Make log target configurable

Right now `env_logger` always logs to stderr. This might be
undesireable in some cases.

This commit adds a way to configure the log target. It can be set to
stdout if desired.

Note that writing to stdout is implemented with `println!` instead of
`writeln!`, so that the output is properly captured in tests.

README.md

@@ -158,3 +158,20 @@ test tests::it_adds_one ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
```

## Configuring log target

By default, `env_logger` logs to stderr. If you want to log to stdout instead,
you can use the `LogBuilder` to change the log target:

```rust
use std::env;
use env_logger::{LogBuilder, LogTarget};

let mut builder = LogBuilder::new();
builder.target(LogTarget::Stdout);
if env::var("RUST_LOG").is_ok() {
    builder.parse(&env::var("RUST_LOG").unwrap());
}
builder.init().unwrap();
```

env/src/lib.rs

@@ -152,11 +152,19 @@ mod filter;
#[path = "string.rs"]
mod filter;

/// Log target, either stdout or stderr.
#[derive(Debug)]
pub enum LogTarget {
    Stdout,
    Stderr,
}

/// The logger.
pub struct Logger {
    directives: Vec<LogDirective>,
    filter: Option<filter::Filter>,
    format: Box<Fn(&LogRecord) -> String + Sync + Send>,
    target: LogTarget,
}

/// LogBuilder acts as builder for initializing the Logger.
@@ -196,6 +204,7 @@ pub struct LogBuilder {
    directives: Vec<LogDirective>,
    filter: Option<filter::Filter>,
    format: Box<Fn(&LogRecord) -> String + Sync + Send>,
    target: LogTarget,
}

impl LogBuilder {
@@ -208,6 +217,7 @@ impl LogBuilder {
                format!("{}:{}: {}", record.level(),
                        record.location().module_path(), record.args())
            }),
            target: LogTarget::Stderr,
        }
    }

@@ -236,6 +246,14 @@ impl LogBuilder {
        self
    }

    /// Sets the target for the log output.
    ///
    /// Env logger can log to either stdout or stderr. The default is stderr.
    pub fn target(&mut self, target: LogTarget) -> &mut Self {
        self.target = target;
        self
    }

    /// Parses the directives string in the same form as the RUST_LOG
    /// environment variable.
    ///
@@ -286,6 +304,7 @@ impl LogBuilder {
            directives: mem::replace(&mut self.directives, Vec::new()),
            filter: mem::replace(&mut self.filter, None),
            format: mem::replace(&mut self.format, Box::new(|_| String::new())),
            target: mem::replace(&mut self.target, LogTarget::Stderr),
        }
    }
}
@@ -337,7 +356,12 @@ impl Log for Logger {
            }
        }

        let _ = writeln!(&mut io::stderr(), "{}", (self.format)(record));
        match self.target {
            LogTarget::Stdout => println!("{}", (self.format)(record)),
            LogTarget::Stderr => {
                let _ = writeln!(&mut io::stderr(), "{}", (self.format)(record));
            },
        };
    }
}