so it is recommended to use function level filters.

Your log filter can be just `debug` to get all `debug!` output and
higher (e.g., it will also include `info!`), or `path::to::module` to get *all*
output (which will include `trace!`) from a particular module, or
`path::to::module=debug` to get `debug!` output and higher from a particular
module.

For example, to get the `debug!` output and higher for a specific module, you
can run the compiler with `RUSTC_LOG=path::to::module=debug rustc my-file.rs`.
All `debug!` output will then appear in standard error.

Note that you can use a partial path and the filter will still work. For
example, if you want to see `info!` output from only
`rustdoc::passes::collect_intra_doc_links`, you could use
`RUSTDOC_LOG=rustdoc::passes::collect_intra_doc_links=info` *or* you could use
`RUSTDOC_LOG=rustdoc::passes::collect_intra=info`.

If you are developing rustdoc, use `RUSTDOC_LOG` instead. If you are developing
Miri, use `MIRI_LOG` instead. You get the idea :)

See the [`tracing`] crate's docs, and specifically the docs for [`debug!`] to
see the full syntax you can use. (Note: unlike the compiler, the [`tracing`]
crate and its examples use the `RUST_LOG` environment variable. rustc, rustdoc,
and other tools set custom environment variables.)

**Note that unless you use a very strict filter, the logger will emit a lot of
output, so use the most specific module(s) you can (comma-separated if
multiple)**. It's typically a good idea to pipe standard error to a file and
look at the log output with a text editor.

So, to put it together:

```bash
# This puts the output of all debug calls in `rustc_middle/src/traits` into
# standard error, which might fill your console backscroll.
$ RUSTC_LOG=rustc_middle::traits=debug rustc +stage1 my-file.rs

# This puts the output of all debug calls in `rustc_middle/src/traits` in
# `traits-log`, so you can then see it with a text editor.
$ RUSTC_LOG=rustc_middle::traits=debug rustc +stage1 my-file.rs 2>traits-log

# Not recommended! This will show the output of all `debug!` calls
# in the Rust compiler, and there are a *lot* of them, so it will be
# hard to find anything.
$ RUSTC_LOG=debug rustc +stage1 my-file.rs 2>all-log

# This will show the output of all `info!` calls in `rustc_codegen_ssa`.
#
# There's an `info!` statement in `codegen_instance` that outputs
# every function that is codegen'd. This is useful to find out
# which function triggers an LLVM assertion, and this is an `info!`
# log rather than a `debug!` log so it will work on the official
# compilers.
$ RUSTC_LOG=rustc_codegen_ssa=info rustc +stage1 my-file.rs

# This will show all logs in `rustc_codegen_ssa` and `rustc_resolve`.
$ RUSTC_LOG=rustc_codegen_ssa,rustc_resolve rustc +stage1 my-file.rs

# This will show the output of all `info!` calls made by rustdoc
# or any rustc library it calls.
$ RUSTDOC_LOG=info rustdoc +stage1 my-file.rs

# This will only show `debug!` calls made by rustdoc directly,
# not any `rustc*` crate.
$ RUSTDOC_LOG=rustdoc=debug rustdoc +stage1 my-file.rs
```

## Log colors

By default, rustc (and other tools, like rustdoc and Miri) will be smart about
when to use ANSI colors in the log output. If they are outputting to a terminal,
they will use colors, and if they are outputting to a file or being piped