chore(traits): added exhaustive test for trait function calls

[nickysn]

This test proves that trait function calls from inside the body of other trait functions are resolved and executed correctly. All combinations of:

default implementation
override with default
override with no default

are tested, for both the caller and callee. For now, this tests method calls only (functions with self as first parameter). Once function calls via Self::function_name() are implemented, the test can be extended to cover that as well.

[jfecher]

I don't think it's very practical to try testing every combination of something. Especially when adding a new dimension of something to test ends up increasing our test cases 4x. Instead of trying to test every single case, lets narrow it down to a select few that are more likely to cause issues for the compiler and aren't already tested by other integration tests.

[nickysn]

I don't think it's very practical to try testing every combination of something. Especially when adding a new dimension of something to test ends up increasing our test cases 4x. Instead of trying to test every single case, lets narrow it down to a select few that are more likely to cause issues for the compiler and aren't already tested by other integration tests.

I know it's not always practical to test every combination of something, unless the number of test conditions is small, yielding a small number of combinations. In this case, we have 3 * 3 = 9 cases, that will be extended to 2 * 2 * 3 * 3 = 27 cases, once function calls via Self:: (a.k.a. static methods) are implemented. This is a very practical number of cases for an automated test and I'm not planning to increase the test conditions for this test any further, because it serves a very specific purpose - it tests whether function call resolution is done correctly with regards to choosing between trait default and trait impl ("override") implementation. This is also a feature that is specific to traits and is different, compared to e.g. struct impl function calls, where there's no such thing as default and override implementations (and combinations). We're not planning to adding any more dimensions to the test, that are unrelated to that (for example, once traits for non-struct types are implemented, we're not going to add every non-struct type as a new dimension to this test, but we'll definitely add a new test that just implements a trait for each type - that's because implementing traits for non-struct types is something that is completely independent from trait function call resolution).