Function sniff_version 

pub fn sniff_version(src: &str) -> Option<RustyfiVersion>

Best-effort detection of a document’s target version from its source text, by inspecting the header-like lines at the top of the file (before any prelude bindings), and, failing that, the first content line.

• A @stage: header line is a real, direct signal: 0.1’s lexer rejects it outright, so seeing it at all yields Some(V0_0).
• @require: / @import: header lines are transparent: byte-identical in both v0.0.6 and dev-0-1-0, so their presence pins neither axis — they are skipped just like a blank/comment line.
• A use-shaped header line — 0.1/Saphe’s module-header syntax, see is_use_header — yields Some(V0_1). This half of the heuristic is best-effort: the exact grammar could not be confirmed against upstream from this sandbox (no network access to GitHub / zenn.dev at the time this was written).
• Blank lines and %-comments (SATySFi’s line-comment syntax, both versions) are skipped while looking for the first header-shaped line.
• Once a non-blank, non-comment, non-header line is reached (headers are only valid at the top of a file), that single line is inspected for a content-level signal (see sniff_content_line) and the result — including None — is returned regardless.
• Returns None if no signal is found at all (e.g. a bare let ... in ... document with no headers, which is valid and version-ambiguous in 0.0.x).