> AFAIU, the fact that the metadata is defined in terms of columns is an
> implementation detail of no particular interest to GDB users.  On the
> user level, this is supposed to step by expressions, is that right?
>
> Perhaps, to avoid too many miscommunications, you could point me to
> the relevant description in the DWARF2 spec, so I could read it, and
> then we could talk based on the same information?
Unfortunately the spec is not too descriptive, it isn't until recent 
years (probably in the last ten-ish) that people (compilers/languages) 
has gotten around to understanding the point of the "Line Number 
Program", that records the line & column meta data, particulary when it 
comes to the column meta data.

There's a new proposal for further extending the LNP that discusses it 
some what more in depth and /perhaps/ that will give you some insight, 
it can be found here: https://dwarfstd.org/issues/140906.1.html (it was 
provided to me by jemarch after discussion on the IRC channel)

Otherwise, the "Line Number Program" can be read about in the latest (as 
well as earlier) specs: https://dwarfstd.org/doc/DWARF5.pdf under the 
chapter "6. Other Debugging Information" (specifically 6.2, the Line 
Number Program). But it won't explain much to you on how this is used by 
compilers.

The DWARF2-5 specs are of few words when it comes to columns, so one has 
to resort to "3rd party" (the proposal link, above) to get any 
explanations of what compilers actually do (or should do) but as 
previously said, GCC, Clang, and Rust all does it, though GCC emits 
somewhat faulty column meta data, see the issue I've filed on bugzilla 
for GCC after discussions on the GDB irc channel: 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109902

The best description of what the column meta data is meant to do, is 
serve as "logical breakpoint locations" on a source code line; this 
includes statements and expressions - though it's never _explicitly_ 
worded like that in the spec (unfortunately, DWARF is not as meticulous 
with it's documentation and describing the point of certain aspects of 
the spec, as "we are", here in the GDB community).

Regards,

Simon