context

Direction: guest implements this interface · Capability: none (pure data / dispatch) · Worlds: context-plugin (exports), treesitter-context-plugin (exports)

The structural-context producer API (treesitter-context.md, TC.2): the scopes a pane pins above its text once their own header lines have scrolled away — the nvim-treesitter-context / sticky-scroll idea.

Scopes cross, not rows. A context-scope is a pure function of the parse tree, so the host caches the set per parse version and resolves "which of these apply to THIS pane right now" itself, per pane, per frame (lattice_cells::context::resolve_context, TC.1). Returning finished rows instead would put a WASM call on the scroll path and give the host a cache keyed on the cursor — one that thrashes by construction. Paramount #1.

Producer, async, host-cached — the decorations shape (PH7.9), for the same reason: the host calls context-scopes OFF the render path on a trigger (a completed reparse), caches the result, and every later read is native. The guest never runs on a keystroke.

Uses

• context-request from types
• context-scope from types
• tree-snapshot from tree-sitter

Functions (1)

context-scopes

context-scopes: func(req: context-request, tree: option<borrow<tree-snapshot>>) -> result<list<context-scope>, string>

Produce the structural context scopes for a buffer.

tree is the buffer's point-in-time parse snapshot, handed in the way grammar.apply-action hands it (TS.1) rather than acquired by the guest: call-scoped access keeps the tree-sitter capability meaning "the tree you were given" instead of "any buffer's tree, any time". It is none when the buffer has no parse (plain text, or a parse still pending), and a guest with nothing to work from should return an empty list.

Async — a produce call suspends the guest and never the render path. The guest is expected to run a whole-buffer run-query here, which is why this must not be synchronous.

Graceful (§8): an err is logged and the buffer KEEPS its previously cached scopes rather than being cleared. A failed refresh must not blank the strip — a transient error would otherwise read as the feature breaking. Same contract as decorations.gutter-decorations.

Example — Produce sticky-context scopes from the tree, bounded by a size option · plugins/treesitter-context/src/lib.rs

fn context_scopes(
    req: ContextRequest,
    tree: Option<&TreeSnapshot>,
) -> Result<Vec<ContextScope>, String> {
    if req.line_count == 0 {
        return Ok(Vec::new());
    }
    // Bound the work BEFORE running the query. Returning empty (not `err`)
    // is deliberate: the host caches an empty set, which is the truth —
    // this file has no context — rather than keeping a stale set from
    // whatever was open before.
    let max_lines = get_option("max-file-lines")
        .and_then(|v| v.parse::<u32>().ok())
        .unwrap_or(DEFAULT_MAX_FILE_LINES);
    if max_lines > 0 && req.line_count > max_lines {
        return Ok(Vec::new());
    }
    // No parse (plain text, or one still pending) is a normal state the
    // host caches as "no scopes" — never an error, which would make it keep
    // the previous buffer's structure.
    let Some(tree) = tree else {
        return Ok(Vec::new());
    };
    scopes_from_tree(tree)
}