When finalizing the coverage map, FunctionCoverage only has the CodeRegions and counters for
the functions that went through codegen; such as public functions and “used” functions
(functions referenced by other “used” or public items). Any other functions considered unused,
or “Unreachable”, were still parsed and processed through the MIR stage, but were not
codegenned. (Note that -Clink-dead-code can force some unused code to be codegenned, but
that flag is known to cause other errors, when combined with -C instrument-coverage; and
-Clink-dead-code will not generate code for unused generic functions.)