TraceId

Stable named-int identifier minted by Tracer for each distinct TensorImpl observed during a trace. Unique within one TraceGraph only — never reused, never persisted.

Design. TraceId is a named std::int64_t rather than a raw alias so that:

• The trace-id namespace is greppable as a type (e.g. find every function that takes a TraceId parameter).
  • Hash specialization is bound to this type (no accidental overload conflicts with other 64-bit identifiers).
  • Future strict-typing upgrades (e.g. removing implicit conversion to int64_t to prevent passing a raw int where a TraceId is expected) only require flipping the operator std::int64_t() conversion. All call sites are already compile-time-verified to use the named type.

Implicit conversion. We deliberately preserve implicit conversion to/from std::int64_t for the first pass — there are ~400 call sites across the compile path that mix raw ints (Python bindings, monotonic counter arithmetic, sentinel -1 checks) with TraceId. Switching to explicit would force a parallel migration we can do in a follow-up. The named type still moves the codebase forward and unblocks the audit-flagged improvement.

Sentinel. TraceId::external_feed() (== -1) signals "not produced by any traced op" — i.e. an external feed (model parameter or user input). Use id == TraceId::external_feed() in new code; id.v < 0 still works for backwards compat.