Adding the `expect` attribute (RFC 2383)#86024

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xFrednet wants to merge 6 commits intorust-lang:masterfrom

xFrednet:54503-expect-lint-attribute

Commits on Jul 31, 2021

Added `expect` attribute tests (RFC 2383)
xFrednet
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Commits on Aug 1, 2021

`expect` attribute: Addressed review feedback
xFrednet
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