pmacs/docs/semantic-frontend-protocol.md

Semantic frontend protocol (design note)

Status: implemented by the M11 arc (M11.1–M11.5), post-v1.0. This note was written as a post-v1.0 design draft — recorded so the v1.0 tag was a conscious decision point — and has since been built out. It originally concluded the v1.0 wire was already safe for this direction: the capability/version scaffolding in protocol.rs (SUPPORTED_PROTOCOL_VERSIONS, negotiate_capabilities, per-session outgoing filters) made the work a non-breaking later addition, mechanically identical to the M10.5–M10.10 CRDT rollout. It was, and the rollout matched the plan. Implementation status against this design:

• M11.1 — wire + capability scaffolding (semantic_render, PROTOCOL_VERSION 3, the SemanticFrame variant family, FrontendEvent::Viewport).
• M11.2 — the instance-side projection seam (SemanticRenderState), selected per session; StyleSpans.
• M11.3 — Decorations from diagnostics + selection.
• M11.4 — full + dirty-segment diffing.
• M11.5 — the headless SemanticClient glue + reconstruction- equivalence and end-to-end tests.

Post-M11 producer arc (the LSP feature arc landed the missing data sources, so the "wire in when those features land" promise came due):

• StyleSpans, second authority — policy A, per-language styling authority. A grammar-backed language stays tree-sitter only; a language with no bundled grammar (C/C++, …) is styled from LSP semantic tokens (lsp_scoped_style_spans, encoding + legend via LspManager::semantic_style_context). Never both on one buffer. Reuses the M11.4 diff pipeline unchanged.
• InlineAdornments — produced from the LSP inlay-hint store (scoped_inline_adornments). The wire variant carries no generation/full/segments, so suppression is M11.2-level (whole-set re-send on change, nothing when byte-identical, never an empty frame).
• FileStyleSummary — resolves Open Q#2 (minimap / whole-file overview). Per-line dominant style for the whole buffer, generation-keyed so an idle buffer pays nothing (file_style_summary_msg / scoped_file_summary). Reuses scoped_style_spans, so policy A's authority pick is inherited automatically.

BlockAdornments / FoldState / ResourceOffer remain declared but deliberately unproduced — pmacs still has no blame / lens / fold / diff source; their producers wire in when those features land (the same "declared, not yet wired" discipline). Open questions #3–#5 remain open by design; #1 is consciously deferred (no real visual-motion command surface or semantic frontend consumer exists yet).

The grid path (TUI, SSH, a future GPU terminal-grade frontend) is unaffected by everything here. This note describes a second projection selected per-session by a negotiated capability, in the VSCode/Zed "two renderers, one core" shape.

The content-model decision this assumes

The exploration weighed two models: semantics-down, layout-local (Monaco / VSCode-Remote) versus layout-down, paint-local (the Emacs glyph-matrix end-state). This note draughts the first. The deciding asymmetry: the optimistic-edit + CRDT-replica substrate v1.0 already shipped (optimistic.rs, the CrdtOp flow, CursorByte, the per-session multi-frontend dispatcher) is only useful if the frontend can lay out a speculative edit locally — which is the semantics-down model. Layout-down would leave that substrate nearly inert. The one cost consciously accepted: a slice of rendering correctness (shaping, wrap, hit-testing) moves into a GPU frontend the instance test harness cannot exercise; the mitigation is golden-testing the semantic projection instance-side (see "Testability").

Contract boundary

One sentence, because everything below derives from it:

The frontend owns the viewport and all visual-motion semantics. The instance owns the document and all edit/command semantics.

The instance never learns a pixel. Not viewport pixel size, not DPI, not font metrics, not glyph advances. This is a deliberate invariant, not an omission: the moment the instance knows pixels, it is tempted to lay out, and the model collapses toward layout-down with per-session reflow caches inside a multi-tenant daemon — the property pmacs's thesis exists to reject. The only spatial fact the instance learns is which buffer byte range is on screen, so it can scope its projection rather than ship a 100k-line file's styling.

Corollary — there is no hit-test round trip. The frontend resolves pixel→offset locally (it has the layout) and only ever emits buffer offsets and edits. Click-to-caret latency is local and therefore zero over SSH. Re-importing a pixel→offset request into the wire would forfeit the entire latency argument; it is prohibited by this contract, not merely discouraged.

Composition with v1.0 primitives

The semantic projection ships no text. A semantic_render session is required to also be a text replica — it holds the rope locally via the existing crdt_replica machinery (BufferSnapshot to bootstrap, CrdtOp to stay live). The semantic frame is purely the interpretation layer over a buffer the frontend already has: styling and decoration keyed by byte range. This mirrors how v1.0 already coupled multi_frontend and crdt_replica, and it keeps the new wire tiny — single-digit KB for a screenful, diffable at span granularity.

Consequently the new surface is small. Cursor reuses the existing InstanceMessage::CursorByte (authoritative cursor as a buffer offset — added for CRDT optimistic-apply, exactly what a layout-local frontend consumes). Peer cursors reuse the existing PresenceUpdate. Edits and local cursor travel the existing FrontendEvent::CrdtOp / presence path. The genuinely new wire is: one capability bit, ~five instance→frontend interpretation variants, and one frontend→instance Viewport variant.

Capability and version mechanics

Identical pattern to crdt_replica:

• New bit semantic_render on FrontendCapabilities and InstanceCapabilities, #[serde(default)] false — every v1.0 wire byte still deserializes.
• negotiate_capabilities AND-combines it into NegotiatedCapabilities; mismatch yields the existing Goodbye(CapabilityMismatch { missing: ["semantic_render"] }).
• semantic_render requires crdt_replica (text-replica dependency above). Negotiation rejects semantic_render: true with crdt_replica: false as a capability mismatch rather than silently degrading.
• PROTOCOL_VERSION 2 → 3; SUPPORTED_PROTOCOL_VERSIONS &[1, 2, 3]. The slice-membership check already in place means v0.1/v1.0 binaries keep connecting unchanged.
• The daemon's per-session outgoing filter gates the entire semantic variant family on the negotiated bit. Postcard's hard-error on unknown variants is mooted exactly as it is for CursorByte (M10.10): a non-semantic session never receives a variant it cannot decode, because the filter never emits it.

Instance → frontend: the SemanticFrame family

New InstanceMessage variants, all gated on negotiated semantic_render, all keyed by BufferId, all anchored in byte offsets (consistent with CursorByte; line/col is a rendering concern the frontend derives, CRDT-position is internal to the replica and not a stable cross-frontend anchor).

A ByteRange is { start: u64, end: u64 } (half-open, like the rope's own ranges).

/// Syntax + face styling over the frontend's current viewport
/// range. `generation` ties the spans to a CRDT version so the
/// frontend can discard styling that predates an edit it has
/// already applied optimistically.
StyleSpans {
    buffer_id: BufferId,
    generation: u64,
    spans: Vec<StyleSpan>, // { range: ByteRange, style: Style }
},

/// Diagnostics, selection, search hits, current-line, and any
/// other "this region means something" overlay, as offset
/// ranges plus a kind. Peer selection is NOT here — it stays on
/// the existing PresenceUpdate path.
Decorations {
    buffer_id: BufferId,
    decorations: Vec<Decoration>, // { range: ByteRange, kind: DecorationKind }
},

/// Inlay hints, blame, lens, virtual text. Anchored at a single
/// offset with a placement; content is text+style or a resource
/// handle (images, see ResourceOffer). Occupies no document
/// bytes — the frontend interleaves it at layout time.
InlineAdornments {
    buffer_id: BufferId,
    items: Vec<InlineAdornment>,
    // { at: u64, placement: BeforeLine|EndOfLine|AtOffset, content: AdornmentContent }
},

/// Diff zones, folded-region placeholders, anything occupying
/// its own vertical band. Anchored to an offset (the line it
/// precedes/replaces); the frontend allocates the vertical space.
BlockAdornments {
    buffer_id: BufferId,
    items: Vec<BlockAdornment>,
},

/// The instance's authoritative fold set, as document facts.
/// The frontend renders the placeholder and adjusts its own
/// layout. Folding is an instance command-semantics concern
/// (Lua can fold); visual collapse is a frontend layout concern.
FoldState {
    buffer_id: BufferId,
    folds: Vec<ByteRange>,
},

/// Out-of-band content an adornment refers to (images, etc.).
/// Sent once, referenced by handle, so a blame avatar or an
/// inline image is not re-shipped per frame.
ResourceOffer {
    handle: u64,
    mime: String,
    body: ResourceBody, // Inline(Vec<u8>) | Uri(String)
},

Each family member diffs against the previous frame the same way CellDelta does today — the instance ships changed spans, not full re-sends, scoped to the viewport range the frontend last declared.

Frontend → instance: Viewport

One new FrontendEvent variant, gated identically:

/// The buffer byte range currently on screen, in buffer
/// coordinates. Replaces the instance-derived grid viewport for
/// semantic sessions. `generation` lets the instance ignore a
/// viewport that races a not-yet-applied edit. NO pixels: see
/// the contract boundary invariant.
Viewport {
    frontend_id: FrontendId,
    buffer_id: BufferId,
    visible: ByteRange,
    generation: u64,
},

That is the entire new frontend→instance surface. Cursor, selection, edits, focus, paste, detach all reuse existing variants. There is deliberately no SemanticResize and no hit-test request — both would leak pixels across the contract boundary.

Instance-side projection seam

SemanticRenderState, a sibling of instance_render::RenderState, reading the same EditorState. RenderState rasterizes to cells and exits late; SemanticRenderState exits earlier — it emits the structured ranges the cell painter would have consumed (tree-sitter spans from syntax.rs/ highlight.rs, overlays from overlay*.rs, diagnostics from diag.rs, LSP adornments from lsp.rs/hover.rs) without the grid-packing step. The dispatcher selects the projection per session, not per buffer, so a grid frontend and a semantic frontend can attach to the same buffer simultaneously — the M10.8 multi-frontend dispatcher already supports per-session fan-out; this is a constraint on SemanticRenderState, not new dispatcher work.

Open questions (deliberately unresolved here)

These are the residue of the responsibility migration; they are design work, not blockers, and none affect the v1.0 tag.

1. Soft-wrap-dependent commands. move-by-visual-line, recenter, scroll-by-page assume the instance knows visual layout. For semantic sessions it does not. Likely resolution: the instance emits intent ("recenter the cursor") and the frontend interprets against its layout; commands that are irreducibly visual become frontend capabilities. Needs a Lua API story so package authors see one model, not two.

2. Minimap / whole-file overview. Resolved. Implemented as InstanceMessage::FileStyleSummary { buffer_id, generation, lines: Vec<Style> } — one dominant style per source line, computed by SemanticRenderState::file_style_summary_msg over whole-buffer spans (policy A inherited from scoped_style_spans). Keyed on CRDT generation so an idle buffer pays nothing; recomputed only on edits. Per-line is the v1 choice because minimap rows naturally correspond to code lines. Future refinements if a real frontend prefers them: fixed-N bands (smaller wire, coarser) or whole-file RLE style runs (highest detail); both straightforward given the existing wire shape.

3. Testability strategy. Recommended: golden-test SemanticFrame sequences instance-side (cleaner than golden cell grids — they are semantic, not pixel). Accept GPUI's layout engine as externally battle-tested. This bounds the untested surface to the frontend↔instance glue, not all rendering. The existing audit/ + proptest discipline extends naturally to semantic-frame goldens.

4. Per-byte tree-sitter / LSP style blend. The producer arc chose policy A (per-language authority) deliberately: C/C++ has no grammar so there is no overlap to resolve, and it reuses the M11.4 diff pipeline untouched. A grammar-backed language refined by LSP semantic precision (mutable-vs-const, etc.) would need tree-sitter-base + LSP-overlay with a per-byte span merge — more code and more async-LSP diff churn. Unresolved; deferred until a concrete language motivates it.

5. Multiple servers, one URI. SemanticTokenStore::for_uri and semantic_style_context pick the lowest-id server when several attach to the same document. Blending styling/adornments across servers (e.g. a typo linter + a language server) is unresolved; the single-representative-server rule is the v1 simplification, not a final answer.