Error Handling & Diagnostics

Use this skill whenever you add, change, or review an error path: a build-time authoring error, a parser/handler failure, a CLI validation error, or anything surfaced to a host (FFI/WASM/Node) or a tool/agent. WebUI errors must be recoverable, actionable, and machine-consumable — for humans and AI agents alike.

1 - Never panic on recoverable input

panic = "abort" in the release profile means a panic kills the process instantly — including any FFI/WASM/Node host embedding the framework. Bad template input, bad CLI input, and bad state are recoverable and must return Result, never panic!/unwrap()/expect().

unwrap()/expect() are banned in library code (clippy.toml disallowed-methods). todo!, unimplemented!, and dbg! are banned workspace-wide (clippy.toml disallowed-macros). Tests opt out with #[allow(clippy::disallowed_methods)].

Enforcement note. unwrap/expect/todo!/unimplemented!/dbg! are caught by clippy. panic! is not lint-banned (too entrenched) — keep it out of recoverable paths by review. The "no regex in core logic" rule is also review-enforced, not deny-banned: actix-web pulls regex transitively, so a crate-level ban would break the build and can't scope to first-party code.

2 - Error type conventions

• Each error layer's Display describes only its own level; the #[source] chain carries the rest, so anyhow's {:#} never double-prints. Provide a flat chain_message() helper for hosts that don't walk the chain (Node, FFI).
• Add dedicated variants instead of overloading a generic Generic(String) / Validation(String) so callers can match programmatically.

3 - Authoring errors are structured Diagnostics

Every "the developer wrote invalid template syntax" mistake is returned as ParserError::Template(Box<Diagnostic>) (crates/webui-parser/src/diagnostic.rs), so all build errors render identically. A Diagnostic carries:

• code — a stable, machine-readable identifier (e.g. invalid-for-each). Defined in diagnostic::codes. Treat codes as a stable API: tools and AI agents branch on them, so rename only with a deliberate migration.
• title — short, lowercase (invalid <for> each expression).
• location — set from a byte offset via .at_offset(source, offset); rendered rustc-style --> owner:line:column (single forward scan, no regex, no recursion), falling back to in component <c> · element <e>.
• snippet — the offending source text.
• help: — an actionable fix (see §4).

Add a new authoring error with the parser helpers (authoring_error, authoring_error_at, html_error) and a new constant in diagnostic::codes. Validate at parse/build time and fail fast — never defer to render time.

4 - Make errors actionable (and typo-aware)

Tell the developer what is wrong and how to fix it. Every Diagnostic should carry a help: line. Where a mistake is likely a typo, suggest the intended name via suggest::closest_match (iterative Levenshtein — no recursion, no regex, cold path only):

• Misspelled directive attribute: <for eahc=…> -> "did you mean each?"
• Unknown component tag: suggest the closest same-namespace registered component (<mp-buton> -> <mp-button>). Prefix-guard the match (text before the first - must match) so a genuine third-party custom element (<md-button>) is never falsely flagged.

5 - Presentation layering: color belongs ONLY in the entry point

Libraries produce plain, color-free data. The entry point decides how to present it. Never embed ANSI in library output or in any machine/host channel.

When one value feeds both a terminal and a non-terminal channel, split it: webui-dev-server's RebuildError { display, message } carries a colorized display for the reporter and a plain message for the browser.

Per-line color: when colorizing multi-line output, style each line independently (open + close the SGR span within the line). A single span that straddles newlines bleeds when the line is later re-prefixed (e.g. [server] under xtask dev).

6 - Machine-readable output and exit codes (webui-cli)

For editors, CI, and AI/agent tooling:

• --format json (global flag) emits each error as one JSON object on stdout (no ANSI; decorative output suppressed): {severity, code, message, file, line, column, snippet, help, chain}. Branch on the stable code, not the human message. Build the object with the serde_json::Map API, not the json! macro (it unwraps internally and trips disallowed_methods).
• Exit codes follow BSD sysexits.h (webui-cli's error::exit_code): 65 data/authoring error, 66 missing input, 69 port in use, 74 I/O, 2 usage (clap), 1 otherwise.
• Replace fragile err_msg.contains("...") dispatch with typed errors that own their hint() and exit_code().

7 - Error construction is COLD - keep it off the hot path

Building a Diagnostic (format strings, suggestions, location scans) is rare, but if it inlines into a hot function it bloats that function and perturbs its code layout — a real, measurable regression (observed ~4-5% on parse benches with no added hot-path work).

• Mark error builders #[cold] + #[inline(never)] (e.g. authoring_error*, html_error, css_diagnostic, *_error constructors, suggest::closest_match).
• Keep hot fast-paths inlinable: a per-element check (e.g. split_once('-')) must stay inlined; only its cold fallback (the registry scan) goes out-of-line.
• Validate layout-sensitive changes with cargo bench -p <crate> against the base branch (see skills/perf/SKILL.md). A "regression" with no added compute is usually layout — fix it with #[cold], don't shrug it off.

8 - Checklist for a new error

• Returned as Result, never panicked (host-safe under panic = "abort").
• Authoring mistake -> ParserError::Template(Box<Diagnostic>) with a new diagnostic::codes constant, location, snippet, and help:.
• help: is actionable; add a "did you mean …?" suggestion if it's a typo.
• No color/ANSI in library, host, browser, or JSON output.
• Surfaced in --format json with the stable code; exit code classified.
• Error construction is #[cold]/#[inline(never)]; hot path unchanged (confirm with a benchmark if it sits near a hot loop).
• Tests: a regression test that fails without the error, asserting on the code (not the prose); JSON stays plain (no \x1b).
• DESIGN.md updated if the error-handling architecture changed; public docs (incl. docs/ai.md) updated if a user-visible code/flag changed.