References to mock objects that live for `static break expectation evaluation

[Will-Low]

Description

From my understanding, expectations are checked on object drop. In cases where objects live for `static and are never dropped, this leads to expectations passing that shouldn't.

This fails, as expected, since my_method() is not being called 200 times:

use mockall::automock;

#[automock]
trait MyTrait {
    fn my_method(&self);
}

fn function_that_does_not_call_my_method<M>(mock: &MockMyTrait)
where M: MyTrait {
    println!("hello");
}

#[test]
fn my_method_is_called() {
    let mut mock = MockMyTrait::default();
    mock.expect_my_method().times(200);
    function_that_does_not_call_my_method::<MockMyTrait>(&mock);
}

However this passes, even though my_method() is not called 200 times:

use mockall::automock;

#[automock]
trait MyTrait {
    fn my_method(&self);
}

fn function_that_does_not_call_my_method<M>(mock: &MockMyTrait)
where M: MyTrait {
    println!("hello");
}

#[test]
fn my_method_is_called() {
    let mock = Box::leak(Box::new(MockMyTrait::default()));
    mock.expect_my_method().times(200);
    function_that_does_not_call_my_method::<MockMyTrait>(mock);
}

Proposals

1. Is there a technical solution that would allow this to be handled by mockall?
2. If not, can an error be raised when trying to do this?
3. Would it make sense to add this exception to the documentation?

[asomers]

Your test fails for me, as expected, like this:

---- my_method_is_called stdout ----
hello
thread 'my_method_is_called' panicked at 'MockMyTrait::my_method: Expectation(<anything>) called 0 time(s) which is fewer than expected 200', mockall/tests/issue_456.rs:3:1
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

How are you running it?

[Will-Low]

I'm just using cargo test.

Here's my output from the first version that I've confirmed is failing:

% cargo test
   Compiling test_doubles_with_mockall v0.1.0 (/Users/<REDACTED>/repos/test_doubles_with_mockall)
warning: unused variable: `mock`
 --> src/lib.rs:8:45
  |
8 | fn function_that_does_not_call_my_method<M>(mock: &MockMyTrait)
  |                                             ^^^^ help: if this is intentional, prefix it with an underscore: `_mock`
  |
  = note: `#[warn(unused_variables)]` on by default

warning: function `function_that_does_not_call_my_method` is never used
 --> src/lib.rs:8:4
  |
8 | fn function_that_does_not_call_my_method<M>(mock: &MockMyTrait)
  |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: `test_doubles_with_mockall` (lib) generated 2 warnings
warning: `test_doubles_with_mockall` (lib test) generated 1 warning (1 duplicate)
    Finished test [unoptimized + debuginfo] target(s) in 0.20s
     Running unittests src/lib.rs (/Users/<REDACTED>/repos/test_doubles_with_mockall/target/debug/deps/test_doubles_with_mockall-af9d419b6703c590)

running 1 test
test my_method_is_called ... FAILED

failures:

---- my_method_is_called stdout ----
hello
thread 'my_method_is_called' panicked at 'MockMyTrait::my_method: Expectation(<anything>) called 0 time(s) which is fewer than expected 200', src/lib.rs:3:1
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

failures:
    my_method_is_called

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

error: test failed, to rerun pass `--lib`

And here is the output from the second version that is not failing:

% cargo test
   Compiling test_doubles_with_mockall v0.1.0 (/Users/<REDACTED>/repos/test_doubles_with_mockall)
warning: unused variable: `mock`
 --> src/lib.rs:8:45
  |
8 | fn function_that_does_not_call_my_method<M>(mock: &MockMyTrait)
  |                                             ^^^^ help: if this is intentional, prefix it with an underscore: `_mock`
  |
  = note: `#[warn(unused_variables)]` on by default

warning: function `function_that_does_not_call_my_method` is never used
 --> src/lib.rs:8:4
  |
8 | fn function_that_does_not_call_my_method<M>(mock: &MockMyTrait)
  |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: `test_doubles_with_mockall` (lib) generated 2 warnings
warning: `test_doubles_with_mockall` (lib test) generated 1 warning (1 duplicate)
    Finished test [unoptimized + debuginfo] target(s) in 0.20s
     Running unittests src/lib.rs (/Users/<REDACTED>/repos/test_doubles_with_mockall/target/debug/deps/test_doubles_with_mockall-af9d419b6703c590)

running 1 test
test my_method_is_called ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

   Doc-tests test_doubles_with_mockall

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

I am using:

• mockall 0.11.3
• stable-aarch64-apple-darwin (default) rustc 1.67.0 (fc594f156 2023-01-24)
• cargo 1.67.0 (8ecd4f20a 2023-01-10)
• macOS 12.6.2
• Apple M1 Pro

[asomers]

Oh, I missed the Box::leak part. In that case, the mock never gets dropped, as you observed. What you should do is add a mock.checkpoint() line at the end of your test.

[Will-Low]

I tried that workaround in my code (not the example here), but it continued to be a problem in my case, since the reference is passed to a new thread, which holds it for `static. At that point, it is no longer possible to call mock.checkpoint(), since .checkpoint() requires &mut self, but the thread still has an active reference to the mock.

[asomers]

It sounds like you need some interior mutability then. Perhaps pass a Arc<Mutex<MyMock>> to the other thread.

[asomers]

Closing this stale issue on the assumption that it's been adequately answered. No code change in Mockall is required.