I'm not sure I fully understand your question, Thomas, but I've been addressing this topic elsewhere, so I might as well continue in this forum.
Weldfast Engineer requires a target. The IPG tells us this about triggered abilities that require a target:
The controller must announce those choices before they next pass priority.
Effectively, you have to choose targets when you put a triggered ability on the stack - so doing something that could only occur after that ability triggered, without mentioning targets, is an indication of a Missed Trigger. Saying “Combat, target FOO with the Engineer” is fine - if FOO is already an artifact creature, that is. Just saying “Combat?” and then trying to crew something is a failure on two fronts - one, you missed your Engineer trigger by not naming a target; two, you're now in Declare Attackers, too late to crew.
Moral of the story: if you control Weldfast Engineer, you should probably crew your Vehicle(s) before you say “Combat?”, and name your target(s) at the proper time.
Many people are getting confused, because of another recent Beginning of Combat trigger -
Toolcraft Exemplar. This is a trigger that affects the game in non-visible ways; it doesn't meet the criteria of the first three bullet points, and it clearly fits the fourth one, so:
The controller must make the change known by the first time the change has an effect on the visible game state.
While I don't like this means of communication, policy allows AP to say “Combat?” and not mention their trigger; when the opponent passes, AP can say "Exemplar trigger resolves, it's now 3/2, I'll tap it to crew my
Heart of Kiran, and attack…“ (NAP can interrupt that and respond to the crew ability, or respond after crew resolves.)
I'd much rather players say ”Combat, Trigger“ and point to whatever is triggering - the Exemplar, in this case. Clear communication is always going to be better for the judge(s) who have to sort things out. However, there's a (very slight?) strategic advantage to making your opponent think you might have forgotten a trigger, so … meh.
Please, teach your players to crew (or animate lands, etc.) in their main phase. There are very few exceptions, and most of them are rare. (Toolcraft Exemplar is an exception, and it's not rare, which makes our job harder.)
Also, encourage clearer communication.
Most of all, make sure they know what they're required to know - not just the ”Combat?" shortcut, but the entirety of the MTR; in 1.10, the MTR says
Players are responsible for
…
Being familiar with the rules contained within this document.
I've heard a lot of complaints that players shouldn't be surprised by how “Combat?” really works. That line in the MTR disagrees, and I do too. I realize that some will still be surprised, and we should be gentle (but firm!) when educating them - but, bottom line, if you're going to play a game competitively, know the rules.
d:^D