A cargo dependency is no longer conditionnal

Works around rust-lang/cargo#6858

tokio/Cargo.toml

@@ -101,7 +101,11 @@ parking_lot = { version = "0.11.0", optional = true }
 
 # Currently unstable. The API exposed by these features may be broken at any time.
 # Requires `--cfg tokio_unstable` to enable.
-[target.'cfg(tokio_unstable)'.dependencies]
+#
+#
+# We're not gating this create behind a cfg flag (defined in a [target.cfg(...)], because cargo would not know it before actually invoking rustc)
+# See https://github.com/rust-lang/cargo/issues/6858
+# [target.'cfg(tokio_unstable)'.dependencies]
 tracing = { version = "0.1.21", default-features = false, features = ["std"], optional = true } # Not in full
 
 [target.'cfg(unix)'.dependencies]