Function launch_via 

pub fn launch_via(
    path: &Path,
    candidates: &[String],
    run: &mut dyn FnMut(&mut Command) -> Result<EditorRunOutcome>,
) -> Result<()>

Try each candidate in order and return once one runs to completion.

run is injected so every arm - including “no editor found” - is reachable under test on every platform without spawning a real, blocking, interactive editor. That matters most on Windows: Command::new("notepad") resolves through the System32 search path that CreateProcess consults before $PATH, so it cannot be made to fail short of tampering with a real system directory, and letting it actually open would hang CI with no timeout.

run is &mut dyn FnMut rather than impl FnMut because several test call sites pass distinct closure literals, and a generic parameter would give each one its own coverage-mapping instantiation. cargo llvm-cov sometimes reports a region as uncovered for one instantiation even when the union of all of them covers every source position.