JIT deserialization overview (facet-format)

Facet’s JIT lives in facet-format and is used by format crates like facet-json.

When to read this

• You’re touching anything under facet-format/src/jit/ or enabling the jit feature.
• You’re investigating performance changes in deserialization (especially “tier2”/JIT benchmarks).
• You’re debugging a JIT crash (SIGSEGV/UB-ish symptoms).

Mental model

facet-format defines a format-neutral deserialization pipeline:

• A FormatParser (implemented by each format crate) produces a stream of ParseEvents.
• A shape-driven layer consumes those events and writes to an output value (often via facet-reflect).

The JIT accelerates this by compiling deserialization code specialized for: