log: Introduce liblog, the old std::logging

This commit moves all logging out of the standard library into an external
crate. This crate is the new crate which is responsible for all logging macros
and logging implementation. A few reasons for this change are:

* The crate map has always been a bit of a code smell among rust programs. It
  has difficulty being loaded on almost all platforms, and it's used almost
  exclusively for logging and only logging. Removing the crate map is one of the
  end goals of this movement.

* The compiler has a fair bit of special support for logging. It has the
  __log_level() expression as well as generating a global word per module
  specifying the log level. This is unfairly favoring the built-in logging
  system, and is much better done purely in libraries instead of the compiler
  itself.

* Initialization of logging is much easier to do if there is no reliance on a
  magical crate map being available to set module log levels.

* If the logging library can be written outside of the standard library, there's
  no reason that it shouldn't be. It's likely that we're not going to build the
  highest quality logging library of all time, so third-party libraries should
  be able to provide just as high-quality logging systems as the default one
  provided in the rust distribution.

With a migration such as this, the change does not come for free. There are some
subtle changes in the behavior of liblog vs the previous logging macros:

* The core change of this migration is that there is no longer a physical
  log-level per module. This concept is still emulated (it is quite useful), but
  there is now only a global log level, not a local one. This global log level
  is a reflection of the maximum of all log levels specified. The previously
  generated logging code looked like:

    if specified_level <= __module_log_level() {
        println!(...)
    }

  The newly generated code looks like:

    if specified_level <= ::log::LOG_LEVEL {
        if ::log::module_enabled(module_path!()) {
            println!(...)
        }
    }

  Notably, the first layer of checking is still intended to be "super fast" in
  that it's just a load of a global word and a compare. The second layer of
  checking is executed to determine if the current module does indeed have
  logging turned on.

  This means that if any module has a debug log level turned on, all modules
  with debug log levels get a little bit slower (they all do more expensive
  dynamic checks to determine if they're turned on or not).

  Semantically, this migration brings no change in this respect, but
  runtime-wise, this will have a perf impact on some code.

* A `RUST_LOG=::help` directive will no longer print out a list of all modules
  that can be logged. This is because the crate map will no longer specify the
  log levels of all modules, so the list of modules is not known. Additionally,
  warnings can no longer be provided if a malformed logging directive was
  supplied.

The new "hello world" for logging looks like:

    #[phase(syntax, link)]
    extern crate log;

    fn main() {
        debug!("Hello, world!");
    }

1 files changed, 0 insertions, 184 deletions

diff --git a/src/libstd/logging.rs b/src/libstd/logging.rs
deleted file mode 100644
index 2271a7c2380..00000000000
--- a/src/libstd/logging.rs
+++ /dev/null
@@ -1,184 +0,0 @@
-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-/*!
-
-Utilities for program-wide and customizable logging
-
-This module is used by the compiler when emitting output for the logging family
-of macros. The methods of this module shouldn't necessarily be used directly,
-but rather through the logging macros defined.
-
-There are five macros that the logging subsystem uses:
-
-* `log!(level, ...)` - the generic logging macro, takes a level as a u32 and any
-                       related `format!` arguments
-* `debug!(...)` - a macro hard-wired to the log level of `DEBUG`
-* `info!(...)` - a macro hard-wired to the log level of `INFO`
-* `warn!(...)` - a macro hard-wired to the log level of `WARN`
-* `error!(...)` - a macro hard-wired to the log level of `ERROR`
-
-All of these macros use the same style of syntax as the `format!` syntax
-extension. Details about the syntax can be found in the documentation of
-`std::fmt` along with the Rust tutorial/manual.
-
-If you want to check at runtime if a given logging level is enabled (e.g. if the
-information you would want to log is expensive to produce), you can use the
-following macro:
-
-* `log_enabled!(level)` - returns true if logging of the given level is enabled
-
-## Enabling logging
-
-Log levels are controlled on a per-module basis, and by default all logging is
-disabled except for `error!` (a log level of 1). Logging is controlled via the
-`RUST_LOG` environment variable. The value of this environment variable is a
-comma-separated list of logging directives. A logging directive is of the form:
-
-```ignore
-path::to::module=log_level
-```
-
-The path to the module is rooted in the name of the crate it was compiled for,
-so if your program is contained in a file `hello.rs`, for example, to turn on
-logging for this file you would use a value of `RUST_LOG=hello`. Furthermore,
-this path is a prefix-search, so all modules nested in the specified module will
-also have logging enabled.
-
-The actual `log_level` is optional to specify. If omitted, all logging will be
-enabled. If specified, the it must be either a numeric in the range of 1-255, or
-it must be one of the strings `debug`, `error`, `info`, or `warn`. If a numeric
-is specified, then all logging less than or equal to that numeral is enabled.
-For example, if logging level 3 is active, error, warn, and info logs will be
-printed, but debug will be omitted.
-
-As the log level for a module is optional, the module to enable logging for is
-also optional. If only a `log_level` is provided, then the global log level for
-all modules is set to this value.
-
-Some examples of valid values of `RUST_LOG` are:
-
-```ignore
-hello                // turns on all logging for the 'hello' module
-info                 // turns on all info logging
-hello=debug          // turns on debug logging for 'hello'
-hello=3              // turns on info logging for 'hello'
-hello,std::option    // turns on hello, and std's option logging
-error,hello=warn     // turn on global error logging and also warn for hello
-```
-
-## Performance and Side Effects
-
-Each of these macros will expand to code similar to:
-
-```rust,ignore
-if log_level <= my_module_log_level() {
-    ::std::logging::log(log_level, format!(...));
-}
-```
-
-What this means is that each of these macros are very cheap at runtime if
-they're turned off (just a load and an integer comparison). This also means that
-if logging is disabled, none of the components of the log will be executed.
-
-## Useful Values
-
-For convenience, if a value of `::help` is set for `RUST_LOG`, a program will
-start, print out all modules registered for logging, and then exit.
-
-*/
-
-use fmt;
-use io::LineBufferedWriter;
-use io;
-use io::Writer;
-use mem::replace;
-use ops::Drop;
-use option::{Some, None, Option};
-use prelude::drop;
-use result::{Ok, Err};
-use rt::local::Local;
-use rt::task::Task;
-
-/// Debug log level
-pub static DEBUG: u32 = 4;
-/// Info log level
-pub static INFO: u32 = 3;
-/// Warn log level
-pub static WARN: u32 = 2;
-/// Error log level
-pub static ERROR: u32 = 1;
-
-/// A trait used to represent an interface to a task-local logger. Each task
-/// can have its own custom logger which can respond to logging messages
-/// however it likes.
-pub trait Logger {
-    /// Logs a single message described by the `args` structure. The level is
-    /// provided in case you want to do things like color the message, etc.
-    fn log(&mut self, level: u32, args: &fmt::Arguments);
-}
-
-struct DefaultLogger {
-    handle: LineBufferedWriter<io::stdio::StdWriter>,
-}
-
-impl Logger for DefaultLogger {
-    // by default, just ignore the level
-    fn log(&mut self, _level: u32, args: &fmt::Arguments) {
-        match fmt::writeln(&mut self.handle, args) {
-            Err(e) => fail!("failed to log: {}", e),
-            Ok(()) => {}
-        }
-    }
-}
-
-impl Drop for DefaultLogger {
-    fn drop(&mut self) {
-        match self.handle.flush() {
-            Err(e) => fail!("failed to flush a logger: {}", e),
-            Ok(()) => {}
-        }
-    }
-}
-
-/// This function is called directly by the compiler when using the logging
-/// macros. This function does not take into account whether the log level
-/// specified is active or not, it will always log something if this method is
-/// called.
-///
-/// It is not recommended to call this function directly, rather it should be
-/// invoked through the logging family of macros.
-pub fn log(level: u32, args: &fmt::Arguments) {
-    // See io::stdio::with_task_stdout for why there's a few dances here. The
-    // gist of it is that arbitrary code can run during logging (and set an
-    // arbitrary logging handle into the task) so we need to be careful that the
-    // local task is in TLS while we're running arbitrary code.
-    let mut logger = {
-        let mut task = Local::borrow(None::<Task>);
-        task.get().logger.take()
-    };
-
-    if logger.is_none() {
-        logger = Some(~DefaultLogger { handle: io::stderr(), } as ~Logger);
-    }
-    logger.get_mut_ref().log(level, args);
-
-    let mut task = Local::borrow(None::<Task>);
-    let prev = replace(&mut task.get().logger, logger);
-    drop(task);
-    drop(prev);
-}
-
-/// Replaces the task-local logger with the specified logger, returning the old
-/// logger.
-pub fn set_logger(logger: ~Logger) -> Option<~Logger> {
-    let mut task = Local::borrow(None::<Task>);
-    replace(&mut task.get().logger, Some(logger))
-}