Confusing import errors for build.rs script

[kraktus]

Code

# Cargo.toml
build = "src/build.rs"

// src/build.rs
mod foo;

use crate::foo::Foo;

fn main() -> std::io::Result<()> {
	Foo;
	Ok(())
}

// src/foo.rs
use crate::Lib;

pub struct Foo;

// src/lib.rs
pub mod foo;

pub use foo::Foo;

pub enum Lib {
    A,
    B,
}

fn main() {
    println!("Hello, world!");
}

Current output

error[E0432]: unresolved import `crate::Lib`
 --> src/foo.rs:1:5
  |
1 | use crate::Lib;
  |     ^^^^^^^^^^ no `Lib` in the root

For more information about this error, try `rustc --explain E0432`.

Desired output

I am not sure, but at least should say the error comes from the build script

+ When compiling the build script `src/build.rs`
error[E0432]: unresolved import `crate::Lib`
 --> src/foo.rs:1:5
  |
1 | use crate::Lib;
  |     ^^^^^^^^^^ no `Lib` in the root

For more information about this error, try `rustc --explain E0432`.
+ Help: you may want to declare that module in the build script

Rationale and extra context

This error got me puzzled for a while before I had the thought to look at the build script.

[weihanglo]

I don't think rustc knows things about build scripts. Let me transfer this to rust-lang/cargo.

[weihanglo]

AFAIK a build script is expected to be built before its package being built¹. I usually debug with cargo build --verbose so that I can clearly see it's a crate named build_script_build, which compiles src/build.rs, that failed.

   Compiling issue-11598 v0.1.0 (/home/user/projects/issue-11598)
     Running `rustc --crate-name build_script_build --edition=2021 src/build.rs ...`
error[E0432]: unresolved import `crate::Lib`
 --> src/foo.rs:2:5
  |
2 | use crate::Lib;
  |     ^^^^^^^^^^ no `Lib` in the root

For more information about this error, try `rustc --explain E0432`.
error: could not compile `issue-11598` due to previous error

Caused by:
  process didn't exit successfully: `rustc --crate-name build_script_build --edition=2021 src/build.rs ...` (exit status: 1)

I don't really have a good idea at hand. We could possibly denote which kind of target is failing. Cargo warnings already got that logic here. Could possiblly reuse it a bit with a refined message like:

   Compiling issue-11598 v0.1.0 (/home/user/projects/issue-11598)
error[E0432]: unresolved import `crate::Lib`
 --> src/foo.rs:2:5
  |
2 | use crate::Lib;
  |     ^^^^^^^^^^ no `Lib` in the root

For more information about this error, try `rustc --explain E0432`.
+ error: could not compile `issue-11598` (build-script) due to previous error

Footnotes

1. (https://doc.rust-lang.org/cargo/reference/build-scripts.html#life-cycle-of-a-build-script ↩

[ehuss]

I think this is essentially the same as #10834, so closing as a duplicate of that.