Introducing Flux

Posted by Ranjit Jhala Nov 14, 2022
Tags: basic

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Types bring order to code. For example, if a variable i:usize then we know i is a number that can be used to index a vector. Similarly, if v:vec<&str> then we can be sure that v is a collection of strings which may be indexed but of course, not used as an index. However, by itself usize doesn’t tell us how big or small the number and hence the programmer must still rely on their own wits, a lot of tests, and a dash of optimism, to ensure that all the different bits fit properly at run-time.

Refinements are a promising new way to extend type checkers with logical constraints that specify additional correctness requirements that can be verified by the compiler, thereby entirely eliminating various classes of run-time problems.

We’re excited to introduce Flux, a refinement type checker plugin that brings this technology to Rust.

Indexed Types

The most basic form of refinement type in flux is a type that is indexed by a logical value. For example

Type Meaning
i32[10] The (singleton) set of i32 values equal to 10
bool[true] The (singleton) set of bool values equal to true


We can already start using these indexed types to start writing (and checking) code. For example we can write the following specification which says that the value returned by mk_ten must in fact be 10

#[flux::sig(fn() -> i32[10])]
pub fn mk_ten() -> i32 {
    5 + 4

but when you compile it, flux will say

error[FLUX]: postcondition might not hold
 --> src/
7 |     5 + 4
  |     ^^^^^

The error says that that the postcondition might not hold which means that the output produced by mk_ten may not in fact be an i32[10] as indeed, in this case, the result is 9! You can eliminate the error by replacing the body with 5 + 5 or just 10.


Here’s a second example that shows how you can use an index to restrict the space of inputs that a function expects.

#[flux::sig(fn (b:bool[true]))]
pub fn assert(b:bool) {
  if !b { panic!("assertion failed") }

Here, the refined specification for assert says that you can only call it with true as the input. So if you write

fn test(){
  assert(2 + 2 == 4);
  assert(2 + 2 == 5); // fails to type check

then flux will complain that

error[FLUX]: precondition might not hold
  --> src/
12 |     assert(2 + 2 == 5); // fails to type check
   |     ^^^^^^^^^^^^^^^^^^

meaning that the call to assert fails to establish that the input is indeed true (as of course, in this case, it is not!)

Index Parameters and Expressions

Its not terribly exciting to only talk about fixed values like 10 or true. To be more useful, flux lets you index types by refinement parameters. For example you can write

#[flux::sig(fn(n:i32) -> bool[0 < n])]
pub fn is_pos(n: i32) -> bool {
    if 0 < n {
    } else {

Here, the type says that is_pos

  • takes as input some i32 indexed by n
  • returns as output the bool indexed by 0 < n

in other words, the output is true exactly when 0 < n.

We might use this function to check that:

pub fn test_pos(n: i32) {
  let m = if is_pos(n) { n - 1 } else { 0 };
  assert(0 <= m);

Existential Types

Often we don’t care about the exact value of a thing – but just care about some properties that it may have. For example, we don’t care that an i32 is equal to 5 or 10 or n but that it is non-negative.

Type Meaning
i32{v: 0 < v} The set of i32 values that positive
i32{v: n <= v} The set of i32 values greater than or equal to n

Flux allows such specifications by pairing plain Rust types with assertions 1 that constrain the value. For example, we can rewrite mk_10 with the output type i32{v:0<v} that specifies a weaker property: the value returned by mk_ten is positive.

#[flux::sig(fn() -> i32{v: 0 < v})]
pub fn mk_ten() -> i32 {
    5 + 5

Similarly, you might specify that a function that computes the absolute value of an i32 with a type which says the result is non-negative and exceeds the input n.

#[flux::sig(fn (n:i32) -> i32{v:0<=v && n<=v})]
pub fn abs(n: i32) -> i32 {
    if 0 <= n {
    } else {
        0 - n

As a last example, you might write a function to compute the factorial of n

#[flux::sig(fn (n:i32) -> i32{v:1<=v && n<=v})]
pub fn factorial(n: i32) -> i32 {
    let mut i = 0;
    let mut res = 1;
    while i < n {
        i += 1;
        res = res * i;

Here the specification says the input must be non-negative, and the output is at least as large as the input. Note, that unlike the previous examples, here we’re actually changing the values of i and res.

Can you guess why the copilot suggestions failed to pass flux, and what refinements were inferred for i and res in the fixed code at the end?


In this post, we saw how Flux lets you

  1. decorate basic Rust types like i32 and bool with indices and constraints that let you respectively refine the sets of values that inhabit that type, and

  2. specify contracts on functions that state pre-conditions on the sets of legal inputs that they accept, and post-conditions that describe the outputs that they produce.

The whole point of Rust, of course, is to allow for efficient imperative sharing and updates, without sacrificing thread- or memory-safety. Next time, we’ll see how Flux melds refinements and Rust’s ownership to make refinements happily coexist with imperative code.

  1. These are not arbitrary Rust expressions but a subset of expressions from logics that can be efficiently decided by SMT Solvers↩︎