Swift Snippets in DocC catalogues aren't rendered in the doc preview

[FranzBusch]

Describe the bug

Swift Snippets in DocC catalogues aren't rendered in the doc preview

To Reproduce

• Create a package with a snippet
• Create a documentation catalogue for a target
• Use the snippet directive to include the snippet

Expected behavior
Snippet gets rendered in the doc preview

[heckj]

To provide a bit of background into the guts here.

The general pattern for supporting snippets in DocC is entirely wrapped into swift-docc-plugin, which does some explicit extra coordination:

• It builds symbol graphs for each of the snippets in the /Snippets directory
• it adds those symbol graphs to the set of symbols provided to DocC to run "docc convert", along side symbols that are provided by the compiler (typically through Swift Package Manager's API)

The relevant tooling for generating a snippet symbol graph is in "snippet-extract" (https://github.com/swiftlang/swift-docc-plugin/tree/main/Sources/snippet-extract). To utilize it, we may want/need to extract it or present it as a library. It was originally built only to be used as a local executable (for example swift run snippet-extract ...) or as the SwiftPM build plugin mechanism.

I'm not sure what the path is that SwiftLSP uses to expose DocC functionality, but I'm betting this isn't on it directly.

[matthewbastien]

I'm not sure what the path is that SwiftLSP uses to expose DocC functionality, but I'm betting this isn't on it directly.

SourceKit-LSP is just passing in the markdown straight to the DocC ConvertService and fulfilling any symbol resolution requests that it comes back with. It does not currently build symbol graphs for the snippets to provide to DocC.