Expose a safe way to efficiently write to an uninitialized, fixed buffer about itoa HOT 3 CLOSED

I like the API in ryu a lot. I would be on board with picking that up here. Please send a PR!

In your use case is it important to use the smallest possible buffer for each integer type or can we just use [u8; 40] everywhere which fits any primitive integer?

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I wouldn't say it is important (bench first and all) but my first thought was just to expose the max size. Other than that it would make it possible to preserve the current internal api which has the explicit per integer sizes so I figured why not do that tiny bit of optimization.

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I would go with [u8; 40] for now even if it means some restructuring internally. If you decide to implement both ways and find measurably better performance from using a smaller buffer for smaller types, then we can go with the faster way.

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