Make convert_while_ascii unsafe

[Zoxc]

convert_while_ascii assumes convert only returns ASCII to ensure the output remains valid UTF-8. This adds that requirement as a safety precondition.

[rustbot]

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[Bryntet]

Could you explain a bit more on how you could get UB by calling convert_while_ascii? This isn't too clear based on your PR description

[Bryntet]

also you should probably reroll, see #155303

[Zoxc]

Could you explain a bit more on how you could get UB by calling convert_while_ascii?

The output of convert is placed unchecked in the out vector which later is passed to String::from_utf8_unchecked.

[teor2345]

Look good to me!

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