[CP] Cherry-pick for restoring using available declarations for completion

[scheglov]

commit(s) to merge: 6207d1d

merge instructions: clean (auto)merge

What is the issue: Computing completion suggestions, when the target library imports libraries that have a lot (thousands) of completion suggestions, is expensive.

What is the fix: The fix is to restore using available declarations feature. In short, we pre-compute and send sets of all declarations in any library that we know about, and at completion request we don't send suggestions for these libraries, we only send the corresponding set ID. This way we compute, and transfer 10x less data.

Why cherrypick: Many users impacted, anyone using Flutter.

Risk: low

Link to original issue(s):
flutter/flutter-intellij#5761

/cc @kevmoo @mit-mit @whesse @athomas @vsmenon @devoncarew

Currently 174 ms to just ask for completion:

1632505265214:Req:{"id"::"32","method"::"completion.getSuggestions","params"::{"file"::"/Users/scheglov/Source/flutter/examples/hello_world/lib/test.dart","offset"::132},
1632505265215:Res:{"id"::"32","result"::{"id"::"14"}}
1632505265299:Noti:{"event"::"completion.existingImports","params"::{"file"::"/Users/scheglov/Source/flutter/examples/hello_world/lib/test.dart","imports"::{"elements"::{
1632505265388:Noti:{"event"::"completion.results","params"::{"id"::"14","replacementOffset"::132,"replacementLength"::0,"results"::[{"kind"::"INVOCATION","relevance"::504

Currently 426 ms when typing M:

1632505689416:Req:{"id"::"59","method"::"completion.getSuggestions","params"::{"file"::"/Users/scheglov/Source/flutter/examples/hello_world/lib/test.dart","offset"::133},
1632505689417:Noti:{"event"::"server.status","params"::{"analysis"::{"isAnalyzing"::true}}}
1632505689507:Res:{"id"::"59","result"::{"id"::"25"}}
1632505689508:Noti:{"event"::"analysis.errors","params"::{"file"::"/Users/scheglov/Source/flutter/examples/hello_world/lib/test.dart","errors"::[{"severity"::"INFO","type
1632505689728:Noti:{"event"::"completion.existingImports","params"::{"file"::"/Users/scheglov/Source/flutter/examples/hello_world/lib/test.dart","imports"::{"elements"::{
1632505689842:Noti:{"event"::"completion.results","params"::{"id"::"25","replacementOffset"::132,"replacementLength"::1,"results"::[{"kind"::"INVOCATION","relevance"::504

With CP 7 ms to just ask for completion:

1632505843052:Req:{"id"::"22","method"::"completion.getSuggestions","params"::{"file"::"/Users/scheglov/Source/flutter/examples/hello_world/lib/test.dart","offset"::132},
1632505843053:Res:{"id"::"22","result"::{"id"::"13"}}
1632505843057:Noti:{"event"::"completion.existingImports","params"::{"file"::"/Users/scheglov/Source/flutter/examples/hello_world/lib/test.dart","imports"::{"elements"::{
1632505843059:Noti:{"event"::"completion.results","params"::{"id"::"13","replacementOffset"::132,"replacementLength"::0,"results"::[{"kind"::"KEYWORD","relevance"::501,"c

With CP 115 ms when typing M:

1632505955583:Req:{"id"::"56","method"::"completion.getSuggestions","params"::{"file"::"/Users/scheglov/Source/flutter/examples/hello_world/lib/test.dart","offset"::133},
1632505955584:Noti:{"event"::"server.status","params"::{"analysis"::{"isAnalyzing"::true}}}
1632505955691:Res:{"id"::"56","result"::{"id"::"22"}}
1632505955692:Noti:{"event"::"analysis.errors","params"::{"file"::"/Users/scheglov/Source/flutter/examples/hello_world/lib/test.dart","errors"::[{"severity"::"INFO","type
1632505955696:Noti:{"event"::"completion.existingImports","params"::{"file"::"/Users/scheglov/Source/flutter/examples/hello_world/lib/test.dart","imports"::{"elements"::{
1632505955698:Noti:{"event"::"completion.results","params"::{"id"::"22","replacementOffset"::132,"replacementLength"::1,"results"::[{"kind"::"KEYWORD","relevance"::501,"c

Note, that actual times are even higher because we measured here only times to compute in DAS, but don't include times to transfer data via stdout, and time to decode this data in IntelliJ. Without available declarations we these times are proportionally higher, probably pushing total completion time beyond 1000 ms, much longer than the target 100 ms.

[devoncarew]

For impact, see also:

• https://www.reddit.com/r/FlutterDev/comments/prvexo/slow_code_suggestion_on_maindart_android_studio/?utm_source=ifttt
• https://twitter.com/MatthewRideout/status/1439972286883315712

[vsmenon]

approved

[athomas]

Merged to stable in 020b3ef (2.14.3).