`cargo test` doesn't mention which file(s) in examples/ is affected when the `main` function is missing

[Cobrand]

Basically, if you have a lib crate with multiple files in "examples", but one of them doesn't have a main, you get this error when running cargo test:

error[E0601]: main function not found

But you don't know which file is affected by this error.

It would be better to have an error message that says "main function not found in examples/something.rs".

Note that it may be related to rustc itself, but just in case I opened an issue here as it seemed more fit.

[alexcrichton]

Thanks for the report! Can you gist the full error message as well as perhaps an example project to reproduce?

[Cobrand]

Extremely simple example: https://github.com/Cobrand/cargo_error_example

Just cargo new --lib and create an empty file in examples/.

Full error message for cargo test:

   Compiling cargo_error_example v0.1.0 (file:///home/cobrand/Code/cargo_error_example)
error[E0601]: main function not found

error: aborting due to previous error

error: Could not compile `cargo_error_example`.

To learn more, run the command again with --verbose.

[alexcrichton]

Hm ok, isn't this covered by

error: Could not compile `cargo_error_example`.

?

[lukaslueg]

The error refers to the name of the package, not the name of the example. Put a file examples/foo.rs and the error still says cargo_error_example.

There are three things going on here:

• We could place an additional (?) warning here and refer to key.target.src_path().to_str_lossy() or something.
• The errors in part come directly from rustc and cargo itself and the output is already quite clobbered. Notice that there are four error:-lines and one warning:-line referring to the same error.
• The overall output is actually different depending on whether or not the queue is empty. Running cargo test the first time (or after cargo clean) gives

Compiling issue4948 v0.1.0 (file:///home/lukas/issue4948)
error[E0601]: main function not found

error: aborting due to previous error

error: Could not compile `issue4948`.
warning: build failed, waiting for other jobs to finish...
error: build failed

Running a second time gives

Compiling issue4948 v0.1.0 (file:///home/lukas/issue4948)
error[E0601]: main function not found

error: aborting due to previous error

error: Could not compile `issue4948`.

To learn more, run the command again with --verbose.

Notice that there is no warning, no error: build failed, yet a info referring to --verbose the second time around. Maybe this needs to get cleaned up?

[ehuss]

One option is to just change the error message (here) to include the crate name. Messages without spans (thus filenames) are very rare, and this is the only one that I think might be confusing. Another example is the lint warning that a crate name should have snake case, but it clearly tells you the name of the crate. Other messages without spans include linker errors and incorrect command-line arguments (presumably the user can figure these out?).

[ehuss]

I think this can be closed now that rust-lang/rust#48706 has been merged.

[alexcrichton]

Great!