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.

4 files changed, 36 insertions, 7 deletions

diff --git a/tests-integration/Cargo.toml b/tests-integration/Cargo.toml
index 265d7d6d..3cca17a7 100644
--- a/tests-integration/Cargo.toml
+++ b/tests-integration/Cargo.toml
@@ -5,11 +5,16 @@ authors = ["Tokio Contributors <team@tokio.rs>"]
 edition = "2018"
 publish = false
 
+[features]
+full = ["tokio/full", "tokio-test"]
+macros = ["tokio/macros"]
+rt-core = ["tokio/rt-core"]
+rt-threaded = ["rt-core", "tokio/rt-threaded"]
+
 [dependencies]
-tokio = { path = "../tokio", features = ["full"] }
+tokio = { path = "../tokio" }
+tokio-test = { path = "../tokio-test", optional = true }
 doc-comment = "0.3.1"
 
 [dev-dependencies]
-tokio-test = { path = "../tokio-test" }
-
 futures = { version = "0.3.0", features = ["async-await"] }
diff --git a/tests-integration/src/lib.rs b/tests-integration/src/lib.rs
index 74795f29..b07714f4 100644
--- a/tests-integration/src/lib.rs
+++ b/tests-integration/src/lib.rs
@@ -1,4 +1,2 @@
-use doc_comment::doc_comment;
-
-// #[doc = include_str!("../../README.md")]
-doc_comment!(include_str!("../../README.md"));
+#[cfg(feature = "full")]
+doc_comment::doc_comment!(include_str!("../../README.md"));
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};