What is an idiomatic way to have shared utility functions for integration tests and benchmarks?

Create a shared crate (preferred)

As stated in the comments, create a new crate. You don't have to publish the crate to crates.io. Just keep it as a local unpublished crate inside your project and mark it as a development-only dependency:

.
├── Cargo.toml
├── src
│   └── lib.rs
├── tests
│   └── integration.rs
└── utilities
    ├── Cargo.toml
    └── src
        └── lib.rs

Cargo.toml

# ...

[dev-dependencies]
utilities = { path = "utilities" }

utilities/src/lib.rs

pub fn shared_code() {
    println!("I am shared code");
}

tests/integration.rs

extern crate utilities;

#[test]
fn a_test() {
    utilities::shared_code();
}

A test-only module

You could place a module inside your crate that is only compiled when a specific feature is passed. This is the same concept used for unit tests. This has the advantage that it can access internals of your library code. It has the disadvantage that you need to pass the flag each time you run the code.

Cargo.toml

# ...

[features]
test-utilities = []

src/lib.rs

#[cfg(feature = "test-utilities")]
pub mod test_utilities {
    pub fn shared_code() {
        println!("I'm inside the library")
    }
}

tests/integration.rs

extern crate the_library;

#[test]
fn a_test() {
    the_library::test_utilities::shared_code();
}

execution

cargo test --features=test-utilities

Use a module from an arbitrary file path

This is just ugly to me, and really goes out of the normal path.

utilities.rs

pub fn shared_code() {
    println!("This is just sitting out there");
}

tests/integration.rs

#[path = "../utilities.rs"]
mod utilities;

#[test]
fn a_test() {
    utilities::shared_code();
}

See also:

• Where should I put test utility functions in Rust?

You could add those utility-functions to a pub-module inside your main crate and use the #[doc(hidden)] or #![doc(hidden)] attribute to hide them from the docs-generator. Extra comments will guide the reader to why they are there.