Janus Formatter: Dogfood Complete — SPEC-FMT Task 7

Janus milestone update · 2026-03-22

The Janus formatter (janus fmt) now formats its own source code idempotently — formatting the formatter produces byte-for-byte identical output on the second pass. This closes SPEC-FMT Task 7 (“Dogfood: formatter formats itself”).

The Bug

The trailing-comma enforcement in the } handler had an idempotency flaw. When the original source had a comma immediately followed by a newline then }:

return Token{
    .kind = .string_lit,
    .text = self.source[start..self.pos],
};

The comma handler detected that the next token was } and did not emit a newline. Then the } handler, seeing at_line_start = false, added a newline before } — producing };\n. The second pass, with } on a new line, would write } directly without adding another newline. First pass: };\n · Second pass: } — not idempotent.

The Fix

Two changes:

1. Line tracking in the tokenizer. Each Token now carries a line: u32 field set to the current source line number when the token is created. This lets the } handler know where the previous token was.

2. Trailing-comma idempotency logic. The } handler now checks:

if (prev_kind == .comma and tok.line == prev_line) {
    // Comma and } on same source line: write } directly
    try state.emitter.write("}");
} else if (!state.at_line_start) {
    try state.emitter.newline();
    state.at_line_start = true;
    try state.emitter.write("}");
}

If the comma and } are on the same line in the original source, } is written directly. If they’re on different lines, a newline is emitted before }. This preserves the original source’s line structure across formatting passes.

Results

• 36/36 formatter tests passing
• formatter.zig is idempotent under itself
• emitter.zig is idempotent under the formatter
• Both files committed to origin/unstable

What This Means

The Janus formatter is now self-hosting at the formatting level. The compiler can format its own source without style drift. This is a prerequisite for the graft syntax work remaining — the parser for graft is implemented; the IR and codegen remain.

What’s Next

The graft syntax (parser done, IR/codegen remaining) is the last open item. After that, Janus enters Phase D — dynamic dispatch polish and stdlib completion.