Auto merge of #4560 - rust-lang:must-use-pure, r=phansch

new lints around`#[must_use]`

changelog: Add `must_use_candidate` lint,  add `must-use-unit` lint, add `double_must_use` lint

The first one checks if an public function or method has no mutable argument and mutates no non-local data and lints if it has no `#[must_use]` attribute. It will skip inner functions, because those are usually highly local and the attribute doesn't have as much benefit there.

The second lints `#[must_use]` attributes on functions and methods that return unit. Those attributes are likely a remnant from a refactoring that removed the return value.

The last one lints for `#[must_use]` attributrs without text on functions that return a type which is already marked `#[must_use]`. This has no auto-suggestion, because while it would be easy to do, there may be value in writing a detailed text for the attribute instead.

This fixes #4526

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