Have you considered implementing InlineArrayType with pure QL changes? One could identify relevant structs by looking at their attributes in QL.

How do we represent inline arrays in MaD rows? Can we use .Element with inline array types?

Yes, I thought about doing that as well, but the nature of an inline array is very different from a struct and it is my impression that the inline array implementation is just a "hack" that is put on structs, so I wanted to keep it separate.
Do you have a strong opinion one way or the other?

I expect that .Element will work as array read and stores are based on ArrayAccess (which we have ensured in the extractor implementation), but I will check. It is my impression that the intention with inline arrays are to mostly do very local performance optimisations and not as such as expose these via an API.

Do you have a strong opinion one way or the other?

I don't have a strong opinion. I tend to avoid DB changes if possible, just because it's cumbersome to add the stats, write downgrade scripts, ... But those are already done in this case, so I don't have a clear preference. At the same time, I think the below could get us pretty far without the added DB structure.

class InlineArrayType extends ValueType {
  Attribute a;
  Field f;
  int length;

  InlineArrayType() {
    a = this.(Attributable).getAnAttribute() and
    a.getType().hasFullyQualifiedName("System.Runtime.CompilerServices", "InlineArrayAttribute") and
    length = a.getArgument(0).getValue().toInt() and
    f = this.getAField()
  }

  Type getElementType() { result = f.getType() }

  ...
}

Do you have a strong opinion one way or the other?

I don't have a strong opinion. I tend to avoid DB changes if possible, just because it's cumbersome to add the stats, write downgrade scripts, ... But those are already done in this case, so I don't have a clear preference. At the same time, I think the below could get us pretty far without the added DB structure.

class InlineArrayType extends ValueType {
  Attribute a;
  Field f;
  int length;

  InlineArrayType() {
    a = this.(Attributable).getAnAttribute() and
    a.getType().hasFullyQualifiedName("System.Runtime.CompilerServices", "InlineArrayAttribute") and
    length = a.getArgument(0).getValue().toInt() and
    f = this.getAField()
  }

  Type getElementType() { result = f.getType() }

  ...
}

This is an excellent point for avoiding to add the extra relation. I will try out something like this, but I would prefer that we introduce the new type kind.