Module coverage 

Line coverage for executed Harn programs.

Harn already stores a source line for every emitted instruction (Chunk::lines), so line coverage needs no separate debug-info pass: the denominator is the set of distinct non-zero lines a chunk (and its nested function bodies) emit, and the numerator is the subset whose instructions actually ran.

How it is wired

Coverage is opt-in and process-global so it captures every VM isolate a run spins up (imports, parallel branches, spawned agents) without threading a flag through every constructor:

• begin_session flips is_enabled on and clears the merged report.
• Each crate::vm::Vm checks is_enabled at construction; when on it carries its own Coverage accumulator and records a hit per executed instruction in the dispatch loop.
• On drop a VM folds its accumulator into the global report.
• end_session flips coverage off and returns the merged Coverage.

File attribution

A chunk compiled from an imported module carries its own source_file; the entry file’s top-level chunk and its same-file function bodies carry None. We attribute a None chunk to the VM’s primary file (the script under execution), and otherwise to the chunk’s source_file. Nested function chunks inherit their parent’s effective file when they carry no source_file of their own, so a module’s uncalled helpers are still counted against the module — not misattributed to the entry script.

Render filters to files that exist on disk, which drops the synthetic paths the embedded stdlib and in-memory eval chunks report.

Structs

Coverage

Accumulated line coverage. Used both as a per-VM accumulator and, after merging, as the whole-run report.

Functions

begin_session

Start a coverage session: clear the merged report and enable recording on every VM constructed until end_session.

end_session

End the coverage session and return the merged report.

is_enabled

True while a coverage session is active. Read once per VM construction and once per executed instruction, so it is a relaxed atomic load — effectively free and branch-predicted “off” when no session is running.