Cold-path, portable validation of values declared by a usage spec.

Expressions use the language implemented by both jdx/expr.rs and the original expr-lang/expr Go package. The spec supplies a raw CLI value as value, so explicit conversions such as int(value) have the same meaning in every runtime.

What this build of the language carries

A validate= expression reads one string and returns a boolean, so this depends on expr-lang with its default features off — the dates, JSON codec and base64 it ships by default are twelve crates a CLI would compile to evaluate int(value) > 0. The operators, the string and array builtins, the numeric conversions and matches are all here.

matches is kept deliberately: the Go runtime has it, and a spec is supposed to mean the same thing in both.

A spec that does want the rest — date(value) > now(), fromJSON(value) — gets it by naming the feature in the application's manifest, which cargo unifies with this crate's copy:

expr-lang = { version = "2.1", features = ["temporal"] }

Without that, such an expression fails the same way nosuchfn(value) always has: not at [check] time, which only parses, but when a value reaches it — with a message naming the feature to add.