stage1: support @alignOf(T) aligned member inside struct T

[kristate]

Fixes #1832

• Implementation
• Documentation (comments)
• Tests

[kristate]

@bdoner All tests passed; can you please test your code regarding #1832 ?

[svc-user]

Will check it out later this evening!

[andrewrk]

Here's a problematic case:

export fn entry() void {
    var n1: Node1 = undefined;
    var n2: Node2 = undefined;
}

const Node1 = struct {
    data: [@alignOf(Node1)]u8,
};

const Node2 = struct {
    data: [1]u8,
};

  %n1 = alloca %Node1, align 8
  %n2 = alloca %Node2, align 1

This will also cause under-alignment problems when #1512 is implemented. The problem is that the assumption that if we hit a recursion loop then the answer is pointer alignment is not always correct.

A more correct solution involves a larger change that introduces "lazy type values" where you can query information, such as, "are you zero bits or not?", "what is your alignment?" while minimally evaluating the sub-expressions in the type. For example, for the expression []align(@alignOf(Node)) Node, we can answer the alignment query without evaluating the expression @alignOf(Node) because the pointer alignment is irrelevant to the query; based on the fact that it is a slice we already know it is pointer size aligned.

That said, this is stage1, and this is an improvement. So I think we can merge this and I'll make a separate issue for the problematic cases.

[andrewrk]

Merged. Thanks @kristate

[kristate]

A more correct solution involves a larger change that introduces "lazy type values" where you can query information, such as ...

Yes, I spent a day thinking about such a system and even thought about piggybacking ontop of ResolveStatus and friends, but in the end I decided on this solution as a stop-gap until copy elision lands.

It would be nice to have some sort of graph to query like you mentioned.

[svc-user]

I tried but failed to build zig on Windows. I trust that you, kristate, and andrewrk are competent enough to test it properly.

Thanks for the swift response and resolution.