The SwiftPM build system integration was the only one in its own modules. Merge it into the `BuildSystemIntegration` modules next to eg. the compilation database build system.
This allows us to flip the default in the future more easily. It also allows users to disable background indexing when it’s enabled by default.
rdar://130280855
The idea here is to unify the different ways in which we can currently set options on SourceKit-LSP in a scalable way: Environment variables, command line arguments to `sourcekit-lsp` and initialization options.
The idea is that a user can define a `~/.sourcekit-lsp/.sourcekit-lsp` file (we store logs in `~/.sourcekit-lsp/logs` on non-Darwin platforms), which will be used as the default configuration for all SourceKit-LSP instances. They can also place a `.sourcekit-lsp` file in the root of a workspace to configure SourceKit-LSP for that project specifically, eg. setting arguments that need to be passed to `swift build` for that project and which thus also need to be set on SourceKit-LSP.
For compatibility reasons, I’m mapping the existing command line options into the new options structure for now. I hope to delete the command line arguments in the future and solely rely on `.sourcekit-lsp` configuration files.
Environment variable will be migrated to `.sourcekit-lsp` in a follow-up commit.
I’d like to qualify `--experimental-prepare-for-indexing` independently of background indexing using `swift build`. Because of this, I think there is value in using SourceKit-LSP using background indexing but without `--experimental-prepare-for-indexing`. It could also be useful to determine if bugs we may find are due to `--experimental-prepare-for-indexing` or also occur when running plain `swift build` commands.
Background indexing probably won’t be the last experimental feature in sourcekit-lsp that we want to gate behind a feature flag. Instead of adding new parameters ad-hoc, introduce a general notion of experimental features.
When looking for a workspace that can handle a file, we were creating full-fledged workspaces along the way, which we would then discard if they couldn’t handle the file being opened. This had multiple problems:
1. When background indexing is enabled, it caused semantic indexing of the workspace, which wrote files to a `.index-build` directory and was a waste of work
2. When background indexing is enabled, it caused package resolution, which also created a `.index-build` folder to be created
3. It caused a syntactic test index of the workspace, which was a waste of work.
To fix this, do multiple things:
1. When creating a workspace, add a check right after build system creation. This allows us to early exit if the build system can’t handle the file and prevents us from generating the `Workspace`, fixing (1) and (3)
2. Don’t call `reloadPackage` when creating a `SwiftPMWorkspace`. Instead, explicitly call `generateBuildGraph` once we committed to creating the workspace.
