[RFC/PATCH] More precise diagnostic locations: dynamic locations for columns vs explicit offset

[Manuel López-Ibáñez <lopezibanez at gmail dot com>]

In some situations, we would like to point to a location which was not
encoded when tokenizing. This happens, for example, in two prominent
cases:

1) To get precise locations within strings
(https://gcc.gnu.org/PR52952) for example, for Wformat warnings.

2) In the Fortran FE, which gives quite precise location information
by tracking the characters that it wants to warn about instead of
relying on the line-map machinery.

The most straightforward way to implement this is by adding variants
of diagnostic functions that take an explicit "offset" argument and
pass this offset through the whole diagnostics machinery. This is what
I implemented in the patch format_offset.diff attached. The downside
is that we would need to add even more variants (with/without offset)
of various diagnostic functions and track the offset/no-offset cases
explicitly.

The nicer/cleaner alternative is to somehow (re)compute a single
location value from a given location plus the new offset. This is what
I implemented in patch fortran-diagnostics-part3.diff in
linemap_redo_position_for_column(). As far as I understand, this
method only works reliably if the location+offset does not jump to a
different line map, that is, if to_column < (1u <<
map->d.ordinary.column_bits). Otherwise, we may need to recompute all
successive line-maps to accommodate the new location. The best way to
do the latter (or to work-around that issue) is not clear to me at the
moment.

Thus, I am putting forward these two alternative implementations and
seeking comments/advice/help in deciding what would be the best way to
fix this key missing piece of GCC diagnostics.

Related to this, perhaps I should make a more general call for help.
Despite the heroic, constant torrent of diagnostic fixes by Paolo,
Marek and others, I have not seen much progress on the key
infrastructure issues in the roadmap
(https://gcc.gnu.org/wiki/Better_Diagnostics). We have had at least
one major item per release since GCC 4.5, but I don't see any
particular item being tackled for GCC 5.0. Are you planning to tackle
any of them?

I have a simple patch to implement Fix-it hints but it needs more
work. Unfortunately, I have very little free time to dedicate to GCC
nowadays, so I'm afraid I might not even be able to finish this in
time. Any item in that list would be a nice major feature for GCC 5.0.
Perhaps we need to ask for help in gcc/gcc-help or some other forum.

Cheers,

Manuel.

Attachment: format-offset.diff
Description: Text document

Attachment: fortran-diagnostics-part3.diff
Description: Text document