Function developer_message 

pub fn developer_message(err: &Error) -> String

A message for whoever is developing the program: user_message’s innermost cause, followed by Lua’s stack traceback: block when the error carries one.

depth.tl:8: attempt to index a nil value (local 'c')
stack traceback:
    depth.tl:8: in function 'depth.field'
    depth.tl:12: in function 'depth.describe'
    boom.tl:3: in main chunk

The innermost line says a value was nil; the frames say which caller passed it, and they name Teal files and Teal lines because a generated chunk is loaded under its source’s own name. This is what htl run and htl test print. The frames are absent only where the debug information is: stripped bytecode, which is what a bundle without --debug and include_tl_bytes! both hold.