logging is an external crate

reference.md

@@ -4243,64 +4243,6 @@ In general, `--crate-type=bin` or `--crate-type=lib` should be sufficient for
 all compilation needs, and the other options are just available if more
 fine-grained control is desired over the output format of a Rust crate.
 
-### Logging system
-
-The runtime contains a system for directing [logging
-expressions](#logging-expressions) to a logging console and/or internal logging
-buffers. Logging can be enabled per module.
-
-Logging output is enabled by setting the `RUST_LOG` environment variable.
-`RUST_LOG` accepts a logging specification made up of a comma-separated list of
-paths, with optional log levels. For each module containing log expressions, if
-`RUST_LOG` contains the path to that module or a parent of that module, then
-logs of the appropriate level will be output to the console.
-
-The path to a module consists of the crate name, any parent modules, then the
-module itself, all separated by double colons (`::`). The optional log level
-can be appended to the module path with an equals sign (`=`) followed by the
-log level, from 1 to 4, inclusive. Level 1 is the error level, 2 is warning, 3
-info, and 4 debug. You can also use the symbolic constants `error`, `warn`,
-`info`, and `debug`. Any logs less than or equal to the specified level will
-be output. If not specified then log level 4 is assumed. Debug messages can be
-omitted by passing `--cfg ndebug` to `rustc`.
-
-As an example, to see all the logs generated by the compiler, you would set
-`RUST_LOG` to `rustc`, which is the crate name (as specified in its `crate_id`
-[attribute](#attributes)). To narrow down the logs to just crate resolution,
-you would set it to `rustc::metadata::creader`. To see just error logging use
-`rustc=0`.
-
-Note that when compiling source files that don't specify a crate name the crate
-is given a default name that matches the source file, with the extension
-removed. In that case, to turn on logging for a program compiled from, e.g.
-`helloworld.rs`, `RUST_LOG` should be set to `helloworld`.
-
-#### Logging Expressions
-
-Rust provides several macros to log information. Here's a simple Rust program
-that demonstrates all four of them:
-
-```
-#![feature(phase)]
-#[phase(plugin, link)] extern crate log;
-
-fn main() {
-    error!("This is an error log")
-    warn!("This is a warn log")
-    info!("this is an info log")
-    debug!("This is a debug log")
-}
-```
-
-These four log levels correspond to levels 1-4, as controlled by `RUST_LOG`:
-
-```sh
-$ RUST_LOG=rust=3 ./rust
-This is an error log
-This is a warn log
-this is an info log
-```
-
 # Appendix: Rationales and design tradeoffs
 
 *TODO*.