Regexps in RUST_LOG as explained in the module documentation should
only allow log messages that match the given expression and not the
other way around.
We may want to add things to the information provided to `log` and
`enabled`. slf4j provides the ability to pass `Marker` objects when
logging which can be used for more advanced filtering for example.
We can't make it fully private but we can mangle the field names and
hide them from the docs. It's important to keep the internals of
`LogLocation` hidden so that we have the option of e.g. adding the
function name if such a syntax extension's added to rustc or switching
the implementation to look at debug info.
