make log an optional dependency

Currently, Mio depends on `log`. In rust-lang/log#552, the `log`
maintainers aim to increase `log`'s MSRV to 1.60. This MSRV increase
would impact Mio, which in turn, would impact Tokio. As of 1.25, Tokio
supports Rust 1.49 with the intent of supporting it until March 2024.

This commit makes `log` an optional dependency for those who wish to
maintain a lower MSRV.

Cargo.toml

@@ -29,7 +29,7 @@ include       = [
 # For documentation of features see the `mio::features` module.
 [features]
 # By default Mio only provides a shell implementation.
-default = []
+default = ["log"]
 
 # Enables the `Poll` and `Registry` types.
 os-poll = []
@@ -43,7 +43,7 @@ os-ext = [
 net = []
 
 [dependencies]
-log = "0.4.8"
+log = { version = "0.4.8", optional = true }
 
 [target.'cfg(unix)'.dependencies]
 libc = "0.2.121"

src/macros.rs

@@ -68,3 +68,27 @@ macro_rules! cfg_any_os_ext {
         )*
     }
 }
+
+macro_rules! trace {
+    ($($t:tt)*) => {
+        #[cfg(feature = "log")]
+        log::trace!($($t)*)
+    }
+}
+
+macro_rules! error {
+    ($($t:tt)*) => {
+        #[cfg(feature = "log")]
+        {
+            log::error!($($t)*)
+        }
+
+        // Silence warnings
+        #[cfg(not(feature = "log"))]
+        {
+            if false {
+                format!($($t)*);
+            }
+        }
+    }
+}

src/poll.rs

@@ -1,5 +1,4 @@
 use crate::{event, sys, Events, Interest, Token};
-use log::trace;
 #[cfg(unix)]
 use std::os::unix::io::{AsRawFd, RawFd};
 use std::time::Duration;

src/sys/unix/selector/epoll.rs

@@ -1,7 +1,6 @@
 use crate::{Interest, Token};
 
 use libc::{EPOLLET, EPOLLIN, EPOLLOUT, EPOLLPRI, EPOLLRDHUP};
-use log::error;
 use std::os::unix::io::{AsRawFd, RawFd};
 #[cfg(debug_assertions)]
 use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};

src/sys/unix/selector/kqueue.rs

@@ -1,5 +1,4 @@
 use crate::{Interest, Token};
-use log::error;
 use std::mem::{self, MaybeUninit};
 use std::ops::{Deref, DerefMut};
 use std::os::unix::io::{AsRawFd, RawFd};