JIT: Execute external command lazily based on the fallbacks order

[adaszko]

For a grammar cmd (--foo | {{{ echo bar }}});, the {{{ echo bar }}} command is executed even when we're matching the first word (i.e. cmd <TAB>), which is premature. {{{ echo bar }}} should be executed lazily only after we know --foo hasn't matched.

The output currently produced:

__complgen_jit () {
    local -a matches=()
    local -a completions=("bar" "foo")
    compadd -Q -a completions
    compadd -O matches -a completions
    [[ ${#matches} -gt 0 ]] && return
}
__complgen_jit

whereas it should look more like (in this case for ZSH)

__complgen_jit () {
    local -a matches=()
    local -a completions=("foo")
    compadd -Q -a completions
    compadd -O matches -a completions
    [[ ${#matches} -gt 0 ]] && return

    completions=("${(@f)$(echo bar)}")
    compadd -Q -a completions
    compadd -O matches -a completions
    [[ ${#matches} -gt 0 ]] && return
}
__complgen_jit

This piece of code is responsible for generating those completions:

complgen/src/jit.rs

Lines 395 to 400 in 5e29dcb

	// Complete `prefix` based on `state`.
	let mut output: Vec<Completion> = Default::default();
	for (input, _) in dfa.iter_transitions_from(state) {
	get_completions_for_input(&input, prefix, shell, &mut output)?;
	}
	output.sort_unstable_by(|left, right| left.get_completion().cmp(&right.get_completion()));