update/extend some comments explaining why tests do not run in Miri

Author: RalfJung

tokio/src/io/stdio_common.rs

@@ -177,7 +177,7 @@ mod tests {
     }
 
     #[test]
-    #[cfg_attr(miri, ignore)]
+    #[cfg_attr(miri, ignore)] // takes a really long time with miri
     fn test_splitter() {
         let data = str::repeat("█", DEFAULT_MAX_BUF_SIZE);
         let mut wr = super::SplitByUtf8BoundaryIfWindows::new(TextMockWriter);
@@ -191,7 +191,7 @@ mod tests {
     }
 
     #[test]
-    #[cfg_attr(miri, ignore)]
+    #[cfg_attr(miri, ignore)] // takes a really long time with miri
     fn test_pseudo_text() {
         // In this test we write a piece of binary data, whose beginning is
         // text though. We then validate that even in this corner case buffer

tokio/src/runtime/tests/mod.rs

@@ -77,7 +77,7 @@ cfg_not_loom! {
     mod inject;
     mod queue;
 
-    #[cfg(not(miri))]
+    #[cfg(not(miri))] // takes a really long time with miri
     mod task_combinations;
 
     #[cfg(miri)]

tokio/tests/process_change_of_runtime.rs

@@ -3,7 +3,7 @@
 // This tests test the behavior of `process::Command::spawn` when it is used
 // outside runtime, and when `process::Child::wait ` is used in a different
 // runtime from which `process::Command::spawn` is used.
-#![cfg(all(unix, not(target_os = "freebsd"), not(miri)))]
+#![cfg(all(unix, not(target_os = "freebsd"), not(miri)))] // Miri cannot run system commands
 
 use std::process::Stdio;
 use tokio::{process::Command, runtime::Runtime};

tokio/tests/process_kill_after_wait.rs

@@ -1,5 +1,5 @@
 #![warn(rust_2018_idioms)]
-#![cfg(all(feature = "full", not(target_os = "wasi"), not(miri)))] // Wasi cannot run system commands
+#![cfg(all(feature = "full", not(target_os = "wasi"), not(miri)))] // Wasi/Miri cannot run system commands
 
 use tokio::process::Command;
 

tokio/tests/process_smoke.rs

@@ -1,5 +1,5 @@
 #![warn(rust_2018_idioms)]
-#![cfg(all(feature = "full", not(target_os = "wasi"), not(miri)))] // Wasi cannot run system commands
+#![cfg(all(feature = "full", not(target_os = "wasi"), not(miri)))] // Wasi/Miri cannot run system commands
 
 use tokio::process::Command;
 use tokio_test::assert_ok;