Le marteau-pilon, forges et aciéries de Saint-Chamond, Joseph-Fortuné LAYRAUD, 1889

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📢 Are you interested in verifying Rust code? Don't know where to start? Please contact me, I'm always looking for case-studies. 📢

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About

Creusot is a tool for deductive verification of Rust code. It allows you to annotate your code with specifications, invariants and assertions and then verify them formally and automatically, proving, mathematically, that your code satisfies your specifications.

Creusot works by translating Rust code to WhyML, the verification and specification language of Why3. Users can then leverage the full power of Why3 to (semi)-automatically discharge the verification conditions!

See ARCHITECTURE.md for technical details.

⚠️ This is research software, and favors progress over stability ⚠️

Citing Creusot

If you would like to cite Creusot in academic contexts, we encourage you to use our ICFEM'22 publication.

Examples of Verification

To get an idea of what verifying a program with Creusot looks like, we encourage you to take a look at some of our test suite:

• Zeroing out a vector
• Binary search on Vectors
• Sorting a vector
• IterMut
• Normalizing If-Then-Else Expressions

More examples are found in creusot/tests/should_succeed.

Projects built with Creusot

• CreuSAT is a verified SAT solver written in Rust and verified with Creusot. It really pushes the tool to its limits and gives an idea of what 'use in anger' looks like.
• Another big project is in the works :)

Installing Creusot as a user

. Set up Rust - Install rustup, to get the suitable Rust toolchain

1. Set up Why3
   • Get opam, the package manager for OCaml
   • Pin why3 to master :

   $ opam pin add why3 https://gitlab.inria.fr/why3/why3.git
   $ opam pin add why3-ide https://gitlab.inria.fr/why3/why3.git
   

   • Install why3 and why3-ide: $ opam install lablgtk3 lablgtk3-sourceview3 ocamlgraph why3 why3-ide
   • Get some SMT solvers: Z3 (available by brew, apt, etc.), CVC4 (brew, apt, etc.), Alt-Ergo (opam, apt, etc.)
   • Configure Why3: $ why3 config detect
     • Troubleshoot: When your z3 is a bit too new (e.g., Why3 supports up to ver. 4.8.10 but yours is 4.8.12), Why3 refuses z3. Then you can try hacking Why3 to make it consider your z3 be of an older version (e.g., 4.8.10), by updating the relevant field of ~/.why3.conf.
2. Clone the creusot repo at any directory you like
3. Build Creusot
   • Enter the cloned directory and run $ cargo install --path creusot, this will build the cargo-creusot and creusot-rustc executables and place them in ~/.cargo/bin.

Verifying with Creusot and Why3

The recommended way for users to verify programs with Creusot is to use cargo-creusot. All you need to do is enter your project and run cargo creusot! This will generate MLCFG files in target/debug/ which can then be loaded into Why3.

This may only work if you're using the same rust toolchain that was used to build creusot: you can copy the rust-toolchain file into the root of your project to make sure the correct toolchain is selected.

To add contracts to your programs you will need to use the creusot-contracts crate by adding it as a dependency:

# Cargo.toml

[dependencies]
creusot-contracts = { path = "/path/to/creusot/creusot-contracts" }

Adding this dependency will make the contract macros available in your code. These macros will erase themselves when compiled with rustc. To add Creusot-only trait implementations or code, you can use cfg(creusot) to toggle.

Proving in Why3

To load your files in Why3, we recommend using the ide script provided in the Creusot repository. You may also copy both this script and the prelude directory in your project to have a fully self contained proof environment.

To load your proofs in Why3, run:

REPO/ide PATH/TO/OUTPUT.mlcfg

From there standard proof strategies of Why3 work. We recommend section 2.3 of this thesis for a brief overview of Why3 and Creusot proofs.

We plan to improve this part of the user experience, but that will have to wait until Creusot gets more stable and complete. If you'd like to help, a prototype VSCode plugin for Why3 is in development, it should make the experience much smoother when complete.

Writing specs in Rust programs

Using Creusot for your Rust code

First, you will need to depend on the creusot-contracts crate, add it to your Cargo.toml and enable the contracts feature to turn on contracts.

Kinds of contract expressions

Currently Creusot uses 4 different kinds of contract expressions: requires, ensures, invariant and variant.

The most basic are requires and ensures, which can be attached to a Rust function declaration like so:

#[requires(... precondition ...)]
#[ensures(... postcondition ...)]
fn my_function(i: u32) -> bool { ... }

You can attach as many ensures and requires clauses as you would like, in any order.

Inside a function, you can attach invariant clauses to loops, these are attached on top of the loop rather than inside, like:

#[invariant(invariant_name, ... loop invariant ...)]
while ... { ... }

Invariants must have names (for now).

A variant clause can be attached either to a function like ensures, or requires or to a loop like invariant, it should contain a strictly decreasing expression which can prove the termination of the item it is attached to.

Controlling verification

We also have features for controlling verification.

First, the trusted marker lets Creusot trust the implementation and specs. More specifically, you can put #[trusted] on a function like the following:

#[trusted]
#[ensures(result == 42u32)]
fn the_answer() -> u32 {
  trusted_super_oracle("the answer to life, the universe and everything")
}

Causing Creusot to assume the contracts are true.

Unbounded integers

By default in Creusot, integers are represented with bounds-checking. This can be tedious or difficult to prove in certain cases, so we can disable bounds checking by passing the --unbounded flag to Creusot.

Pearlite

Contracts and logic functions are written in Pearlite, a specification language for Rust we are developing. Pearlite can be seen as a pure, immutable fragment of Rust which has access to a few additional logical operations and connectives. In practice you have:

• Base Rust expressions: matching, function calls, let bindings, binary and unary operators, tuples, structs and enums, projections, primitive casts, and dereferencing
• Logical Expressions: quantifiers (forall and exists), logical implication ==>, logical equality a == b, labels
• Rust specific logical expressions: access to the final value of a mutable reference ^, access to the model of an object @

We also provide two new attributes on Rust functions: logic and predicate. Marked #[logic] or #[predicate], a function can be used in specs and other logical conditions (requires/ensures and invariant). The two attributes have the following difference.

• A logic function can freely have logical, non-executable operations, such as quantifiers, logic equalities, etc. Instead, this function can't be called in normal Rust code (the function body of a logic function is replaced with a panic). You can use pearlite syntax for any part in the logic function by marking that part with the pearlite! { ... } macro.
• A predicate is a logical function which returns a proposition (in practice, returns a boolean value).

When you write recursive logic or predicate functions, you have to show that the function terminates. For that, you can add #[variant(EXPR)] attribute, which says that the value of the expression EXPR strictly decreases (in a known well-founded order) at each recursive call. The type of EXPR should implement the WellFounded trait.

You can also give a custom model to your type. To do that, you just implement the Model trait (provided in creusot_contracts) specifying the associated type Model. You give a trusted spec that defines the model (which can be accessed by @) on primitive functions. For example, the following gives a spooky data type MyPair<T, U> a nice pair model.

impl<T, U> Model for MyPair<T, U> {
    type Target = (T, U);
}
#[trusted]
#[ensures(@result == (a, b))]
fn my_pair<T, U>(a: T, b: U) -> MyPair<T, U> {
  ...
}