Macro composite_id 

macro_rules! composite_id {
    ($name:ident, $local:ty) => { ... };
}

Declares a composite IR ID: a { func: FunctionId, local: LocalX } pair.

SSA is intra-function, so every instruction/block/param/edge handle carries the owning function plus a function-local index. Unlike the Identifier newtypes these do not index a global Registry — they route through the owning FunctionBody’s arena. Derived Ord compares func then local.