eval: support arbitrary map types?

[Ethan Reesor]

Adding something like this:

if mtyp, ok := expr.(*ast.MapType); ok {
	keyTyp, err := bi.findTypeExpr(mtyp.Key)
	if err != nil {
		return nil, err
	}
	elemTyp, err := bi.findTypeExpr(mtyp.Value)
	if err != nil {
		return nil, err
	}

	typ := new(godwarf.MapType)
	typ.KeyType = keyTyp
	typ.ElemType = elemTyp
	typ.Name = fmt.Sprintf("map[%v]%v", keyTyp, elemTyp)
	typ.Type = typ
	return typ, nil
}

to (*BinaryInfo).findTypeExpr would allow evaluating arbitrary map types. I can't think of a scenario where this would matter, but it also seems a bit odd that evaluating a map[Key]Value expression will fail if that specific type does not exist in the DWARF section.

[Alessandro Arzilli]

Why would we do this? To have a better error when doing type assertions to map types? If there is a way to successfully use this constructed type (for example by using typecasts) it will be impossible to do anything with it because the Type field needs to be a StructType with an actual description of the hashtable's layout (the typ.Type = typ line is wrong).

[Ethan Reesor]

It was just something that I ran into while I was working on #3691. As I said, I can't think of a scenario where this would matter. I'm fine with closing this, unless you think it's worth the added complexity to return a better error; e.g. if findType returns an error and the expression is a MapType, return "I can't make that map" or something instead of "I can't find that type".