feat: make tracing dependency optional (default: enabled)

Supersedes #30. The `tracing` dependency of the `httpsig` crate is now
gated behind a new `tracing` feature, enabled by default so that existing
users keep their log output.

Unlike #30, log call sites are left untouched: the `trace` facade module
now provides no-op macros when the feature is disabled, so a missing cfg
guard can never break a feature combination (#30 failed to compile with
`--features rsa-signature` alone). The dependency is also declared with
`default-features = false`, which drops `tracing-attributes` from the
dependency graph since only the bang-style macros are used.

Verified: cargo check/test across default, --no-default-features,
rsa-signature with and without tracing, --all-features, and the whole
workspace.

Co-authored-by: keskalukasfri <270303466+keskalukasfri@users.noreply.github.com>

Author: junkurihara

httpsig/Cargo.toml

@@ -13,12 +13,13 @@ rust-version.workspace = true
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [features]
-default = []
+default = ["tracing"]
 rsa-signature = ["rsa"]
+tracing = ["dep:tracing"]
 
 [dependencies]
 thiserror = { version = "2.0.19" }
-tracing = { version = "0.1.44" }
+tracing = { version = "0.1.44", default-features = false, optional = true }
 rustc-hash = { version = "2.1.3" }
 indexmap = { version = "2.14.0" }
 rand = { version = "0.10.2" }
@@ -56,4 +57,3 @@ base64 = { version = "0.22.1" }
 
 # for rfc8941 structured field values
 sfv = { version = "0.15.0" }
-

httpsig/src/trace.rs

@@ -1,2 +1,43 @@
+#[cfg(feature = "tracing")]
 #[allow(unused)]
-pub use tracing::{debug, error, info, trace, warn};
+pub(crate) use tracing::{debug, error, info, trace, warn};
+
+/// No-op macros used when the `tracing` feature is disabled.
+/// They expand to `()` so that they are usable both in statement and
+/// expression positions, like the original `tracing` macros.
+/// `warn` is defined as `warn_` and re-exported, since a macro named
+/// `warn` conflicts with the built-in `warn` attribute (E0659).
+#[cfg(not(feature = "tracing"))]
+mod noop {
+  #![allow(unused_macros)]
+  macro_rules! debug {
+    ($($arg:tt)*) => {
+      ()
+    };
+  }
+  macro_rules! error {
+    ($($arg:tt)*) => {
+      ()
+    };
+  }
+  macro_rules! info {
+    ($($arg:tt)*) => {
+      ()
+    };
+  }
+  macro_rules! trace {
+    ($($arg:tt)*) => {
+      ()
+    };
+  }
+  macro_rules! warn_ {
+    ($($arg:tt)*) => {
+      ()
+    };
+  }
+  #[allow(unused_imports)]
+  pub(crate) use {debug, error, info, trace, warn_ as warn};
+}
+#[cfg(not(feature = "tracing"))]
+#[allow(unused)]
+pub(crate) use noop::*;