Let cargo test and cargo commands with --examples ignore examples that are not for the current target

[najamelan]

Describe the problem you are trying to solve
Currently cargo test --target wasm32-unkown-unkown will try to compile all examples. This is a problem if there is code that uses dev-dependencies that are not available on this target, like networking crates.

Describe the solution you'd like
This already works for files in the tests directory, but not for examples:
I would like to see an example ignored when it has: #![ cfg(not( target_arch = "wasm32" )) ] at the top of the file.

Currently this throws an error like:

error[E0601]: `main` function not found in crate `basic`
  |
  = note: consider adding a `main` function to `examples/basic.rs`

The only workaround I have found is to put a cfg attribute on each toplevel statement and to add a dummy main tagged with a cfg for wasm. This is problematic if you have a bunch of free standing functions and type definitions, especially since examples are a form of documentation.

Ideally also cargo would have a nice error message if a user tries to run an example on the wrong target. That is, if there is a positive cfg limiting the example to a single target, telling them to run it with cargo run --example basic --target wasm32-unknown-unknown and if there is a negative cfg, telling them: This example cannot run on target xxx.

[najamelan]

ps: I haven't tested what the score is with features, but I imagine it would be handy to in/exclude examples depending on whether a feature is turned on or not.

[najamelan]

I have a similar problem with a crate that compiles on stable, but has examples that require nightly. Now cargo test can no longer be run on stable.

[najamelan]

I made some progress without modifying cargo:

• I tried package.autoexamples = false + [[target."cfg(not( target_arch = \"wasm32\" ))".example]] but that failed. It seems that the target selection is specific to dependencies?
• I managed a workaround with package.autoexamples = false, a notwasm feature and using required-features = notwasm in the example section. The downside of this is that users can only run examples with: cargo run --example basic --features notwasm, which is unfortunate and also one needs to list all examples in Cargo.toml. I suppose this technique could also work with a nightly feature to allow tests to run in stable.

I suppose a solution to #4663 would solve most of the user facing problem here.

[4LT]

I'm having the similar problem, but with an example that uses #![cfg(feature = "std")]. Thought it would be worth mentioning since the issue is broader than just targets, but not worth creating a separate issue for.

[epage]

#9208 sounds like its wanting to solve the same problem, so I'm going to close this to consolidate the conversation. If there is something that I missed, let us know!