This is a way to shrink call spans that doesn't involve mixing different spans,
and avoids overlap with argument spans.
This patch also removes some low-value comments that were causing rustfmt to
ignore the match arms.
When preparing a function's coverage counters and metadata during codegen, any
part of the original coverage graph that was removed by MIR optimizations can
be treated as having an execution count of zero.
Somewhat counter-intuitively, if we give those unreachable nodes a _higher_
priority for receiving physical counters (instead of counter expressions), that
ends up reducing the total number of physical counters needed.
This works because if a node is unreachable, we don't actually create a
physical counter for it. Instead that node gets a fixed zero counter, and any
other node that would have relied on that physical counter in its counter
expression can just ignore that term completely.
When making changes that have a large impact on coverage counter creation, this
makes it easier to see whether the number of physical counters has changed.
(The highest counter ID seen in coverage maps is not necessarily the same as
the number of physical counters actually used by the instrumented code, but
it's the best approximation we can get from looking only at the coverage maps,
and it should be reasonably accurate in most cases.)
For coverage tests, splitting code across multiple lines often makes the
resulting coverage report easier to interpret, so we force rustfmt to retain
line breaks by adding dummy line comments with `//`.
Some of these cases currently don't occur in practice, but are included for
completeness, and to avoid having to add them later as branch coverage and
MC/DC coverage start building more complex expressions.
These tests deliberately use non-standard formatting, so that the line
execution counts reported by `llvm-cov` reveal additional information about
where code regions begin and end.
In some cases we need to prepare a coverage expression that is the sum of an
arbitrary number of other terms. This patch simplifies the code paths that
build those sums.
This causes some churn in the mappings, because the previous code was building
its sums in a somewhat idiosyncratic order.
