macros: fix `#[tokio::main]` without rt-core (#2139)

The Tokio runtime provides a "shell" runtime when `rt-core` is not
available. This shell runtime is enough to support `#[tokio::main`] and
`#[tokio::test].

A previous change disabled these two attr macros when `rt-core` was not
selected. This patch fixes this by re-enabling the `main` and `test`
attr macros without `rt-core` and adds some integration tests to prevent
future regressions.

2 files changed, 26 insertions, 0 deletions

diff --git a/tests-integration/tests/macros_main.rs b/tests-integration/tests/macros_main.rs
new file mode 100644
index 00000000..182e2779
--- /dev/null
+++ b/tests-integration/tests/macros_main.rs
@@ -0,0 +1,25 @@
+#![cfg(feature = "macros")]
+
+#[tokio::main]
+async fn basic_main() -> usize {
+    1
+}
+
+#[cfg(feature = "rt-core")]
+mod spawn {
+    #[tokio::main]
+    async fn spawning() -> usize {
+        let join = tokio::spawn(async { 1 });
+        join.await.unwrap()
+    }
+
+    #[test]
+    fn main_with_spawn() {
+        assert_eq!(1, spawning());
+    }
+}
+
+#[test]
+fn shell() {
+    assert_eq!(1, basic_main());
+}
diff --git a/tests-integration/tests/process_stdio.rs b/tests-integration/tests/process_stdio.rs
index de7cf025..8bf2c149 100644
--- a/tests-integration/tests/process_stdio.rs
+++ b/tests-integration/tests/process_stdio.rs
@@ -1,4 +1,5 @@
 #![warn(rust_2018_idioms)]
+#![cfg(feature = "full")]
 
 use tokio::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
 use tokio::process::{Child, Command};