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Rust core specification

This project is about adding specifications to Rust's core lib, by decorating it with thorough functional tests, item-wise. This project aims at generating tests for the core library and at facilitating verification of Rust programs.

Approach

For every function f, we want to write a comprehensive and documented list of contracts that are satisfied by f. Each contract exercises one aspect of the behavior of a function. Ideally, the set of contracts for a given function should be complete, i.e. every input falls into at least one contract.

Contracts are universally quantified predicates about functions: for each of them, we generate a list of concrete tests, exercising the validity of each contract.

Reduced manual effort and clarity

We need to write contracts manually: contracts define the specifications of functions.

Independenlty, we generate automatically test cases for those contracts.

Quickstart

The various contracts are defined in the src/specs folder.

Running cargo build generates the tests under two different formats.

  1. generated_tests/doc.rs: a "literate" Rust module which contains tests as documentation examples. This format is especially useful for browsing the tests: just run cargo doc --no-deps --open and browse the tests module.
  2. generated_tests/bin.rs: a binary that packs all the tests as plain asserts!, running them in sequence. No TCB involved (e.g. rustdoc), just vanilla Rust.

Tests are being generated via cargo build using a build script located in build/main.rs by instantiating the contracts with generated test vectors. More information is available in the "Contracts" section.

Tests can be run in two different ways:

  • cargo test will ask rustdoc to test all the documentation examples from generated_tests/doc.rs. The downside is that you need to trust that rustdoc indeed executes all the tests.
  • cargo run will run the binary built from generated_tests/bin.rs, which runs all the tests in a very simple and understandable way.

Contracts

Definition

A contract is defined by the following data.

  • Some metadata (title, english description).
  • A sequence of typed input variables x₁: T₁...xᵢ: Tᵢ. Note: let's call domain the tuple type (T₁, ..., Tᵢ)
  • A precondition (a boolean predicate on the contract's domain);
  • A postcondition (a boolean predicate on the contract's domain).

Test vector

A contract can be instantiated with values of its domain.

Abstractions

To define contracts, we usually use abstractions.

For instance, we model the addition on machine integer of some size with the addition on mathematical integers: whenever x↑ + y↑ < 256 holds, then u8::add(x, y) == int::add(x↑, y↑)↓ holds. Note ↑ and ↓ lift up and down machine integers to mathematical ones.

While such abstractions are great at the level of contracts, when we instantiate them with test vector, the resulting tests should be very concrete and self contained.

Let's continue with addition example. Instantiating our contract with (x, y) = (40, 2), we would like to test that u8::add(40, 2) == 42 holds, not u8::add(40, 2) == int::add(40↑, 2↑)↓.

The final tests we will run against core should be as concrete as possible: we don't want to test core modulo the semantics of our abstraction. We want to test core itself, and the valididty of the generated tests should be obvious to the reader.

Partial computations: have your cake and eat it too

Here, we want both to have abstract contracts and very concrete tests: it seems a bit contradictory.

We need a way of collapsing our abstractions, but only the abstractions. It is the job of the proc-macro crate partial_compute. Given a function of type (T₁, …, Tᵢ, …, Tⱼ) → U it is able to produce a function (T₁, …, Tᵢ) → Expr: given the i first arguments, it produces a rust expression (a string) where the i first arguments are inlined and where some parts are pre-computed.

Through this mechanics, we can concretize a contract in a flexible way.

Test vector generation

Currently test vectors are a combination of (1) an optional, user-defined list of tests and (2) randomly generated values.

gWe want to leverage the hax toolchain to automatically find edge cases, given the actual core implementation and the specification.