noprop

An imperative property-based testing library for Rust.

• No dependencies
• Expressive without a DSL
  • A small, orthogonal imperative API — the property is a plain Rust function that samples values ("generators" in PBT parlance) and asserts on the results.
  • Ordinary Rust control flow (if / match / for / recursion) and interior mutability express any generator directly — no combinator DSL or derive macros to learn.
• Stateful (model-based) PBT built in
  • The same API covers one-line properties, dependent generators, and command loops that compare a model against a system under test — no separate stateful framework or dependent-generation syntax for the harder cases.
• Guided sampling for hard-to-reach states
  • Feedback-guided search steers sampling toward the semantic states the property reports as interesting — deep protocol phases, hard-to-hit branches, long command sequences a uniform run would only find by chance.

Example

#[test]
fn addition_is_commutative() -> noprop::TestResult {
    noprop::Runner::new(0xDEAD_BEEF).run(1024, |ctx| {
        let a = noprop::sample_u32(ctx);
        let b = noprop::sample_u32(ctx);
        assert_eq!(a.wrapping_add(b), b.wrapping_add(a));
        Ok(())
    })?;
    Ok(())
}

The seed is caller-supplied, so a failure is reproducible: rerunning with the seed from the failure report reproduces the identical case. See docs/recipes.md for the seed / env-variable scaffolding, sampling patterns, stateful properties, feedback-guided search, and the failure-reproduction workflow.

When to use noprop

noprop is imperative-first and suits properties that are naturally sequential — dependent generation, model-based (stateful) commands, protocol traces — and where writing the generator as plain Rust reads more clearly than a combinator DSL. The API stays small so a project can adopt it as a dev-dependency without pulling in a graph of crates.

If you need automatic shrinking or file-based failure persistence, another PBT library will fit better today; noprop deliberately leaves those out in v0.1 (see below).

Main constraints

• panic=unwind is required. noprop catches property panics and uses panic-based unwinding for TestCaseContext::reject_case; panic=abort is not supported.
• No automatic shrinking in v0.1. The failure report instead carries an automatic value trace — every noprop::sample_* call recorded at its source location — so the failing input is visible without extra plumbing. Reproduce the failing case from the seed and case budget; if you want a frozen regression, hand-simplify the trace into a plain #[test].
• No file-based failure persistence. The caller manages the seed and case budget; there is no on-disk seed corpus.

Documentation

• Recipes — task-oriented recipes for common property shapes: seed / run scaffolding, sampling primitives and collections, rejection scopes, dependent generators, stateful properties, feedback-guided search, and reproducing a failing seed.
• Generator design — the small design decisions every sample_* generator has to make (support, distribution, termination, rejection scope, valid-by-construction).
• Feedback-guided search design — the design of Runner::run_feedback_guided, the corpus admission and eviction rules, and how the feature registry is bounded.
• API reference — every function and type on docs.rs.

The doc modules (docs::recipes, docs::generator_design, docs::feedback_guided_search) render the same Markdown on docs.rs, so the recipes and design notes appear alongside the API rustdoc.

Examples

The examples/ directory contains runnable end-to-end demos of the larger recipes (each runs with cargo run --example <name>):

• basics.rs — the minimal property shape against a real function, the common pitfalls (Fn closures and interior mutability, environment-controlled seeds via seed_from_env_or_time), and the short idioms for random-length collections and boundary values
• stateful.rs — model-based (stateful) property testing of an LRU cache with a bounded command loop
• feedback_guided.rs — steering the search toward interesting inputs (long log lines that exercise truncation) with event / bucket / transition
• rejection.rs — sample_with_rejection for constrained draws and reject_case for whole-case preconditions, parsing key=value config lines
• reproduce.rs — reproducing a failing seed via NOPROP_SEED and the failure report's reproduce hint (this one fails on purpose)

Benchmark

The detection benchmark harness lives in the benchmark/ workspace crate; see its README.md for how to run it.