External specifications

Since the Rust standard library and external libraries do not specify contracts for their functions, Prusti allows specifying the contract for functions separately from where they are implemented. Such a specification looks like a regular impl, with the exception that there are no bodies in the implementation functions, and that the impl has an #[extern_spec] attribute.

The standard library type std::option::Option could be specified as follows:

use prusti_contracts::*;

impl<T> std::option::Option<T> {
    #[ensures(matches!(*self, Some(_)) == result)]
    pub fn is_some(&self) -> bool;

    #[ensures(self.is_some() == !result)]
    pub fn is_none(&self) -> bool;

    pub fn unwrap(self) -> T;

    // ...

Any function in an external specification is implicitly trusted (as if marked with #[trusted]). It is possible to specify multiple #[extern_spec] implementations for the same type, but it is an error to externally specify the same function multiple times.

The extern_spec attribute accepts an optional argument to provide the module path to the function being specified. For example, to specify std::mem::swap, the argument is std::mem:

use prusti_contracts::*;

#[ensures(*a === old(snap(b)) && *b === old(snap(a)))]
fn swap<T>(a: &mut T, b: &mut T);