Show a better error when using --test with #[proc_macro_derive]

[keeperofdakeys]

Fixes #37480

Currently using --test with a crate that contains a #[proc_macro_derive] attribute causes an error. This PR doesn't attempt to fix the issue itself, or determine what tesing of a proc_macro_derive crate should be - just to provide a better error message.

[rust-highfive]

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[alexcrichton]

Thanks for the PR! I do think though that cargo test should work out of the box for proc-macro crates somehow at least. One option was to make it work in the compiler but otherwise we'll need to add knowledge to Cargo to not test the library of proc-macro crates.

This to me seems like the most conservative option with respect to the compiler, however, so seems reasonable to me.

@rust-lang/compiler thoughts about taking this strategy? (erroring on --test in the compiler and not testing in Cargo by default)

[keeperofdakeys]

I agree that it should work, but there are probably a few questions that need answering first. Fixing the misleading errors is probably a good start.

[alexcrichton]

True yeah, I'll go ahead and r+ and we can discuss further strategies in parallel.

@bors: r+

[bors]

📌 Commit 3254fab has been approved by alexcrichton

[bors]

⌛ Testing commit 3254fab with merge bfa709a...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-rustbuild
Approved by: alexcrichton
Pushing bfa709a to master...