feature: zig equivalent of @fieldParentPtr()

[pierrec]

Just had a case where I need to do pointer arithmetic, which is brittle and forces me to use a specific struct layout:

type Tree struct {
  click     ClickList
  core      treeCore
  // ...
}

func openTreeNode(click *ClickList, i int) {
  tree:= c.PtrAs[Tree](click)
  t := c.PtrAdd(tree, c.Sizeof[ClickList]()) // *treeCore
  // ...
}

In this case, Zig sidesteps this with the @fieldParentPtr() builtin, which I find elegant and is comptime safe: it checks the field actually exists and computes the offset for you.

func openTreeNode(click *ClickList, i int) {
 // Give me the Tree pointer from the pointer of its click field
 tree := c.FieldParentPtr[Tree](click, "click") // *Tree
 t := &tree.core
 // ...
}

[nalgeon]

This is probably doable with C's offsetof, but it won't be completely safe. All c calls happen at runtime, not compile time.

[pierrec]

If its not comptime, I dont really see the point. Will see if this is doable in a simple way.

[nalgeon]

Maybe I was overstating this :) The field offset is comptime. The pointer arithmetic is runtime. It's the same in Zig, I think.