Previously, if a diagnostic context spanned multiple lines, any lines
which did not overlap with the highlight range would be displayed as
blank. This commit fixes the bug.
The problem was caused by the unconditional use of `PartitionAround` to
split the line into before/highlighted/after ranges. When two ranges
don't overlap, this method returns empty ranges, which results in a
blank line. Instead, we first check if the ranges do overlap, and if not
we print the entire line from the context.
When warnings appear in isolation (not accompanied by an error) it's
reasonable to want to defer resolving them for a while because they are
not actually blocking immediate work.
However, our warning messages tend to be long by default in order to
include all of the necessary context to understand the implications of
the warning, and that can make them overwhelming when combined with other
output.
As a compromise, this adds a new CLI option -compact-warnings which is
supported for all the main operation commands and which uses a more
compact format to print out warnings as long as they aren't also
accompanied by errors.
The default remains unchanged except that the threshold for consolidating
warning messages is reduced to one so that we'll now only show one of
each distinct warning summary.
Full warning messages are always shown if there's at least one error
included in the diagnostic set too, because in that case the warning
message could contain additional context to help understand the error.
Alisdair McDiarmid