Rollup merge of #109985 - blyxyas:is_test_crate, r=compiler-errors

Add little `is_test_crate` function

Ok, this is quite a story.
I'm mainly a Clippy contributor, so I was fixing [this Clippy issue](https://github.com/rust-lang/rust-clippy/pull/10584) about a lint having to ignore test modules but that wasn't ignoring test files (integration test, `test/` dirs and such).

As test **files** don't tend to have an inner `#[cfg(test)]` module inside them, I tried everything, looking for filenames, looking for item's parents in the HIR Map, doing black magic...

I even asked [on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/.E2.9C.94.20Checking.20if.20file.20is.20integration.20test), and jyn answered something about `--cfg test`. Aha! That's something that I might be looking for, so I started looking at `rustc_driver_impl` flag parsing and configuration and all that.

Then, I stumbled on [this function right here](2e486be8d2/compiler/rustc_driver_impl/src/lib.rs (L174-L181)), and noticed the argument `config: Config`. That's a hint.

So [Config](https://doc.rust-lang.org/beta/nightly-rustc/rustc_interface/interface/struct.Config.html) has the field `opts: Options`, and [`Options`](https://doc.rust-lang.org/beta/nightly-rustc/rustc_session/options/struct.Options.html) has the field `test`.

This journey has been ~7 or 8 hours in 3 days, it's a very hard thing to find, so this PR adds a mini-function to check if the current crate is a testing one. So that no one has to travel through the same as me, and can just search for `is_test_crate` in the documentation.

compiler/rustc_builtin_macros/src/test_harness.rs

@@ -53,7 +53,7 @@ pub fn inject(krate: &mut ast::Crate, sess: &Session, resolver: &mut dyn Resolve
     // even in non-test builds
     let test_runner = get_test_runner(span_diagnostic, &krate);
 
-    if sess.opts.test {
+    if sess.is_test_crate() {
         let panic_strategy = match (panic_strategy, sess.opts.unstable_opts.panic_abort_tests) {
             (PanicStrategy::Abort, true) => PanicStrategy::Abort,
             (PanicStrategy::Abort, false) => {

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -2301,7 +2301,7 @@ fn add_native_libs_from_crate(
                         || (whole_archive == None
                             && bundle
                             && cnum == LOCAL_CRATE
-                            && sess.opts.test);
+                            && sess.is_test_crate());
 
                     if bundle && cnum != LOCAL_CRATE {
                         if let Some(filename) = lib.filename {

compiler/rustc_interface/src/passes.rs

@@ -230,7 +230,7 @@ fn configure_and_expand(
             features: Some(features),
             recursion_limit,
             trace_mac: sess.opts.unstable_opts.trace_macros,
-            should_test: sess.opts.test,
+            should_test: sess.is_test_crate(),
             span_debug: sess.opts.unstable_opts.span_debug,
             proc_macro_backtrace: sess.opts.unstable_opts.proc_macro_backtrace,
             ..rustc_expand::expand::ExpansionConfig::default(crate_name.to_string())
@@ -292,7 +292,7 @@ fn configure_and_expand(
     }
 
     sess.time("maybe_create_a_macro_crate", || {
-        let is_test_crate = sess.opts.test;
+        let is_test_crate = sess.is_test_crate();
         rustc_builtin_macros::proc_macro_harness::inject(
             &mut krate,
             sess,

compiler/rustc_passes/src/stability.rs

@@ -530,7 +530,7 @@ struct MissingStabilityAnnotations<'tcx> {
 impl<'tcx> MissingStabilityAnnotations<'tcx> {
     fn check_missing_stability(&self, def_id: LocalDefId, span: Span) {
         let stab = self.tcx.stability().local_stability(def_id);
-        if !self.tcx.sess.opts.test
+        if !self.tcx.sess.is_test_crate()
             && stab.is_none()
             && self.effective_visibilities.is_reachable(def_id)
         {

compiler/rustc_resolve/src/check_unused.rs

@@ -393,7 +393,7 @@ impl Resolver<'_, '_> {
             // If we are in the `--test` mode, suppress a help that adds the `#[cfg(test)]`
             // attribute; however, if not, suggest adding the attribute. There is no way to
             // retrieve attributes here because we do not have a `TyCtxt` yet.
-            let test_module_span = if tcx.sess.opts.test {
+            let test_module_span = if tcx.sess.is_test_crate() {
                 None
             } else {
                 let parent_module = visitor.r.get_nearest_non_block_module(

compiler/rustc_session/src/config.rs

@@ -1258,7 +1258,7 @@ pub fn build_configuration(sess: &Session, mut user_cfg: CrateConfig) -> CrateCo
     // some default and generated configuration items.
     let default_cfg = default_configuration(sess);
     // If the user wants a test runner, then add the test cfg.
-    if sess.opts.test {
+    if sess.is_test_crate() {
         user_cfg.insert((sym::test, None));
     }
     user_cfg.extend(default_cfg.iter().cloned());

compiler/rustc_session/src/session.rs

@@ -294,6 +294,11 @@ impl Session {
         self.crate_types.get().unwrap().as_slice()
     }
 
+    /// Returns true if the crate is a testing one.
+    pub fn is_test_crate(&self) -> bool {
+        self.opts.test
+    }
+
     pub fn needs_crate_hash(&self) -> bool {
         // Why is the crate hash needed for these configurations?
         // - debug_assertions: for the "fingerprint the result" check in