rust-project: remove hardcoded mode file in OSS build

[jazzdan]

With this patch I am able to run the following command in a non-fbcode buck project and get a functioning rust-project.json file:

rust-project develop --prefer-rustup-managed-toolchain //my:target

Fixes #519

[jazzdan]

I have this test too but not sure how useful it would be

fn test_select_mode() {
    // Test default behavior without the fbcode_build feature
    assert_eq!(select_mode(None), None);
    assert_eq!(
        select_mode(Some("custom-mode")),
        Some("custom-mode".to_owned())
    );

    // Test behavior with the fbcode_build feature enabled
    #[cfg(all(feature = "fbcode_build", target_os = "macos"))]
    {
        assert_eq!(select_mode(None), Some("@fbcode//mode/mac".to_owned()));
        assert_eq!(
            select_mode(Some("custom-mode")),
            Some("custom-mode".to_owned())
        );
    }

    #[cfg(all(feature = "fbcode_build", target_os = "windows"))]
    {
        assert_eq!(select_mode(None), Some("@fbcode//mode/win".to_owned()));
        assert_eq!(
            select_mode(Some("custom-mode")),
            Some("custom-mode".to_owned())
        );
    }

    #[cfg(all(
        feature = "fbcode_build",
        not(any(target_os = "macos", target_os = "windows"))
    ))]
    {
        assert_eq!(select_mode(None), None);
        assert_eq!(
            select_mode(Some("custom-mode")),
            Some("custom-mode".to_owned())
        );
    }
}

[davidbarsky]

I have this test too but not sure how useful it would be

fn test_select_mode() {
    // Test default behavior without the fbcode_build feature
    assert_eq!(select_mode(None), None);
    assert_eq!(
        select_mode(Some("custom-mode")),
        Some("custom-mode".to_owned())
    );

    // Test behavior with the fbcode_build feature enabled
    #[cfg(all(feature = "fbcode_build", target_os = "macos"))]
    {
        assert_eq!(select_mode(None), Some("@fbcode//mode/mac".to_owned()));
        assert_eq!(
            select_mode(Some("custom-mode")),
            Some("custom-mode".to_owned())
        );
    }

    #[cfg(all(feature = "fbcode_build", target_os = "windows"))]
    {
        assert_eq!(select_mode(None), Some("@fbcode//mode/win".to_owned()));
        assert_eq!(
            select_mode(Some("custom-mode")),
            Some("custom-mode".to_owned())
        );
    }

    #[cfg(all(
        feature = "fbcode_build",
        not(any(target_os = "macos", target_os = "windows"))
    ))]
    {
        assert_eq!(select_mode(None), None);
        assert_eq!(
            select_mode(Some("custom-mode")),
            Some("custom-mode".to_owned())
        );
    }
}

It'd be great to have this test because the CI will be running with and without fbcode build :)

[jazzdan]

@davidbarsky done!

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