prometeu-studio/discussion/lessons/DSC-0044-pbs-lsp-code-actions/LSN-0073-pbs-quick-fixes-are-frontend-repairs-transported-by-the-lsp.md

---
id: LSN-0073
ticket: pbs-lsp-code-actions
title: PBS quick fixes are frontend repairs transported by the LSP
created: 2026-09-22
tags: [studio, lsp, vscode, compiler-pbs, editor, code-actions, quick-fix]
---

# PBS quick fixes are frontend repairs transported by the LSP

## Original Problem

PBS diagnostics already reach the editor, but the server did not advertise `codeActionProvider`. A user could see a duplicate attribute or a bad `InitAllowed` and still had to repair it by hand.

The tempting shortcut was a code-to-edit table in the LSP. That shortcut does not work. Compiler `Diagnostic` values carry a stable code, phase, template, placeholders, span, and related locations, but the reserved-attribute reports use the short constructor: empty placeholders, and the English sentence is the only fine discriminator. `E_SEM_MALFORMED_RESERVED_ATTRIBUTE` covers Doc without `markdown`, a wrong payload, empty markdown, an unknown argument, `InitAllowed` with arguments, an out-of-range `AssetLowering`, and a bad `Host` shape. A switch on the code cannot pick a safe edit without reading the message. Diagnostic wording is rendering, not identity. Code actions are an optional editorial capability: the frontend owns the meaning, and the LSP transports it.

## Consolidated Decision

PBS quick fix is one `WorkspaceEdit` tied to one diagnostic of the requested physical document. The frontend produces the repair from the syntax tree. The LSP does not invent it.

Durable locks from `DEC-0057` (AGD-0047: scope A, producer B, residence B, first wave C):

1. This wave is quick fix only. No organize-imports, source action, refactor, function or struct stub, code lens, or server command.
2. The server does not write files. The client applies the edit.
3. Each repair names the stable diagnostic code, the primary span of that diagnostic, and exactly one local text edit. The LSP must not keep a code-to-edit table and must not read the diagnostic message to build the edit.
4. The repair is computed on `textDocument/codeAction` from the current document analysis. It is not a required field of compiler `Diagnostic`, and it is not published inside `publishDiagnostics`, including `data`.
5. `publishDiagnostics` carries the stable compiler code on `Diagnostic.code` so the client can echo it. Source, severity, message, and related information stay as they were. Those presentation questions remain with `AGD-0054`.
6. A repair is returned only when the request context contains a diagnostic with the same code and the same primary range. An empty diagnostic list returns an empty action list. An `only` filter that does not include quick fix returns an empty action list.
7. Each item is a QuickFix `CodeAction`, not a `Command`. The edit touches one file, the requested physical document, and contains one `TextEdit`. There is no lazy resolve.
8. A path that is not a compiler-known regular file, including `/virtual/stdlib`, gets no edit.
9. If the frontend does not expose the capability, the server does not advertise `codeActionProvider`. A request that arrives anyway returns an empty list, not a protocol error. Absence of a fix is not a refused cursor.
10. PBS exposes the capability. Another frontend does not gain it merely by having a language service.
11. The first wave is two shapes, not three diagnostic codes.
    - For every extra reserved attribute the declaration validator already reports as `E_SEM_DUPLICATE_RESERVED_ATTRIBUTE`, delete exactly that occurrence's span. Do not delete the first occurrence, the declaration, or neighboring whitespace.
    - For an `InitAllowed` attribute with a non-empty argument list on a host signature, the shape the validator reports as `E_SEM_MALFORMED_RESERVED_ATTRIBUTE` when the file is an interface module, replace that attribute span with `[InitAllowed]`. Do not emit that repair for any other attribute that shares the code. `[InitAllowed()]` is not diagnosed today and gets no repair.
12. Nothing else gets a quick fix in this wave. In particular: inserting `[Doc(markdown = """""")]` or any other Doc rewrite, inserting an empty or incomplete `[Host]` or `[BuiltinType]`, `W_SEM_IGNORED_VALUE`, unresolved imports, an invalid reserved-attribute target, parse errors, unresolved overloads, and method stubs.

`AGD-0048` still owns formatting. `AGD-0049` still owns import assistance and organize-imports. `AGD-0054` still owns range, related locations, and diagnostic presentation. The PBS diagnostics spec does not gain a mandatory remediation field.

Spec 23 §8.3 now states the shared contract: code actions are quick fixes only; the frontend produces each repair from the document syntax; tooling transports it and does not parse the message or switch on codes; the published diagnostic carries the stable code; a repair is returned only for a matching context diagnostic; the edit is one physical file; the server does not write files, return a command, or require resolve; a missing capability is an empty list, not an error; virtual documents get no edit.

## Final Implementation

| Layer | What landed |
|---|---|
| PBS repairs | `PbsQuickFixCollector` walks the same attribute surfaces as `PbsDeclarationSemanticsValidator`, including the interface-module gate. Duplicate reserved attributes become an empty replacement of the extra span. The first `InitAllowed` with arguments on an interface host signature becomes `[InitAllowed]`. |
| Source kind | `PbsSemanticReadSurface.sourceKindByFile` records the kind used to parse each file. A missing kind means project rules, so a project `declare host` does not pretend the validator emitted the interface-only `InitAllowed` diagnostic. |
| Generic contract | `FrontendLanguageService.codeActionsSupported` defaults false. `codeActions` defaults empty. `FrontendCodeAction` is title, diagnostic code, path, offsets, and `newText`. Empty `newText` is a deletion and is allowed. |
| PBS mapping | `PBSFrontendLanguageService` exposes the capability and returns repairs only for the requested regular file. `/virtual/stdlib` fails `Files.isRegularFile` and produces nothing. |
| LSP | `BaselineDocumentIssue.code` is copied onto `Diagnostic.code`. `data` stays unset. The bridge drops repairs whose code and range do not match a context diagnostic. `PrometeuTextDocumentService.codeAction` honors the quick-fix `only` filter and never fails the request for “no fix”. |
| Protocol | `codeActionProvider` is `CodeActionOptions` with kind `quickfix` and `resolveProvider` false, and only when the frontend reports support. Mapped actions are `CodeAction` values with a `WorkspaceEdit`. They are not `Command`s. |

Titles are `Remove duplicate reserved attribute` and `Remove InitAllowed arguments`.

## Examples

Duplicate `Doc` on a project file. The first copy stays. Each later copy is deleted at its own span:

```pbs
[Doc(markdown = """
First.
""")]
[Doc(markdown = """
Second.
""")]
fn duplicated() -> void { return; }
```

The quick fix removes the second attribute and leaves `First.`. A context diagnostic with the same range but code `E_SEM_MALFORMED_RESERVED_ATTRIBUTE` produces no action. An empty diagnostic list produces no action.

Malformed or empty Doc is visible and not repairable here:

```pbs
[Doc(text = """
Wrong argument.
""")]
fn malformed() -> void { return; }
```

`InitAllowed` with arguments is repaired only on an interface-module host signature:

```pbs
declare host Gfx {
  [Host(module = "gfx2d", name = "clear", version = 1)]
  [InitAllowed(flag = 1)]
  fn clear() -> void;

  [Host(module = "gfx2d", name = "fill", version = 1)]
  [InitAllowed()]
  fn fill() -> void;
}
```

`clear` becomes `[InitAllowed]`. `fill` is already valid to the validator, so `[InitAllowed()]` is left alone. The same `InitAllowed(flag = 1)` inside a project file does not get that repair, because the validator does not emit the malformed-attribute diagnostic there.

The same collector deletes the extra copy of `BuiltinType`, `IntrinsicCall`, `BuiltinConst`, `Host`, and `AssetLowering` when the validator reports `E_SEM_DUPLICATE_RESERVED_ATTRIBUTE` for that surface. A malformed `Host` that merely shares the malformed-attribute code gets no edit.

## Pitfalls

Do not choose the edit in `prometeu-lsp` by switching on `E_SEM_MALFORMED_RESERVED_ATTRIBUTE` or by parsing the English message. The code is too coarse, and the message is not the identity.

Do not store the repair on compiler `Diagnostic` or in `Diagnostic.data`. Compilation and quick fix stay different surfaces. The published code exists so the client can echo it back.

Do not match on `Diagnostic.source` as if it were the long-term identity field. Today the bridge still copies the code into `source` for the existing presentation. The code-action match uses `Diagnostic.code`. `AGD-0054` may change `source` later.

Do not delete the first duplicate, and do not trim the whitespace around the deleted span. That cleanup is formatting and belongs to `AGD-0048`.

Do not insert `[Doc(markdown = """""")]`, an empty `[Host]`, or an empty `[BuiltinType]`. Those edits trade one diagnostic for another, or invent a declaration.

Do not treat `W_SEM_IGNORED_VALUE` as a source edit. Discard exists in lowering, not as PBS syntax.

Do not offer the `InitAllowed` rewrite for `[InitAllowed()]` or for a project file. The validator does not diagnose those shapes as “arguments not allowed”, so there is no diagnostic to attach.

Do not return a protocol error when there is no fix. Rename refuses a bad cursor with an error. Quick fix answers “nothing to do” with an empty list.

Do not advertise `codeActionProvider` for every frontend that has a language service. The default capability is unsupported.

Do not treat this lesson as closing formatting, import assistance, diagnostic presentation, snapshots, folding, document links, or hierarchy.

## References

- Agenda: `AGD-0047` (scope A, producer B, residence B, first wave C)
- Decision: `DEC-0057`
- Plan: `PLN-0136`
- Spec 23 §8.3 — optional code actions; frontend-owned repair; match by code and range; empty list when unsupported
- Spec: `docs/specs/compiler-languages/pbs/12. Diagnostics Specification.md` — wording is not identity; remediation stays outside the v1 diagnostic minimum
- `LSN-0056` — code actions are an optional editorial capability
- `LSN-0058` — generic `FrontendLanguageService` for the LSP
- `LSN-0067` — no new editorial SPI without a real consumer; PBS implements the existing language service
- Still open: `AGD-0048` formatting, `AGD-0049` import assistance, `AGD-0054` diagnostics UX, `AGD-0051` snapshots, `AGD-0056` hierarchy