feat: TOML-based filter DSL for community contributions

[pszymkowiak]

Problem

Adding a new filter to RTK currently requires writing a full Rust module (~100-200 lines), adding a variant to the Commands enum, updating main.rs, and passing the full quality gate. This is a high barrier for contributors who just want to add simple output filtering for a tool like pre-commit, helm show values, or ansible-playbook.

There are currently 30+ open PRs, many of which are simple line-oriented filters that follow the same pattern.

Proposal

A declarative TOML filter DSL for the ~45% of commands with simple line-oriented output. Contributors add a TOML block instead of writing Rust:

[[filter]]
command = "pre-commit run"
strip_lines_matching = ["^Passed$", "^\\[INFO\\]"]
keep_lines_matching  = []
strip_ansi           = true
max_lines            = 100
on_empty             = "✓ All hooks passed"

[[filter]]
command = "helm show values"
strip_lines_matching = ["^#", "^\\s*$"]
max_lines            = 100

The Rust engine interprets and applies these rules. Built-in rules ship with the binary (include_str!). Users can add custom rules at multiple levels:

Filter lookup priority

1. Rust built-in filters          (highest priority, always win)
2. .rtk/filters.toml              (project-level, in current directory)
3. ~/.config/rtk/filters.toml     (user-level, global custom rules)
4. Built-in TOML rules            (shipped with binary)
5. Passthrough                    (no filter → raw execution)

Project-level .rtk/filters.toml allows teams to define custom filters for their own tools and scripts without polluting the global config — similar to .eslintrc or .prettierrc.

Scope

TOML-feasible (~45% of open issues): commands with line-oriented text output — pre-commit, helm, terraform fmt, gcloud, ansible-playbook, make, du, stat, jj, oxlint...

Rust-required (~55%): JSON/NDJSON parsing, multi-line state machines, complex grouping — docker inspect, psql, go test, aws ec2 describe-instances...

DSL primitives (8 max — no feature creep)

strip_lines_matching, keep_lines_matching, strip_ansi, max_lines, head_lines, tail_lines, truncate_lines_at, on_empty

No line_format, no group_by, no transforms — if a filter needs structured data parsing, it belongs in a Rust module.

Architecture

• New rtk run -- <command> dispatch path (additive, no breaking changes)
• Rust filters keep priority over TOML filters
• rtk rewrite registry extended with TOML fallback
• schema_version = 1 from day one for future migrations
• .rtk/ directory convention for project-level customization