x/tools/gopls: ParseExported mode and go/analysis

[stamblerre]

go/analysis requires code from source in order to get facts for all dependencies.
Our current strategy of parsing dependencies in exported mode deletes function bodies, thereby preventing go/analysis from getting correct facts for dependencies.

Some possible solutions:

• Parse dependencies in full mode
  • Will fix everything easily
  • Takes up a lot of memory
• Parse dependencies in full mode, run analyses, trim ASTs, and re-cache as ParseExported
  • This will be hard to get right
  • This will require deleting items from the cache explicitly
  • It may throw off position information
  • This will still keep a lot of unnecessary information around in the *types.Info map
• Have a separate path for type-checking language features like completion, definition, etc. and analysis.
  • Language feature path: Type-checks only a specific function in a package as well as the package's dependencies in exported form
  • Analysis path: Type-checks everything from source, caching only analysis facts

/cc @ianthehat @dominikh @muirrn

[gopherbot]

Change https://golang.org/cl/202623 mentions this issue: internal/lsp: use source.ParseFull for all AST parsing

[muirdm]

Maybe a wacky idea, but has anyone ever thought about "compressing" the type info? In my experience, a lot of the memory usage is from map overhead for all the AST overlay maps in *types.Info. Maybe we could compress the *types.Info data down into slice form when not being used to save space. The assumption is that thawing the data would still be much faster than re-type checking the package.