warn on empty precision, but only when width present

[hkBst]

Reworking of #136638.

Fixes #131159 by warning on missing precision only if there is a width specified.

This is the only case where a previous crater run caught a bug because "{:2.?}" was written but "{:.2?}" was meant.

r? @m-ou-se

Feel free to close if this is still deemed undesirable.

[rust-bors]

☔ The latest upstream changes (presumably #157558) made this pull request unmergeable. Please resolve the merge conflicts.

[Enselic]

This PR doesn't seem to do what was last discussed in #136638, namely to add a clippy lint for this. Was that agreement changed in discussions that took place later elsewhere , perhaps?

[hkBst]

@Enselic No, this is just a continuation of that discussion, with a proposal for a much quieter version of the original. Like you said it is not the preferred solution, so there's a good chance this will not go anywhere, but I wanted to present it anyway, since it was a simple change of the original.

I'm not planning on doing the preferred (clippy) version, since it seems too big of a project for me right now, but someone else should definitely feel free to go for that. You?

[Enselic]

Ok, I see, thanks for explaining. Not sure it makes sense to keep this PR open though, since changing the compiler for this has already been decided against.

Since you said

Feel free to close if this is still deemed undesirable.

I will go ahead and close this.