internal/planner: Insert general ref head objects starting from the leaves, not root

[srenatus]

This way the object insert operations can return a new object instance.

Before, the object construction for a rule like

p[a][b] := ...

would look like this:

*ir.BlockStmt BlockStmt (1 blocks)
  *ir.Block Block (3 statements)
    *ir.BlockStmt BlockStmt (1 blocks)
      *ir.Block Block (2 statements)
        *ir.DotStmt &{Source:{Value:Local<2>} Key:{Value:Local<10>} Target:Local<14>}
        *ir.BreakStmt &{Index:1}
    *ir.MakeObjectStmt &{Target:Local<14>}
    *ir.ObjectInsertOnceStmt &{Key:{Value:Local<10>} Value:{Value:Local<14>} Object:Local<2>}
*ir.ObjectInsertOnceStmt &{Key:{Value:Local<11>} Value:{Value:Local<13>} Object:Local<14>}

Now, it'll look like

*ir.BlockStmt BlockStmt (1 blocks)
  *ir.Block Block (2 statements)
    *ir.BlockStmt BlockStmt (1 blocks)
      *ir.Block Block (2 statements)
        *ir.DotStmt &{Source:{Value:Local<2>} Key:{Value:Local<10>} Target:Local<14>}
        *ir.BreakStmt &{Index:1}
    *ir.MakeObjectStmt &{Target:Local<14>}
*ir.ObjectInsertOnceStmt &{Key:{Value:Local<11>} Value:{Value:Local<13>} Object:Local<14>}
*ir.ObjectInsertStmt &{Key:{Value:Local<10>} Value:{Value:Local<14>} Object:Local<2>}

so the object in Local<14> is built first, and the added to object Local<2>.

[johanfylling]

This makes more sense 👍.

It would seem the original implementation was relying on an implementation detail in the Wasm engine/transpilation it shouldn't have.

I recall there being an issue with an earlier, similar approach to this, that this change doesn't appear to suffer from. But that earlier approach did some other things differently also, so I think those issues came from elsewhere.

There are yaml tests covering this; but I see we don't have tests for more than two vars in the ref. Maybe we should add at least one more case there covering a longer ref with more vars; e.g. p[a][b][c][d] := ..., where each var enumerates to more than one value.

[srenatus]

Thanks for the review, I'll add some tests there tomorrow

[srenatus]

@johanfylling if you could have another look, I'd appreciate it 😃

[johanfylling]

Thanks! LGTM 👍