As mentioned in #9456, the format! syntax extension would previously consider an
empty format as a 'Unknown' format which could then also get coerced into a
different style of format on another argument.
This is unusual behavior because `{}` is a very common format and if you have
`{0} {0:?}` you wouldn't expect them both to be coereced to the `Poly`
formatter. This commit removes this coercion, but still retains the requirement
that each argument has exactly one format specified for it (an empty format now
counts as well).
Perhaps at a later date we can add support for multiple formats of one argument,
but this puts us in at least a backwards-compatible situation if we decide to do
that.
This large commit implements and `html` output option for rustdoc_ng. The
executable has been altered to be invoked as "rustdoc_ng html <crate>" and
it will dump everything into the local "doc" directory. JSON can still be
generated by changing 'html' to 'json'.
This also fixes a number of bugs in rustdoc_ng relating to comment stripping,
along with some other various issues that I found along the way.
The `make doc` command has been altered to generate the new documentation into
the `doc/ng/$(CRATE)` directories.
This renames the syntax-extension file to format from ifmt, and it also reduces
the amount of complexity inside by defining all other macros in terms of
format_args!
