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Modeline language detection — side quest

Status: Revision 2, approved for implementation by the user on 2026-07-22. No implementation was present at approval. Base: githubsucks/main at d5d9b9c; protocol v18.

Problem

Language inference currently uses four signals, in order:

bundled grammar extension;
pmacs.lsp.filetypes extension;
exact basename;
shebang.

That chain cannot classify a deliberately misleading or extensionless file when its author supplied editor metadata such as -*- mode: python -*- or vim: set ft=python:. Mode-system wiring #129 gives the result a persistent per-buffer home and makes it observable through key dispatch, help, and the statusline, but no modeline parser exists.

The goal is one bounded, non-executing modeline detector whose result drives the same initial language used by syntax, LSP, and Buffer.major_mode. This is a language-detection feature, not a general file-local settings system.

Ground truth

Detection is duplicated today

builtin/runtime/syntax.lua::resolve_active_language and builtin/runtime/lsp.lua::buffer_language independently implement the same extension → filetype → filename → shebang chain. They already have small but important differences:

syntax uses buf:name(), preserving historical grammar-by-name behavior for pathless buffers;
LSP requires buf:path(), because it cannot construct a URI or project root for a pathless buffer;
syntax pins the grammar in parse_lang_by_buffer after dispatch, while the public LSP language query re-sniffs mutable shebang text on every call.

Adding the fifth signal to both copies would create a third convention and let syntax, LSP, mode initialization, auto-pairing, and comment commands disagree. The implementation must consolidate the actual inference in syntax.lua and leave only the LSP path-eligibility guard in lsp.lua.

Hook order is already useful

src/editor.rs loads syntax.lua before lsp.lua. Their buffer.after-load callbacks therefore run in that registration order:

syntax detects the language, initializes the major mode, and dispatches a grammar when one exists;
LSP resolves the same buffer language and attaches a server when configured.

The modeline result must be computed and pinned by step 1 so step 2 cannot independently reinterpret the file.

Major mode and parser language have different later lifecycles

At first load, the detected language is the initial major-mode name. Afterward:

explicit pmacs.buffer.set_major_mode changes dispatch/statusline only;
syntax and LSP remain attached to their initially selected language;
buffer.after-switch restores views without re-detecting;
edits do not change the selected grammar.

Modelines follow that same load-time contract. Editing a cookie is not a live mode or parser switch. Close/reopen re-evaluates it. A future true reload that fires buffer.after-load re-evaluates it as a fresh load; explicit mode overrides and clears are not reload-persistent, matching the #129 framing.

Existing Lua reads are sufficient

BufferIdLua exposes byte-length and byte-slice operations. Detection can read bounded prefix/suffix windows without copying the whole buffer or adding a Rust binding. The rope is byte-addressed, so a bounded slice does not need UTF-8 boundary repair before Lua pattern matching.

Compatibility references

The supported subset follows the documented, non-evaluating pieces of:

GNU Emacs, “Specifying File Variables”: https://www.gnu.org/software/emacs/manual/html_node/emacs/Specifying-File-Variables.html
Vim, :help modeline and :help 'modelines': https://vimhelp.org/options.txt.html#modeline

Compatibility is deliberately bounded below. pmacs does not become an Emacs file-variable evaluator or a Vim :set interpreter.

Scope

In scope:

Emacs -*- mode: NAME -*- and mode-only -*- NAME -*- cookies;
Vim/Vi ft=NAME and filetype=NAME modelines;
first/last-line scanning with fixed byte and line limits;
canonical aliases for common external filetype names;
modeline precedence over inferred path/shebang language;
one shared, pinned per-buffer language decision;
initial major mode, grammar, LSP, and language-aware Lua consumers agreeing;
focused parser and end-to-end acceptance on both Lua backends.

Out of scope:

Emacs Local Variables: tail blocks;
variables other than Emacs mode or Vim ft/filetype;
eval, Vim commands, option mutation, directory-local variables, or project trust prompts;
Vim ex: markers, version predicates, escaped option values, and combined dotted filetypes;
live re-detection after edits, saves, renames, or buffer switches;
buffer.after-mode-change, minor modes, mode-scoped settings, describe-mode, or session persistence of explicit mode overrides;
a protocol change or frontend-specific work.

Decisions

Q#MD1 — Scan only bounded edge lines

Read at most 8 KiB from each end of the buffer and inspect:

Emacs: line 1, or line 2 only when line 1 begins with #!;
Vim/Vi: the first five and last five logical lines, matching Vim's default 'modelines'=5 behavior.

When the prefix and suffix overlap, deduplicate lines before parsing. Strip one trailing \r so CRLF and LF behave identically. Discard the suffix window's leading fragment when its line begins before the 8 KiB boundary; that fragment does not count toward the five complete logical lines counted backward from buffer end. No truncated candidate is parsed. Detection therefore allocates at most 16 KiB per fresh load, independent of file size, and an adversarial giant edge line cannot force a whole-buffer copy.

The Emacs 3000-character tail Local Variables: mechanism is a separate parser with comment-prefix/suffix rules and is excluded.

Q#MD2 — Recognize a conservative syntax subset

Emacs:

require a complete pair of -*- delimiters on the eligible line;
accept mode: NAME in a semicolon-separated property list;
accept a mode-only payload such as -*- Lisp -*-;
ignore every property except mode;
when a cookie contains multiple valid mode properties, the last wins.

Vim/Vi:

accept vim:, vi:, and Vim: at line start or preceded by ASCII space or tab; Vim: requires the set form, matching Vim;
accept only exact ft=NAME and filetype=NAME assignments; ft:NAME and filetype:NAME are not modeline assignment forms and are rejected;
in the direct form, split option tokens on ASCII whitespace and :, so the common vim:ft=python:sw=4: form yields ft=python;
in the set / se form, end the option section at the first : and split only the preceding text on ASCII whitespace; vim: set sw=4: ft=python therefore contains no live filetype assignment;
ignore all other live option tokens rather than interpreting them;
require that terminating colon for the set / se form, so a comment suffix is never consumed as an option value;
reject ex:, Vim version predicates, and marker substrings embedded in a word.

Across all eligible lines, the last valid mode assignment in document order wins. This matches Emacs's “final defined mode” behavior and ordinary sequential option assignment. A footer Vim modeline can intentionally override a header Emacs cookie; conflicting metadata does not depend on Lua table iteration.

Q#MD3 — Modeline names are data, never code

Trim ASCII edge whitespace, ASCII-lowercase the name, require [a-z0-9][a-z0-9+_-]*, and cap it at 128 bytes. Empty, non-ASCII, control, whitespace-containing, or overlong values are ignored silently.

The restriction is intentionally narrower than pmacs.buffer.set_major_mode, which continues accepting arbitrary Lua strings for trusted configuration. Untrusted file content receives no path to control characters, huge statusline values, Lua evaluation, or option mutation.

Q#MD4 — Normalize common external names through one alias table

Expose a user-extensible Lua table:

pmacs.parse.modeline_aliases = {
  ["c++"] = "cpp",
  cxx = "cpp",
  sh = "bash",
  shell = "bash",
  ["shell-script"] = "bash",
  zsh = "bash",
  py = "python",
  js = "javascript",
  js2 = "javascript",
  jsx = "javascriptreact",
  ts = "typescript",
  tsx = "typescriptreact",
  yml = "yaml",
  makefile = "make",
  docker = "dockerfile",
}

A normalized name absent from the table passes through unchanged. Users may add, replace, or remove aliases in init.lua; defaults use or-style seeding so preconfigured entries are not overwritten. Alias outputs must satisfy the same 128-byte token rule before use.

This keeps canonical pmacs names stable without pretending that extension maps and editor-mode names are the same namespace. Do not strip a trailing -mode: GNU Emacs explicitly specifies the value without that suffix, and silent stripping would make custom names ambiguous.

Q#MD5 — Explicit modelines override inference

The final fresh-load order is:

modeline;
bundled grammar extension;
pmacs.lsp.filetypes extension;
exact basename;
shebang.

A modeline is explicit file metadata; the remaining signals are inference. Thus a template.txt containing vim: set ft=python: selects python, and a misnamed .py file containing -*- mode: lua -*- selects lua consistently for mode, parser, and LSP.

This interprets the backlog's “fifth layer after extension → filetype → filename → shebang” as a layer added after that work, not as a lowest-priority fallback. Making explicit metadata lose to a suffix would defeat the feature's primary use case.

Q#MD6 — One resolver owns the effective language

syntax.lua owns:

pmacs.parse.language_from_modeline(buf): parse current content and return the normalized modeline language or nil;
the private fresh inference chain;
pmacs.parse.buffer_language(buf): return the language pinned for this buffer's current load, resolving once only for an unseen buffer.

The syntax buffer.after-load path forces a fresh inference, records either the language or an explicit “resolved none” sentinel, then uses that same value for mode initialization and grammar dispatch. buffer.after-switch consumes the pin and resolves only the pre-existing hidden-buffer case that never received an after-load event.

pmacs.lsp.buffer_language(buf) keeps its current path requirement, then delegates to pmacs.parse.buffer_language(buf). The active-buffer wrapper stays unchanged. This preserves pathless LSP behavior while deleting the duplicate extension/filetype/filename/shebang chain.

The pin also closes an existing shebang inconsistency: editing #!/bin/sh to #!/usr/bin/env lua no longer leaves a bash parse tree while making later auto-pair/comment queries report Lua. Raw parser tests may call language_from_modeline; behavior-driving consumers use the pin.

On buffer.after-load, set the buffer's major mode to the freshly resolved language, including nil when no signal resolves. This replaces the current “only if nil” guard and makes the already-documented reload contract exact: a fresh after-load decision replaces an earlier explicit override or clear.

Then:

dispatch a grammar only if pmacs.parse._has_language(lang);
let LSP attach only if the same language has a configured server and a real file path;
retain a valid unknown language as the major mode, while silently skipping grammar/LSP attachment.

After load, explicit set_major_mode remains independent: it immediately changes mode key dispatch and statusline display but does not rewrite the pinned parser/LSP language. Switches preserve both values.

Q#MD8 — Malformed or unsupported metadata is fail-closed and quiet

A malformed marker, invalid name, unsupported form, or truncated candidate never raises from buffer.after-load and never emits an *errors* entry. The detector returns nil and the existing inference chain continues.

A syntactically valid unknown name is different: it is a legitimate major mode and is pinned, but _has_language and LSP config gates prevent a bogus parser or server launch. This preserves #129's custom-mode capability without executing file content.

A file can already select a configured language—and therefore which LSP server pmacs starts—through its extension or a content-sniffed shebang. Modelines add another bounded language selector within that existing capability class; they do not introduce automatic execution beyond what current language detection already permits.

No enable/disable setting is added. The supported input can only select a bounded string already consumed as passive mode/language identity; it cannot run hooks or set options. If a future buffer.after-mode-change hook makes mode selection executable, modeline trust must be revisited in that feature's framing.

Q#MD9 — No Rust or protocol surface is required

Expected implementation touch set:

builtin/runtime/syntax.lua — bounded parser, aliases, shared resolver, pin, and after-load initialization;
builtin/runtime/lsp.lua — delegate language inference while retaining the path guard;
tests/m4_acceptance.rs — parser, precedence, lifecycle, and end-to-end regression coverage;
this framing and the side-quest/handoff state documents when the feature lands.

No changes are expected in src/buffer.rs, Lua bindings, frontends, pmacs-protocol, or protocol version 18.

Bets

Sixteen KiB of edge text is sufficient. Real modelines are short; files with an 8 KiB first/last candidate line are better treated as malformed than copied wholesale during load.
One canonical language should drive mode, parser, and LSP initially. A future distinction between editor mode and parser language needs a real consumer and an explicit mapping contract, not accidental divergence.
ASCII-lowercased identifiers cover interoperable modelines. Trusted Lua remains available for arbitrary UTF-8 custom mode names.
Load-time detection is enough. Live cookie edits would require orderly parser teardown, LSP didClose/didOpen, overlay replacement, mode-change notification, and failure rollback; that is not a one-shot detector.
Pathless LSP buffers stay ineligible. Syntax may still infer from a buffer name, but spawning a server without a URI/project root remains wrong.

Acceptance

All end-to-end fixtures clear pmacs.lsp.config unless the case intentionally observes server selection, so opening a test file never starts a machine-local language server.

Emacs forms: first-line property and shorthand cookies resolve; a cookie on line 2 resolves only after a shebang; unrelated properties are ignored; the last mode property wins.
Vim forms: vim:/vi: direct and set forms resolve ft and filetype in the first/last five lines; CRLF works; Vim: requires set. The direct vim:ft=sh:et:sw=2: form resolves sh, while vim: set sw=4: ft=python ignores the assignment after the terminating colon. ft:python and filetype:python resolve nothing in either form.
Boundary rejection: sixth-line, sixth-from-end, middle-of-file, word-embedded, unterminated, truncated, invalid-character, and overlong candidates do not resolve and do not log errors. A partial line at the start of the suffix byte window is discarded without consuming one of the five complete tail-line slots.
Conflict order: overlapping edge windows are deduplicated and the last valid assignment in document order wins deterministically.
Alias behavior: seeded aliases map sh/zsh → bash, c++ → cpp, js2 → javascript, tsx → typescriptreact, and docker → dockerfile; a user override wins; an invalid alias output is ignored.
Explicit precedence: a .py file with a Lua modeline yields lua from pmacs.parse.buffer_language, pmacs.lsp.active_buffer_language, the parse tree, and pmacs.buffer.major_mode.
Unknown valid mode: a .txt file with mode: prose receives major mode prose, creates no parse view, starts no LSP server, and produces no error.
No-modeline regression: extension, filetype, filename, and shebang cases retain their present precedence and outputs.
Shebang pin regression: open an extensionless #!/bin/sh file, replace its shebang with #!/usr/bin/env lua, and assert the parse tree and pmacs.lsp.buffer_language both remain bash.
Pinned modeline lifecycle: changing a loaded modeline does not change the pinned language, parser, or major mode; switch-away/back remains stable; close and reopen re-evaluates the changed on-disk cookie.
Explicit override independence: set_major_mode after load changes dispatch/statusline but not pmacs.parse.buffer_language; switches preserve the override.
Pathless preservation: syntax-by-buffer-name behavior remains, while pmacs.lsp.buffer_language still returns nil without a backing path.
Backend parity: focused acceptance passes with default LuaJIT and --no-default-features --features lua54; protocol remains v18.

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