Rust build.rs "OUT_DIR not set" / Generated File Missing in CI

Code that reads OUT_DIR - typically an include!(concat!(env!("OUT_DIR"), "/generated.rs")) - failed because OUT_DIR wasn’t set, or the build script didn’t write the file the code includes. OUT_DIR only exists for code invoked by Cargo with a build script present.

What this error means

The build fails with environment variable OUT_DIR not defined, or a No such file or directory for a generated file under OUT_DIR. It often appears when invoking rustc/a tool directly outside Cargo, or when build.rs didn’t run or didn’t emit the expected file.

cargo output

error: environment variable `OUT_DIR` not defined at compile time
 --> src/proto.rs:1:14
  |
1 | include!(concat!(env!("OUT_DIR"), "/items.rs"));
  |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = help: use `std::env::var("OUT_DIR")` to read at run time

Common causes

OUT_DIR only set by Cargo with a build script

Cargo sets OUT_DIR for the crate when a build.rs exists. Compiling the file directly with rustc, or in a crate that has no build script, leaves env!("OUT_DIR") undefined at compile time.

The build script didn’t write the included file

The code includes a file the build script is supposed to generate into OUT_DIR. If build.rs failed, was skipped, or wrote to the wrong path, the include can’t find it.

How to fix it

Build through Cargo with a real build.rs

Ensure the crate has a build.rs and is built by Cargo (not a bare rustc call), and that the script writes into OUT_DIR.

build.rs

// build.rs
use std::{env, fs, path::Path};
fn main() {
    let out = env::var("OUT_DIR").unwrap();
    fs::write(Path::new(&out).join("items.rs"), "pub const N: u32 = 1;").unwrap();
}

Confirm the script ran and emitted the file

Run cargo build -vv and check the build-script ran and its output path.
Verify the generated file exists under target/.../build/<crate>-*/out/.
Make sure you’re compiling via Cargo, not invoking rustc on the file directly.

How to prevent it

Only read OUT_DIR from code Cargo compiles for a crate that has a build.rs.
Have the build script create the included file deterministically every run.
Use cargo build -vv to confirm build-script output when diagnosing.