cargo test --test <testname> builds binaries

[alkim0]

Problem
The documentation for cargo seems to suggest that if I run: cargo test --test apple, then,
this should run test apple without compiling any of the binaries. However, this doesn't seem to be the case.

Steps

1. Build a new rust project using: cargo new banana.
2. Create a simple lib.rs file in src/. My contents were:

pub struct A {}

3. Create a binary directory bin/ in src/ then add a file run.rs; my contents were:

use banana::A;
fn main() {
    let a = A {};
    println!("Hi");
}

4. Create a tests directory tests/ at the project root. Add a simple test file sample.rs with contents:

use banana::A;
#[test]
fn sample() {
    let a = A {};
    println!("Hi");
}

5. Run cargo test --test sample
6. bin/run.rs is compiled. We can tell since we can warnings about unused variables from it.

Notes

Output of cargo version:

cargo 1.43.0-nightly (e57bd0299 2020-02-21)

[ehuss]

This is done intentionally so that integration tests can execute the binaries to exercise their behavior. Seems like it would be good to document that! In general the Cargo documentation on writing tests is a little shallow.

[alkim0]

Is there a way to build the tests without the binaries?

Also, regarding documentation, according to the Cargo Book, it says:

Passing target selection flags will test only the specified targets.

and under the --tests flag, it says:

By default this includes the library and binaries built as unittests, and integration tests.

implying there is a way to avoid the default behavior, and it also states that --all-targets is equivalent to:

--lib --bins --tests --benches --examples

implying that there is a way to build tests without binaries (since --bins is specifically specified).

I propose these parts be rewritten for clarity.

[ehuss]

There is not currently a way to skip building the binaries.

[weihanglo]

Seems a explanation for auto-build binaries is already there?

Binary targets are automatically built if there is an integration test or benchmark. This allows an integration test to execute the binary to exercise and test its behavior.

https://github.com/rust-lang/cargo/blob/7b7d80e84ae74dd620c0f294d6f12ada16cdb94e/src/doc/man/cargo-test.md#target-selection

[weihanglo]

#10726 improves the doc a little. I'll close this issue as resolved. If you feel we need a fix other than a doc update, please file a new issue. Thank you!

[rambip]

There is not currently a way to skip building the binaries.

Oh, that was exactly the feature I was looking for 😕
I have a Yew project (so it compiles to webassembly, and then generates some javascript glue)
I wanted to write unit tests for this project, but I was not able to make it work:

• if I run cargo test, cargo will say it cannot run a wasm file
• if I try to change the target, it will fail to compile parts of the code
• if I decide to give up the unit tests and write integration tests instead, cargo will still complain it cannot run (if I chose the wasm target) or compile (if I chose the linux target) the tests ...