0.0.16: clangd checks a file once typing pauses, an unchanged build input is no change, a module that failed in an import is named, build.mcpp beside the mcpp extension

[Sunrisepeak]

Part of a cross-repository plan (.agents/docs/2026-10-14-cross-repo-build-mcpp-and-typing-cpu-plan.md) with mcpp-community/mcpp-vscode#18 and mcpp-community/mcpp-plugins#47, both merged.

What changes

Typing (A4). clangd built the file at every key: its debounce follows its build time, which is about a person's pace between keys, and VS Code's semantic tokens / highlights / code actions / hints after each edit force a build of the latest version anyway. The clangd engine now sends each version of a file being typed in with wantDiagnostics: false, holds the requests that build the file (the latest of each kind; replaced ones answered ContentModified), and once the keys stop for clamp(1.2 x last build, 300 ms, 1 s) sends the same version with empty contentChanges and wantDiagnostics: true. Build programs, module units and a change that leaves the text unchanged are not held. Setting mcppls.engine.diagnosticsWhileTyping (auto default, always = 0.0.15).

• Verified first against the bundled clangd: wantDiagnostics: false publishes nothing; the same-version empty change with true publishes the latest text's diagnostics, also when a request already built the AST.
• Bench (Qt headers + import std + import nlohmann.json, typing std::vec, delete, arr, delete, fin at 200 ms with VS Code's request mix, 3 rounds): clangd CPU 5.23-5.36 s -> 2.90-3.06 s (-43%), BuildAST 16 -> 3, semanticTokens builds 14 -> 1.
• Fixture typing-pause (all four platforms): 0 publications during 21 keys (0.0.15: 15), the latest version diagnosed within 5 s, the last semanticTokens answered with tokens.

Idle (A1, A2). mcpp rewrites the lock of a build program's synthesised package on every run and lists it in data.watch (mcpp-community/mcpp#807): with a build.mcpp, the project was described every 2 s of an idle session. A build file or producer input holding the bytes the last description read is now no change (changed or created; deleted always counts). mcpp-emit-watch gains W0 (mockmcpp rewriteEachRun), passing on both watch paths; 0.0.15 fails both. A prefetch that came back empty is not asked again until a file or the plan changes (test_answer).

Failed modules (A3). An error inside an imported module or file starts In module 'M' imported from <unit>:<line>: / In file included from <unit>:<line>:; the first <unit> is the failed one. The importer now gets module-failed with the reason; a failure whose unit no plan names is still said on the importer; a hint BMI that was never built says so. Unit tests with real log lines (anonymised, incl. Windows nesting) and fixture module-fails-in-import (0.0.15: ast_file_not_found).

build.mcpp beside mcpp-vscode (A6). The client also selects **/*.mcpp, so an extension that gives build.mcpp its own language id (mcpp-vscode <= 0.6.0) no longer keeps it from mcppls; the server treats .mcpp as C++. The extension API gains otherCppFeatures() ({ active: [{ extensionId, displayName, settable }], canRestore }), asserted in suite-conflicts.

Checked locally

mcpp test (38/38), devtools check all, npm run compile, unit tests (129), and conformance: the new fixtures plus typing-autosave, typing-import, typing-import-spin, module-edit-autosave, module-faults, build-program-*, mcpp-emit-watch (both watch paths), inferred.