feat(prompt): ${var@P} renders through the PS1 renderer; prompt preview uses it

[LESdylan]

What & why

Section 1 of #134: prompt preview showed prompts that never appear.

$ V='100%done'; PS1='[${V}] '
prompt:           [100%done]
prompt preview:   [100/home/me/projone]      # %d read as an escape

The preview rendered a PS1 theme with print -rP "${PS1//%/%%}". print -P is zsh's, so it substitutes a variable first and then reads the value for % escapes. A PS1 does not do that, because hellish follows bash's order there. A segment holding 100%done, 50%off or \w therefore previewed as something else. Doubling every % also hid the bilingual PS1's own %~ from the preview.

print -P is right to behave like zsh. What was missing is a way to render a PS1 exactly as the prompt does. bash has one, ${var@P}, but hellish parsed it and returned the value unchanged (a documented v1 scope-out).

Change. prompt_expand_p (new file, src/infrastructure/prompt_expand.c) implements @P with the PS1 rule itself:

• it calls zsh_to_ps1 in the mode the dialect bit selects, then ps1_render, the same pair prompt_normal uses for PS1;
• under set -o zsh, where PS1 is PROMPT, a % in a value is an escape again, as in the live prompt;
• it uses ps1_render, not ps1_animated, so an @P from a hook or inside a PS1 does not rebuild the live prompt's animation cells;
• \[ \] render as nothing outside a line editor, as in bash. HELLISH_DBG_PROMPT_MARKS keeps them, the same window print -P already gives the width tests.

prompt preview now draws a PS1 theme with "${PS1@P}". PROMPT themes stay on print -rP, which is the live PROMPT's renderer.

Refs #134

How I verified it

• Reproduced first.

  • tests/scripts/61_prompt_expand_P.sh is graded against bash --posix 5.3.9. It covers:

    • the escapes both shells have, and \[ \];
    • \nnn and \D{};
    • $VAR, ${…} and $((…)), with values holding \w, %d and $HOME left as they are;
    • unset and empty values, positional parameters and a function's arguments;
    • quoting and assignment contexts.

    develop prints the values raw. This branch matches bash.

  • tests/prompt_expand_p_test.py runs at a real prompt with both the in-process and the forked readers. It checks that:

    • the live PS1 and ${PS1@P} are the same bytes, and stay the same after the variable changes;
    • the same holds under set -o zsh, where the value is read again;
    • prompt preview prints the live prompt for a PS1 theme.

    develop fails 8 of its 14 checks. Its preview line is the issue's bug verbatim: [100/tmp/…one.

  • tests/prompt_themes_test.py renders PS1 themes the way the preview now does. Every check passes.

• Local gates on this commit (ASan debug build, on develop with docs(prompt): say that $( ) is not run in a prompt, and what to do instead #154 and fix(prompt): PS2 is the prompt of every continuation line #155):
  • golden tests/tester: 5363/5363;
  • tests/run_scripts.sh against bash --posix: 126/126;
  • verify_alloc.sh: identical output on both heaps;
  • alloc_stress.sh: all clean;
  • tests/pty_suite.sh: 112 ok, 6 skipped, 4 failed. None of the four is this change:
    • prompt_compat_matrix, prompt_drift_matrix and prompt_jobs_badge expect the non-root %/$ prompt and get #, because the container runs as root. They fail the same way on develop, and CI runs them as a normal user.
    • hxp_framework_test hit the 420 s per-file limit on this 4-core container. Run alone, it passes every check in 426 s, and develop's binary takes 440 s on the same machine. CI's runners finish it inside the limit (the pty job is green on feat(env): $EPOCHREALTIME and $SRANDOM #157).
• norminette is OK on every touched file.
• wiki/interactive.md documents ${PS1@P}.

Notes / trade-offs

• @P on a whole list, "${@@P}" or "${a[@]@P}", is still a bad substitution. The same is true of every transform (@Q, @U, …): hellish applies the operators to one value, never element by element. That gap is older than this PR and not part of Prompt renderer, extglob and shopt: 8 findings from building a 30-theme config on 2.10.2 #134, so it is left for its own change. ${a[1]@P} works.
• A value is rendered once, not rescanned, exactly as bash does. Under set -o zsh it is rescanned, because that is what the live zsh prompt does.

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