Missing coverage files for src files

I’m trying to do code coverage analysis for my new package, and I run the runtests.jl in following ways:
julia --code-coverage test/runtests.jl
this only generated .cov files for the code in test/ but no coverage data for code in src/
julia> using Pkg
julia> Pkg.test(coverage=True)
this doesn’t generate and .cov files at all. In my understanding .cov files are the coverage data that sends to Codecov for coverage analysis, thus there is only coverage data for test/ codes.