feat(trace): add internal tracing macro module

Introduce several macros that act as wrappers around the `tracing`
module's macros to allow for more concise syntax for conditional
compilation of `tracing` macro calls. As `tracing` is unstable,
the new `trace` module will facilitate transitioning `tracing`
to an optional feature, as outlined in hyperium#2874 and hyperium#3326.

BREAKING CHANGES: Integration of macros requires the replacement of
uses of `trace` macros.

src/lib.rs

@@ -68,6 +68,8 @@ pub use crate::error::{Error, Result};
 #[macro_use]
 mod cfg;
 #[macro_use]
+mod trace;
+#[macro_use]
 mod common;
 pub mod body;
 mod error;

src/trace.rs

@@ -0,0 +1,107 @@
+#![allow(unused_macros)]
+
+macro_rules! debug {
+    ($($arg:tt)+) => {
+        #[cfg(feature = "tracing")]
+        {
+            println!($($arg)+);
+            tracing::debug!($($arg)+);
+        }
+    }
+}
+
+macro_rules! debug_span {
+    ($($arg:tt)*) => {
+        #[cfg(feature = "tracing")]
+        {
+            let span = tracing::debug_span!($($arg)+);
+            let _ = span.enter();
+        }
+    }
+}
+
+macro_rules! error {
+    ($($arg:tt)*) => {
+        #[cfg(feature = "tracing")]
+        {
+            tracing::error!($($arg)+);
+        }
+    }
+}
+
+macro_rules! error_span {
+    ($($arg:tt)*) => {
+        #[cfg(feature = "tracing")]
+        {
+            let span = tracing::error_span!($($arg)+);
+            let _ = span.enter();
+        }
+    }
+}
+
+macro_rules! info {
+    ($($arg:tt)*) => {
+        #[cfg(feature = "tracing")]
+        {
+            tracing::info!($($arg)+);
+        }
+    }
+}
+
+macro_rules! info_span {
+    ($($arg:tt)*) => {
+        #[cfg(feature = "tracing")]
+        {
+            let span = tracing::info_span!($($arg)+);
+            let _ = span.enter();
+        }
+    }
+}
+
+macro_rules! trace {
+    ($($arg:tt)*) => {
+        #[cfg(feature = "tracing")]
+        {
+            tracing::trace!($($arg)+);
+        }
+    }
+}
+
+macro_rules! trace_span {
+    ($($arg:tt)*) => {
+        #[cfg(feature = "tracing")]
+        {
+            let span = tracing::trace_span!($($arg)+);
+            let _ = span.enter();
+        }
+    }
+}
+
+macro_rules! span {
+    ($($arg:tt)*) => {
+        #[cfg(feature = "tracing")]
+        {
+            let span = tracing::span!($($arg)+);
+            let _ = span.enter();
+        }
+    }
+}
+
+macro_rules! warn {
+    ($($arg:tt)*) => {
+        #[cfg(feature = "tracing")]
+        {
+            tracing::warn!($($arg)+);
+        }
+    }
+}
+
+macro_rules! warn_span {
+    ($($arg:tt)*) => {
+        #[cfg(feature = "tracing")]
+        {
+            let span = tracing::warn_span!($($arg)+);
+            let _ = span.enter();
+        }
+    }
+}