Completion response says `"isIncomplete": true` even when the total number of completion items is two

[bstaletic]

Yesterday (so 11. 02. 2021.) I updated rust-analyzer to the latest version available in rustup. This is a part of the log that shows the completion request/response:

2021-02-11 14:00:01 - DEBUG - TX: Sending message: b'Content-Length: 211\r\n\r\n{"id":22,"jsonrpc":"2.0","method":"textDocument/completion","params":{"position":{"character":12,"line":13},"textDocument":{"uri":"file:///home/bstaletic/work/ycmd/ycmd/tests/rust/testdata/common/src/main.rs"}}}'
2021-02-11 14:00:01 - DEBUG - RX: Received message: b'{"jsonrpc":"2.0","id":22,"result":{"isIncomplete":true,"items":[{"label":"into","kind":2,"detail":"-> T","documentation":{"kind":"markdown","value":"Performs the conversion."},"deprecated":false,"filterText":"into","insertTextFormat":1,"textEdit":{"range":{"start":{"line":13,"character":12},"end":{"line":13,"character":12}},"newText":"into"},"additionalTextEdits":[]},{"label":"build_rocket","kind":2,"detail":"-> ()","documentation":{"kind":"markdown","value":"Do not try at home"},"deprecated":false,"filterText":"build_rocket","insertTextFormat":1,"textEdit":{"range":{"start":{"line":13,"character":12},"end":{"line":13,"character":12}},"newText":"build_rocket"},"additionalTextEdits":[]},{"label":"build_shuttle","kind":2,"detail":"-> ()","deprecated":false,"filterText":"build_shuttle","insertTextFormat":1,"textEdit":{"range":{"start":{"line":13,"character":12},"end":{"line":13,"character":12}},"newText":"build_shuttle"},"additionalTextEdits":[]}]}}'

The project used for testing: https://github.com/ycm-core/ycmd/tree/master/ycmd/tests/rust/testdata/common
The specific file open: https://github.com/ycm-core/ycmd/blob/master/ycmd/tests/rust/testdata/common/src/main.rs#L14
The expected completions: https://github.com/ycm-core/ycmd/blob/master/ycmd/tests/rust/testdata/common/src/test.rs#L7-L8

The new rust-analyzer does work, but if it is always returning true for isIncomplete, it circumvents ycmd's cache and impacts performance.

On the bright side, this exposed a bug in ycmd.

[flodiebold]

We set isIncomplete to true unconditionally, because with the fuzzy autoimport completions the set may change with each letter. In some situations that's overly careful of course, but since the goal is for our completions to be fast enough to be requested on every keystroke (#7542) we'll probably not put effort into implementing more complicated logic there.

One possible consideration would be to only set it to true if import autocompletions are actually enabled, though.

[bstaletic]

I'm concerned that 68ms is a performance regression for our users. Hitting the ycmd cache, right now, roundtrips in <6ms.

[lnicola]

Those 68ms are on a cold RA cache. A second invocation should be fast.

[bstaletic]

Alright, thanks for correcting me. That doesn't sound too bad.

[flodiebold]

I think we can close this?

[bstaletic]

Sure! I should have closed it yesterday.