Treat #[test] like #[cfg(test)] in non-test builds

[jseyfried]

This PR treats #[test] like #[cfg(test)] in non-test builds. In particular, like #[cfg(test)],

• #[test] nodes are stripped during cfg processing, and
• #[test] is disallowed on non-optional expressions.

Closes #33946.
r? @nrc

[jseyfried]

cc @LeoTestard

[LeoTestard]

👍

[LeoTestard]

Could you add a test similar to the #33946 case?

[nrc]

I'd be against doing this (but not strongly so) - I like the orthogonal-ness of #[cfg(test)] vs #[test]. Probably wants a lang team decision though.

[jseyfried]

@nrc These are my reasons for supporting this:

• It would allow us to use #[cfg(test)] macros in #[test]s (we can already use other #[cfg(test)] items in #[test]s). For example,

#[cfg(test)] fn f() {}
#[cfg(test)] macro_rules! m { () => {} }

#[test]
fn test() {
    f(); //< this is OK,
    m!(); //< but this is currently an error (on non-test builds).
}

• It would allow us to enable features only for tests, for example

#![cfg_attr(test, feature(stmt_expr_attributes))]

#[test]
fn test() {
    let _ = #[attr] (); //< Currently, this is a gated feature error (on non-test builds).
}

• If we expand an item, which requires resolving macro invocation paths, I think we should also resolve other paths. Since we can't backwards compatibly resolve all paths in #[test]s, I think we should not resolve any paths in #[test]s, meaning no expansion (on non-test builds).

[nikomatsakis]

lang-team thinks this is good :)

[nrc]

We discussed this at the language meeting last week. The important point I missed earlier was that to some extent #[test] already has the behaviour of stripping the code away in non-test compiles, it just happens later in compilation, so all this PR does is bring that behaviour earlier and thus avoid some errors. It therefore seems like pure win.

[nrc]

@bors: r+

(Would be good to add a test for 33946, but not essential, keep r=me with the test too)

[bors]

📌 Commit 369a996 has been approved by nrc

[bors]

⌛ Testing commit 369a996 with merge 6af2e10...

[bors]

💔 Test failed - auto-win-msvc-64-cargotest

[jseyfried]

@bors r=nrc

[bors]

📌 Commit 29c4b67 has been approved by nrc

[bors]

⌛ Testing commit 29c4b67 with merge 945ba12...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-32cross-opt, auto-linux-64-cargotest, auto-linux-64-cross-armhf, auto-linux-64-cross-armsf, auto-linux-64-cross-freebsd, auto-linux-64-cross-netbsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, 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-nopt-t, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-cross-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild