Function fn_id_for_decl 

pub fn fn_id_for_decl(ctx: &CodegenContext, fd: &FnDef) -> Option<FnId>

Resolve &FnDef to the opaque crate::ir::FnId from the symbol table. Pointer-eq scope detection routes module-owned fns through their canonical Module.fn key. Returns None when the fn isn’t registered (built-ins / synthesized variants the table excludes by design).