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logging is an external crate

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Steve Klabnik 2014-09-19 16:59:43 -04:00
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@ -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*.