proposal: strconv: Make FormatInt accept any integer type

[misha-ridge]

Description

• Change strconv.FormatInt to be a generic that accepts any integer type.
• Deprecate strconv.FormatUint and suggest to use strconv.FormatInt instead.

Motivation

During a review of a medium-sized codebase I have noticed that there are two causes of typecasts to the arguments of strconv.Format[U]Int: casting integers to a wider type, and casting non-integers (floats, types based on integers). These two cases are hard to distinguish and as a result of the review we had to add a comment to every typecast.

First type of casts (wider-type casts) ought not exist, they are an artifact of Format[U]Int been created before generics. This proposal gets rid of the useless typecasts of first type, leaving only second, meaningful, typecasts in place.

As far as I can tell this proposal does not break backward compatibility, as it makes FormatInt accept strictly more types than before.

[zephyrtronium]

This kind of change is not backward compatible, unfortunately. Code today can write var f = strconv.FormatInt, which would become illegal if FormatInt were given type parameters. See #48287.

[ianlancetaylor]

Unfortunately we can't do this, so closing.