rustc: buffer the output writer for -Z ast-json[-noexpand]. #12999
  Add this suggestion to a batch that can be applied as a single commit.
  This suggestion is invalid because no changes were made to the code.
  Suggestions cannot be applied while the pull request is closed.
  Suggestions cannot be applied while viewing a subset of changes.
  Only one suggestion per line can be applied in a batch.
  Add this suggestion to a batch that can be applied as a single commit.
  Applying suggestions on deleted lines is not supported.
  You must change the existing code in this line in order to create a valid suggestion.
  Outdated suggestions cannot be applied.
  This suggestion has been applied or marked resolved.
  Suggestions cannot be applied from pending reviews.
  Suggestions cannot be applied on multi-line comments.
  Suggestions cannot be applied while the pull request is queued to merge.
  Suggestion cannot be applied right now. Please check back later.
  
    
  
    
rustc: buffer the output writer for -Z ast-json[-noexpand].
This takes the time for
rustc libstd/lib.rs -Z ast-json-noexpand > file.jsonfrom 9.0s to 3.5s (~0.5s spent parsing etc.) and-Z ast-jsonfrom 11s to 5s (~1.5s spent parsing and expanding).