Module json 

What a value file is written in.

The probe’s value representation is JSON, because a probe has to be callable from a shell script and diffable by a human. This is the whole of what that costs: two trait implementations, and the four rules that are JSON’s rather than IDL’s.

The four rules

• A 64-bit integer is a string. A JSON number is a double, so 9007199254740993 read back through one is 9007199254740992. A value file that spells a 64-bit integer as a number therefore says a different number than its author typed and every reader of the file agrees on the wrong one. Refused on the way in (EXACT_JSON_INTEGER) and never produced on the way out — for every value of those two widths and not only the large ones, because a form that changed shape at 2^53 would compare unequal across the boundary.
• A long double is a string. Sixty of a binary128 significand’s 113 bits do not fit a double. A number is still accepted on the way in, because 1.5 in a value file means 1.5 and every double widens into binary128 exactly; the way out is always the exact decimal.
• A fixed<P, S> is a string, for the same reason: a fixed<31, 2> is exactly the value a double cannot carry.
• NaN and infinity have no spelling. JSON has neither, so a sample carrying one has no canonical form and is refused out loud rather than written as null, which would compare equal to an absent member.

Everything else about what a value means is in crate::contract and is the same in every representation.

Why this is here and not in zerodds-wire-probe

It belongs with the probe, which is the crate that reads value files; Rust’s orphan rule puts it here instead. Both traits and serde_json::Value are foreign to the probe, so the probe cannot implement one for the other. Behind the default serde-json feature, so a caller that brings its own representation does not take the dependency.

Constants

EXACT_JSON_INTEGER

The largest integer a JSON number carries without loss.