cfg_attr(not(feature = "nightly"),
+           should_panic(expected = "Returning default values requires"))]
+fn return_default() {
+    let mut mock = MockFoo::new();
+    mock.expect_foo();
+    let r = mock.foo(0);
+    assert_eq!(u32::default(), *r);
+}
+
 #[test]
 fn return_var() {
     let mut mock = MockFoo::new();

mockall/tests/mock_return_reference.rs

@@ -39,6 +39,16 @@ fn return_const() {
     assert_eq!(5, *mock.foo());
 }
 
+#[test]
+#[cfg_attr(not(feature = "nightly"),
+           should_panic(expected = "Returning default values requires"))]
+fn return_default() {
+    let mut mock = MockFoo::new();
+    mock.expect_foo();
+    let r = mock.foo();
+    assert_eq!(u32::default(), *r);
+}
+
 mod sequence {
     use super::*;
 

mockall/tests/mock_struct.rs

@@ -184,8 +184,9 @@ fn return_const() {
     assert_eq!(42, mock.foo(5));
 }
 
+#[cfg_attr(not(feature = "nightly"),
+           should_panic(expected = "Returning default values requires"))]
 #[test]
-#[cfg_attr(not(feature = "nightly"), ignore)]
 fn return_default() {
     let mut mock = MockFoo::new();
     mock.expect_foo();

mockall/tests/mock_struct_with_static_method.rs

@@ -20,6 +20,18 @@ fn checkpoint() {
     panic!("Shouldn't get here!");
 }
 
+// Until we get some kind of context for free functions, the return_default
+// feature won't work if there are any other tests that mock the same function.
+// https://github.com/asomers/mockall/issues/30
+#[cfg_attr(not(feature = "nightly"),
+           should_panic(expected = "Returning default values requires"))]
+#[test]
+fn return_default() {
+    MockFoo::expect_baz();
+    let r = MockFoo::baz(5);
+    assert_eq!(u8::default(), r);
+}
+
 #[test]
 fn returning() {
     MockFoo::expect_bar()