Cargo scripts try to build a nearby `src/lib.rs` if present

[kornelski]

Problem

If I place a script in the same directory as a crate, it uses crate's source files, but does not use crate's dependencies, causing unexpected compilation errors.

Steps

./Cargo.toml:

[package]
name = "mainpackage"
version = "0.1.0"
edition = "2021"

[dependencies]
hex = "0.4.3"

./src/lib.rs:

use hex;

./script.rs:

#!/usr/bin/env cargo +nightly -Zscript

//! ```cargo
//! [package]
//! edition = "2021"
//! ```

fn main() {
}

---

Running ./script.rs causes:

warning: `package.edition` is unspecified, defaulting to `2021`
   Compiling script v0.0.0 (…/mainpackage)
error[E0432]: unresolved import `hex`
 --> src/lib.rs:1:5
  |
1 | use hex;
  |     ^^^ no external crate `hex`

This looks broken, because src/lib.rs is compiled with scripts dependencies, not its usual crate dependencies.

Possible Solution(s)

I would expect Cargo.toml and src/ to be completely ignored by the scripts. Or if they're not ignored, to be compiled correctly as a library and included as an external crate in the script (like tests/ or main.rs get). But compiling the crate, but with a wrong Cargo.toml context seems like the worst of both.

Notes

BTW, "warning: package.edition is unspecified, defaulting to 2021" is printed, even though I've tried to specify it in all places.

Version

cargo 1.82.0-nightly (8f40fc59f 2024-08-21)
release: 1.82.0-nightly
commit-hash: 8f40fc59fb0c8df91c97405785197f3c630304ea
commit-date: 2024-08-21
host: aarch64-apple-darwin
libgit2: 1.8.1 (sys:0.19.0 vendored)
libcurl: 8.7.1 (sys:0.4.74+curl-8.9.0 system ssl:(SecureTransport) LibreSSL/3.3.6)
ssl: OpenSSL 1.1.1w  11 Sep 2023
os: Mac OS 15.0.0 [64-bit]

[epage]

BTW, "warning: package.edition is unspecified, defaulting to 2021" is printed, even though I've tried to specify it in all places.

It looks like this is because you are using an older syntax

//! ```cargo
//! [package]
//! edition = "2021"
//! ```

We dropped support for the doc-comment syntax in favor of rust-lang/rfcs#3503

I've verified the following does not produce the warning:

#!/usr/bin/env nargo
---
[package]
edition = "2021"
---

fn main () {
}

[epage]

I would expect Cargo.toml and src/ to be completely ignored by the scripts. Or if they're not ignored, to be compiled correctly as a library and included as an external crate in the script (like tests/ or main.rs get). But compiling the crate, but with a wrong Cargo.toml context seems like the worst of both.

We do, for bins, tests, benches, examples, and build scripts I overlooked that we don't have a way to control auto-discovery of lib targets. We'll either need to hack this in or add a control mechanism for lib auto-discovery.