Refactor preview doesn't show preview with clangd

[hokein]

Coming from clangd/vscode-clangd#376

• VS Code Version: 1.71.2
• OS Version: MacOS

Steps to Reproduce:

1. opened a dummy t.cc (auto S = 1;) in VSCode with vscode-clangd extension, waiting for clangd to warm up.
2. Move the cursor to auto, click the 💡 and get a CodeAction menu
3. Move the cursor to the Expand auto type Code ACtion, and press Command + Enter

Expected result: a preview pane will be shown
Actual result: we apply the code action without showing the preview

A special bit about clangd which may be related: the CodeAction returned from clangd doesn't have the edit field propagated, instead we use the applyTweak command to apply a workspace edit.

The detailed logs in case it is useful:

V[] <<< {"id":9,"jsonrpc":"2.0","method":"textDocument/codeAction","params":{"context":{"diagnostics":[],"triggerKind":2},"range":{"end":{"character":2,"line":0},"start":{"character":2,"line":0}},"textDocument":{"uri":"file:///tmp/t3.cpp"}}}
I[] <-- textDocument/codeAction(9)
I[] --> reply:textDocument/codeAction(9) 0 ms
V[] >>> {"id":9,"jsonrpc":"2.0","result":[{"command":{"arguments":[{"file":"file:///tmp/t3.cpp","selection":{"end":{"character":2,"line":0},"start":{"character":2,"line":0}},"tweakID":"ExpandAutoType"}],"command":"clangd.applyTweak","title":"Expand auto type"},"kind":"refactor","title":"Expand auto type"}]}


V[] <<< {"id":10,"jsonrpc":"2.0","method":"workspace/executeCommand","params":{"arguments":[{"file":"file:///tmp/t3.cpp","selection":{"end":{"character":2,"line":0},"start":{"character":2,"line":0}},"tweakID":"ExpandAutoType"}],"command":"clangd.applyTweak"}}
I[] <-- workspace/executeCommand(10)
I[] --> workspace/applyEdit(1)
V[] >>> {"id":1,"jsonrpc":"2.0","method":"workspace/applyEdit","params":{"edit":{"changes":{"file:///tmp/t3.cpp":[{"newText":"int","range":{"end":{"character":4,"line":0},"start":{"character":0,"line":0}}},{"newText":"\n","range":{"end":{"character":0,"line":2},"start":{"character":11,"line":0}}}]}}}}

[mjbvz]

The refactor preview only supports previewing code actions that have an edit. This example shows that the code action is instead using a command, which we are not able to preview (since running the command may have other side effects)

Can vscode-clangd investigate switching use using edits instead?

[hokein]

The refactor preview only supports previewing code actions that have an edit.

This makes sense, but this seems more like a UX issue in VSCode -- it'd be better for VSCode to disable the preview feature if the code action that don't have edit.

Can vscode-clangd investigate switching use using edits instead?

In clangd, we want to avoid paying the cost of generating edits for each code action (as code action is triggered when evert the cursor moves in VSCode).
I think we could support the newly-added codeAction/resolve in clangd, but it is not our priority at the moment.