ast+topdown+planner: non-built-in function mocking

[srenatus]

Follow-up to #4540

We can now mock functions that are user-defined:

package test

f(_) = 1 {
    input.x = "x"
}
p = y {
    y := f(1) with f as 2
}

...following the same scoping rules as laid out for built-in mocks.
The replacement can be a value (replacing all calls), or a built-in,
or another non-built-in function.

Also addresses bugs in the previous slice:

• topdown/evalCall: account for empty rules result from indexer
• topdown/eval: capture value replacement in PE could panic

Note: in PE, we now drop 'with' for function mocks of any kind:

These are always fully replaced in the saved support modules, so
this should be OK.

When keeping them, we'd also have to either copy the existing definitions
into the support module; or create a function stub in it.

Fixes #4449.

[johanfylling]

This might be a somewhat contrived example, but when the same function is mocked at multiple layers in the "call stack", my initial expectation would have been that the outer-most with would "win".

package mocking.user_defined

f(_) = 1 {
    input.x = "x"
}

p = y { 
    y := f(1) with f as 2 
}

test_p {
    v := p with f as 3
    v == 2 # is 3 expected here?
}

Is there a line of thought here that I'm not grasping, or thinking of?

[srenatus]

This might be a somewhat contrived example, but when the same function is mocked at multiple layers in the "call stack", my initial expectation would have been that the outer-most with would "win".

Yeah, it's the other way around, I'm afraid. Every new with encountered adds a new frame onto the with-stack, and when a function is to be evaluated, the lookups (for replacements) go top-to-bottom.

So when you evaluate f(1), the stack of withs is [ {f: 3}, {f: 2} ], and the rightmost one "wins".

It's the same as plain data mocks, e.g.

package ex

f = 1 {
    input.x = "x"
}

p = y { 
    y := f with f as 2 
}

test_p {
    v := p with f as 3
    v == 2 # is 3 expected here?
}

[johanfylling]

LGTM! 👍

[johanfylling]

Just a personal preference, but I like it when the note is the first field. Helps me getting an overview, as I then can see it as the "title", with the "body" of the test case following it.

But browsing some other test cases, I see this is the norm.