Allow mocking methods that return references to trait objects

Fixes #210

CHANGELOG.md

@@ -6,6 +6,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ## [Unreleased] - ReleaseDate
 ### Added
 
+- Added the ability to mock methods returning references to trait objects.
+  ([#213](https://github.com/asomers/mockall/pull/213))
+
 - `mock!` supports a new syntax: "impl Trait for".  It has two benefits:
   * It can implement a generic trait for specific generic type(s).
   * It allows mocking a non-local trait.

mockall/src/lib.rs

@@ -507,6 +507,25 @@
 //! assert_eq!("abcd", mock.name());
 //! ```
 //!
+//! Similarly, Mockall will use a Boxed trait object for the Expectation of
+//! methods that return references to trait objects.
+//!
+//! ```
+//! # use mockall::*;
+//! # use std::fmt::Display;
+//! #[automock]
+//! trait Foo {
+//!     fn name(&self) -> &dyn Display;
+//! }
+//!
+//! # fn main() {
+//! let mut mock = MockFoo::new();
+//! mock.expect_name().return_const(Box::new("abcd"));
+//! assert_eq!("abcd", format!("{}", mock.name()));
+//! # }
+//! ```
+//!
+//!
 //! ## Impl Trait
 //!
 //! Rust 1.26.0 introduced the `impl Trait` feature.  It allows functions to

mockall/tests/mock_return_dyn_trait.rs

@@ -0,0 +1,49 @@
+// vim: tw=80
+//! a method that returns a reference to a trait object
+#![deny(warnings)]
+
+use mockall::*;
+use std::fmt::Debug;
+
+trait T: Debug + Sync {
+    fn mutate(&mut self) {}
+}
+
+impl T for u32 {}
+
+impl<Q> T for Q where Q: Debug + Sync + AsMut<dyn T> {}
+
+mock!{
+    Foo {
+        fn foo(&self) -> &dyn Debug;
+        fn bar(&self) -> &'static dyn T;
+        fn baz(&mut self) -> &mut dyn T;
+    }
+}
+
+#[test]
+fn return_const() {
+    let mut mock = MockFoo::new();
+    mock.expect_foo()
+        .return_const(Box::new(42u32) as Box<dyn Debug>);
+
+    assert_eq!("42", format!("{:?}", mock.foo()));
+}
+
+#[test]
+fn static_ref() {
+    let mut mock = MockFoo::new();
+    mock.expect_bar()
+        .return_const(&42u32 as &dyn T);
+
+    assert_eq!("42", format!("{:?}", mock.bar()));
+}
+
+#[test]
+fn return_var() {
+    let mut mock = MockFoo::new();
+    mock.expect_baz()
+        .return_var(Box::new(42u32) as Box<dyn T>);
+
+    mock.baz().mutate();
+}

mockall_derive/src/mock_function.rs

@@ -3,6 +3,29 @@ use super::*;
 
 use quote::ToTokens;
 
+/// Convert a trait object reference into a reference to a Boxed trait
+fn dedynify(ty: &mut Type) {
+    if let Type::Reference(ref mut tr) = ty {
+        if let Type::TraitObject(ref tto) = tr.elem.as_ref() {
+            if let Some(lt) = &tr.lifetime {
+                if lt.ident == "static" {
+                    // For methods that return 'static references, the user can
+                    // usually actually supply one, unlike nonstatic references.
+                    // dedynify is unneeded and harmful in such cases.
+                    //
+                    // But we do need to add parens to prevent parsing errors
+                    // when methods like returning add a `+ Send` to the output
+                    // type.
+                    *tr.elem = parse2(quote!((#tto))).unwrap();
+                    return;
+                }
+            }
+
+            *tr.elem = parse2(quote!(Box<#tto>)).unwrap();
+        }
+    }
+}
+
 /// Convert a special reference type like "&str" into a reference to its owned
 /// type like "&String".
 fn destrify(ty: &mut Type) {
@@ -206,6 +229,7 @@ impl<'a> Builder<'a> {
             ReturnType::Type(_, ref ty) => {
                 let mut output_ty = supersuperfy(&**ty, self.levels);
                 destrify(&mut output_ty);
+                dedynify(&mut output_ty);
                 output_ty
             }
         };