Diagnostics for formatting issues

[award999]

Description

Had a request for the vscode-swift extension for showing formatting diagnostics while typing: swiftlang/vscode-swift#1220

Since sourcekit-lsp is handling the formatting itself, would it be possible to include swift-format linting diagnostics in the publishDiagnostics notification? Seemingly swift-format would be able to produce that info

/path/to/main.swift:244:23: warning: [TrailingWhitespace] remove trailing whitespace

[ahoppen]

Launching a swift-format subprocess to get formatting issues every time you get diagnostics is probably not viable (especially on Windows where process launching overhead is much bigger than on Unix).

This might be an argument to re-consider running swift-format in process, or run swift-format as a long-running process. The key challenge that needs to be solved for that is to avoid a version skew between the swift-format binary and the version of swift-format that SourceKit-LSP uses. The two solutions I see here are:

1. Ship swift-format as a library into the toolchain, provide a stable ABI and make SourceKit-LSP dlopen that library.
2. Add a long-running process mode to swift-format in which SourceKit-LSP can send formatting requests to it. We then just need to keep that API stable.

I’m personally leaning towards (2), also because it means that a crashing swift-format wouldn’t take down sourcekit-lsp.

[award999]

@ahoppen ya I assumed that swift-format had a library target that the LSP was using. Definitely agree we don't want to spawn to get these diagnostics