doc(logging): update mentions of log to tracing

Author: weihanglo

src/doc/contrib/src/implementation/debugging.md

@@ -2,11 +2,11 @@
 
 ## Logging
 
-Cargo uses the [`env_logger`] crate to display debug log messages. The
-`CARGO_LOG` environment variable can be set to enable debug logging, with a
-value such as `trace`, `debug`, or `warn`. It also supports filtering for
-specific modules. Feel free to use the standard [`log`] macros to help with
-diagnosing problems.
+Cargo uses the [`tracing`] crate to display debug log messages.
+The `CARGO_LOG` environment variable can be set to enable debug logging, with a value such as `trace`, `debug`, or `warn`.
+It also supports filtering for specific modules with comma-separated [directives].
+Feel free to use [shorthand macros] to help with diagnosing problems.
+We're looking forward to making Cargo logging mechanism more structural!
 
 ```sh
 # Outputs all logs with levels debug and higher
@@ -22,5 +22,6 @@ CARGO_HTTP_DEBUG=true CARGO_LOG=cargo::ops::registry=debug cargo fetch
 CARGO_LOG=cargo::core::compiler::fingerprint=trace cargo build
 ```
 
-[`env_logger`]: https://docs.rs/env_logger
-[`log`]: https://docs.rs/log
+[`tracing`]: https://docs.rs/tracing
+[directive]: https://docs.rs/tracing_subscriber/filter/struct.EnvFilter.html#directives
+[shorthand macros]: https://docs.rs/tracing/index.html#shorthand-macros

src/doc/src/reference/environment-variables.md

@@ -9,7 +9,7 @@ with them:
 You can override these environment variables to change Cargo's behavior on your
 system:
 
-* `CARGO_LOG` --- Cargo uses the [`env_logger`] crate to display debug log messages.
+* `CARGO_LOG` --- Cargo uses the [`tracing`] crate to display debug log messages.
   The `CARGO_LOG` environment variable can be set to enable debug logging,
   with a value such as `trace`, `debug`, or `warn`.
   Usually it is only used during debugging. For more details refer to the
@@ -389,7 +389,7 @@ let out_dir = env::var("OUT_DIR").unwrap();
   the environment; scripts should use `CARGO_ENCODED_RUSTFLAGS` instead.
 * `CARGO_PKG_<var>` --- The package information variables, with the same names and values as are [provided during crate building][variables set for crates].
 
-[`env_logger`]: https://docs.rs/env_logger
+[`tracing`]: https://docs.rs/tracing
 [debug logging]: https://doc.crates.io/contrib/architecture/console.html#debug-logging
 [unix-like platforms]: ../../reference/conditional-compilation.html#unix-and-windows
 [windows-like platforms]: ../../reference/conditional-compilation.html#unix-and-windows