v0.79.0

Likely to cause new Flow errors:

• A fix in requiring annotations on exports: Flow uses type variables for
  unannotated program parts. To facilitate inference, these type variables may
  not appear in input positions of exports. Before, Flow would not complain if a
  type variable appeared in both an output and an input position of an export
  (in that order), as the first occurrence marked the use of the type variable
  as legitimate. Now, Flow analyzes exports in a polarity-sensitive way and will
  require an annotation in the above scenario.

New Features:

• Saved state:
  • Added an initialization path, where instead of parsing every file dependency,
    it loads the expected results for each available file from the saved state.
  • A hash is stored for each file in the saved state to determine whether it has been
    modified and therefore needs to be reparsed after the state is loaded. This way,
    saved state servers can skip parsing files that are unchanged.
  • If loading saved state fails, Flow falls back to a full initialization, unless
    --saved-state-no-fallback is passed in which case Flow fails.
  • Saved state tests are included in the automated testing framework.
• Profiling was refactored to support hierarchical profiling. That is:
  • Support running timers inside of other timers.
  • Support merging a finished profiling object (like from a recheck) into the
    currently running profiling object (like from handling a command).
• Cancelable workloads (jobs that handle commands). If Flow determines that a file
  has changed while handling a command, it stops, performs a recheck, and then
  re-runs the workload from scratch.
• The Flow diff checker became more fine-grained, by including comparisons at the
  level of loops and variable declarations.
• Added a name field in the [options] portion of .flowconfig.

Notable bug fixes:

• Send the server logging context to the LSP command and use that when logging
  success and failure events instead of the LSP command's context (which is logged
  as the client context).
• Fixed stack overflow when running flow --json with a large error output, by
  moving to a tail-recursive implementation.
• Fixed string literal printing in flow suggest.

Misc:

• Library definition improvements for React$Context.Provider and
  CanvasRenderingContext2D.imageSmoothingQuality.
• Typed AST: The constraint generation phase returns a version of the AST that
  includes a type for each program node.
• Added doc details for $Shape.
• The changes in saved state were followed by code refactorings in various checking
  modules (types_js.ml, merge_service.ml, rechecker.ml, etc.).