Implement baml language server code completions

docs/lsp-completions-design.md

@@ -0,0 +1,104 @@
+## BAML Language Server: Code Completions Design
+
+### Goals
+- Provide fast, context-aware completions for `.baml` files:
+  - Top-level declarations: `function`, `class`, `enum`, `client`, `generator`, `retry_policy`, `template_string`, `type`
+  - Attributes: `@alias`, `@description`, `@check`, `@assert`, `@stream.done`, `@stream.not_null`, `@stream.with_state`, block attributes `@@dynamic`, `@@alias`, `@@assert`
+  - Prompt helpers inside template bodies: `_.role("system"|"user"|"assistant")`, `ctx.output_format`, `ctx.client`, `ctx.client.name`, `ctx.client.provider`
+  - IR-derived symbols (runtime-aware): function names, class/enum/type alias names
+- Respect trigger characters: '@', '.', '"'
+- Avoid blocking the main loop; reuse existing runtime caching and session/project mechanisms
+
+### Current State
+- `engine/language_server/src/server/api/requests/completion.rs` returns `Ok(None)`.
+- Server capabilities already advertise Completion with triggers `@`, `"`, `.`.
+- Hover, go-to-definition, and diagnostics already use `Project::runtime()` and `get_word_at_position()` utilities.
+
+### Approach
+1. Parse context
+   - Get the document and cursor position
+   - Extract current line and token using `get_word_at_position`, `get_symbol_before_position`
+   - Detect simple contexts:
+     - Attribute context: prefix `@` or `@@` at current token
+     - Dot context: `ctx.` or `ctx.client.` or `_.role(` prefix
+     - String-start context: after `"` in `client` shorthand or enum values
+2. Suggest sets
+   - Attributes
+     - Field attributes (single `@`): `alias`, `description`, `check`, `assert`, `stream.done`, `stream.not_null`, `stream.with_state`
+     - Block attributes (double `@@`): `dynamic`, `alias`, `assert`
+   - Prompt helpers
+     - For `_.role(` propose `system`, `user`, `assistant` snippet variants
+     - For `ctx.` propose `output_format`, `client`
+     - For `ctx.client.` propose `name`, `provider`
+   - Keywords/top-level declarations
+     - `function`, `class`, `enum`, `client`, `generator`, `retry_policy`, `template_string`, `type`
+   - Runtime IR symbols (uses cached runtime): function names, class names, enum names, type aliases
+3. Build LSP items
+   - Use `CompletionResponse::List(CompletionList { is_incomplete: false, items })`
+   - Provide `kind`, `detail`, and `insertText` where appropriate; snippets for `_.role("${1:system}")` and `@alias("${1:name}")`
+   - Optionally set `filterText` to support minimal prefix filtering
+
+### File Changes
+- `engine/language_server/src/server/api/requests/completion.rs`
+  - Implement `SyncRequestHandler::run` using `Session::get_or_create_project`, `DocumentKey::from_url` and current file contents
+  - Detect context and construct a list of `CompletionItem`
+  - Query runtime via `project.lock().runtime()` for IR names
+- `engine/language_server/src/baml_project/position_utils.rs`
+  - Already contains `get_word_at_position` and helpers; reuse as-is
+
+### Examples
+- Attribute completions
+  - Typing `@a` -> `@alias`, `@assert`, `@alias("...")` (snippet)
+- Prompt helpers
+  - Typing `{{ _.ro` -> `_.role("system")`, `_.role("user")`, `_.role("assistant")`
+  - Typing `{{ ctx.` -> `output_format`, `client`
+  - Typing `{{ ctx.client.` -> `name`, `provider`
+- Top-level declarations
+  - At file start: `function`, `class`, `enum`, `client`, `generator`, `retry_policy`
+- IR symbols
+  - In references: suggest available `FunctionName`, `ClassName`, `EnumName`, `TypeAliasName`
+
+Code sample (shape only):
+```rust
+// completion.rs (excerpt)
+let symbol_before = get_symbol_before_position(&doc.contents, &pos);
+let word = get_word_at_position(&doc.contents, &pos);
+let cleaned = trim_line(&word);
+let mut items = Vec::new();
+match () {
+  _ if cleaned.starts_with("@@") || symbol_before == "@" && cleaned.starts_with("@") => {
+    items.extend(block_or_field_attribute_items(cleaned));
+  }
+  _ if cleaned.ends_with("_.role(") || cleaned.contains("_.role(") => {
+    items.extend(role_items());
+  }
+  _ if cleaned.ends_with("ctx.") || cleaned.contains("ctx.") => {
+    items.extend(ctx_items(cleaned));
+  }
+  _ if is_top_level_context(&doc.contents, &pos) => {
+    items.extend(top_level_keywords());
+  }
+  _ => {
+    // IR-driven symbols
+    if let Ok(rt) = guard.runtime() {
+      items.extend(ir_symbol_items(rt));
+    }
+  }
+}
+Ok(Some(CompletionResponse::List(CompletionList { is_incomplete: false, items })))
+```
+
+### Testing Strategy
+- Unit tests in `engine/language_server/src/tests.rs` using in-memory LSP harness:
+  - Open a `.baml` doc and request completion at various contexts
+  - Assert returned items include expected labels and kinds
+- Run `cargo test --lib` at `engine/`
+
+### Performance
+- Reuse runtime caching via `BamlProject::runtime` (already hashed across files and flags)
+- Avoid expensive work when no project can be resolved
+
+### Future Enhancements
+- Snippet completions for function templates and scaffolding
+- Type-aware suggestions inside blocks (e.g., class fields and types)
+- Completion resolve support for detailed docs

engine/language_server/src/logging.rs

@@ -64,8 +64,12 @@ pub(crate) fn init_logging(_log_level: LogLevel, log_file: Option<&std::path::Pa
             }),
     );
 
-    tracing::subscriber::set_global_default(subscriber)
-        .expect("should be able to set global default subscriber");
+    if let Err(e) = tracing::subscriber::set_global_default(subscriber) {
+        #[allow(clippy::print_stderr)]
+        {
+            eprintln!("logging already initialized; continuing without resetting subscriber: {e}");
+        }
+    }
 
     match baml_log::set_running_in_lsp(true) {
         Ok(_) => (),