Add example, bin configurations

[lure23]

Problem

I've been looking for a way to automatically enable certain features, when running examples (cargo run --example ...). There seem to be at least three 1 2 3 issues open on this matter, over the years - all are still open.

If it was possible to use #[cfg_attr(example, cfg(all()))] in build.rs, I could add a feature in there and be happy.

Tried it:

warning: unexpected `cfg` condition name: `example`
  --> build.rs:44:16
   |
44 |     #[cfg_attr(example, cfg(all()))]
   |                ^^^^^^^
   |
   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `docsrs`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`

From the error message it seems we have cfg(doc, ...) and cfg(test, ...) but not example and bin.

Is this an oversight or due to historical pedigree? It would feel consistent for me to have (at least) all these four equally covered (perhaps adding benchmark as well).

Proposed Solution

Have these be available reflecting the use of --example, --bin (and --benchmark) parameters of the cargo command:

#[cfg(example, ...)]
#[cfg(bin, ...)]

Notes

There are two work-arounds for this.

1. Manually provide features

   This is somewhat tedious and personally, I've wrapped cargo run --example ... with a Makefile to keep things short. Would rather have cargo run itself be able to do the right thing, consistently.

2. Explicitly making targets of each example in Cargo.toml

   Have not tried this. Mainly because I'd like to treat any file in examples/ the same; needing to mention them in Cargo.toml creates two tiers of examples.

[epage]

build.rs is for the package, not individual build targets, so providing a --cfg for the build target kind wouldn't work. If we changed it so it did, we'd need to rebuild and re-run the build script fr every build target type currently being built. If this were done by default, this would be prohibitively slow.

Related issues

• Specify a build script for only binaries #1430
• Optional (dev) build.rs #4511
• Allow specifying target-dependent build.rs scripts #4932
• Allow a build script (build.rs) in a virtual manifest Cargo.toml #8732

As the use case is to workaround the lack of other features, rather than being a need on its own, I would be inclined that we do not go in this direction.

From the error message it seems we have cfg(doc, ...) and cfg(test, ...) but not example and bin.

We don't even do cfg(test), see #13294. Unsure about cfg(doc).

[lure23]

Thank You for the explanation. A holistic view on how the tooling should be is appreciated!

Closing.