Avoid subtracting `Duration` from `Instant` about rust-clippy HOT 8 OPEN

@rustbot claim

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This seemingly already exists as a pedantic lint: https://rust-lang.github.io/rust-clippy/master/#/unchecked_duration_subtraction

@rustbot release-assignment

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TIL, yeah so I guess this can be closed then.

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Is pedantic with allow by default the right approach for it though? You basically should never subtract from Instant is what I think. In fact I was contemplating opening an issue directly in the Rust repo to possibly even deprecate it.

To me it should be in correctness and possibly even deny by default.

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Might as well make this change and see what people think. On the whole I agree.

@rustbot claim

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FWIW, this lint does differ in what you suggested by suggesting the use of checked_sub instead of rewriting to use addition. We could either remove this suggestion and essentially suggest that the user rewrite the operation entirely, or keep the suggestion as-is and just change the category to correctness?

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Yeah there's definitely different solutions, but in general it should probably be avoided, because there's likely not a good way to recover from the checked operation failing. Not subtracting at all in case it fails is probably the wrong behavior. Rewriting in terms of an addition is generally the safer solution, just not one that clippy can offer automatically probably.

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See also #10061 and the linked zulip discussions. Becoming on by default again is not impossible but the help message would have to be improved to suggest avoiding the operation rather than replacing it with a slightly worse one

I think a concrete example for the description could also help, when I read things like "certain platforms" I assume it means uncommon ones but that may not be the case

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