TransparentCompiler: scripts read #load'ed files through the global FileSystem, and DocumentSource.Custom gives new versions on every call #20750

FsAutoComplete installs a process-wide FileSystemAutoOpens.FileSystem shim so FCS reads the in-memory text of open files. We would like to stop doing that and give each checker its own view of the files (ionide/FsAutoComplete#1555). With the TransparentCompiler, project files already come from snapshots we build ourselves. Scripts are the part that still goes through the global file system, and the per-checker alternative, DocumentSource.Custom, does not work well there yet.

Links are to main at 409c7a6.

1. GetProjectSnapshotFromScript ignores the checker's document source

FSharpChecker.GetProjectSnapshotFromScript falls back to DocumentSource.FileSystem when no documentSource is passed, also when the checker was created with DocumentSource.Custom (service.fs#L533). The TransparentCompiler's GetProjectOptionsFromScript always passes DocumentSource.FileSystem (TransparentCompiler.fs#L2377). So the files a script #loads are read through the global file system (FSharpProjectSnapshot.fs#L81-L89).

2. A custom document source gives every file snapshot a new version

FSharpFileSnapshot.CreateFromDocumentSource uses DateTime.Now.Ticks as the version (FSharpProjectSnapshot.fs#L95). Each call creates new versions for files that did not change, so the TransparentCompiler caches miss. This happens for:

• the #loaded files when GetProjectSnapshotFromScript is given DocumentSource.Custom (TransparentCompiler.fs#L2483);
• every FSharpProjectOptions based call on a TransparentCompiler created with DocumentSource.Custom, through FSharpProjectSnapshot.FromOptions (FSharpProjectSnapshot.fs#L715-L738).

3. None from a custom document source throws

When the function returns None, CreateFromDocumentSource fails with "Couldn't get source for file" (FSharpProjectSnapshot.fs#L104). The BackgroundCompiler reads the file from disk in that case (FSharpSource.fs#L69-L77). An editor only knows the text of its open files, so with the TransparentCompiler it has to read every other file itself.

Possible changes

• Use the checker's document source in GetProjectSnapshotFromScript when none is passed.
• Let a custom source return a version with the text, or use a stable version, for example the last write time for files read from disk.
• Treat None as "read from the file system", as the BackgroundCompiler does.
• Or, for the snapshot API: let the caller create the snapshots of the #loaded files (a string -> Async<FSharpFileSnapshot> callback), as it already does for project files.

documentSource on FSharpChecker.Create is marked experimental and "likely to be removed in the future" (service.fsi#L53). If that is still the plan, which way should an editor give the TransparentCompiler the text of open files that a script #loads?