Enable 'clippy::must_use_candidate' lint

[evdokimovs]

Synopsis

Currently, we suppress clippy::must_use_candidate lints because it given us many false positives at the start. At this moment this lint doesn't throw many false positives, so we can use it.

Solution

Remove all #[allow(clippy::must_use_candidate)] and fix all lints.

Checklist

• Created PR:
  • [x] In draft mode
  • [x] Name contains Draft: prefix
  • [x] Name contains issue reference
  • [x] Has k:: labels applied
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• [x] Documentation is updated (if required)
• [x] Tests are updated (if required)
• [x] Changes conform code style
• [x] CHANGELOG entry is added (if required)
• [x] FCM (final commit message) is posted
  • [x] and approved
• [x] Review is completed and changes are approved
• Before merge:
  • [x] Milestone is set
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  • [x] Draft: prefix is removed
  • [x] All temporary labels are removed

[evdokimovs]

FCM

Enable and satisfy 'clippy::must_use_candidate' lint (#185)

[alexlapa]

@tyranron ,

Я тут в паре мест убрал #[must_use] над async функциями, которые Вы в прошлой задаче добавили. Он, по идее, не нужон, так как async возвращает футуру, которая и так #[must_use]. И клиппи на это вот таким линтом ругается: double_must_use.

Иногда натравливаю клиппи на тесты ради душевного спокойствия.