Add leading/trailing zeros to floats

[alok]

Essentially, these reformats should be done for consistency and readability:

x. -> x.0
.x -> 0.x

So 1. would become 1.0 and .1 would become 0.1.

[amatveiakin]

Sent a pull request to implement this.

Note that .x is not a concern, since it's not a valid floating-point literal in Rust (in contrast to C++).

Also I believe it would be nice to support different formatting styles. I came up with the following options:

• Preserve (default). Leave the literal as-is.
• Always. Add a trailing zero to the literal. E.g. 1.0, 2.0e10, 3.0f32.
• IfNoPostfix. Add a trailing zero by default. If the literal contains an exponent or a suffix, the zero and the preceding period are removed. E.g. 1.0, 2e10, 3f32.
• Never. Remove the trailing zero. If the literal contains an exponent or a suffix, the preceding period is also removed. E.g. 1., 2e10, 3f32.

[ytmimi]

Thanks for taking the time to look into this and for submitting a PR! I think this is a configuration rustfmt could support, and the different options you've outlined here make sense to me. I'll try to carve out some time over the next few days to review and provide feedback on your implementation 😁

[Keavon]

The proposed Never mode would perhaps need a choice for what to do in range syntax. Some possibilities include:

• 0.0..1.0 (when used in range, uses Always)
• 0.0..1. (when used in range, uses Always for the LHS and Never for the RHS)
• (0.)..1. (when used in range, wraps the LHS in parentheses)
• Any more cases I'm forgetting?

That said, I really appreciate having Never since I currently have to manually enforce the removal of trailing zeros during code review at the moment in my open source code base. I point these out, but I hope the optionality here doesn't slow down or de-scope the overall effort by having perfect be the enemy of good. Picking any of those would be appreciated over skipping or delaying the Never option entirely.

[amatveiakin]

@Keavon Thanks for pointing this out!
I went with another option in #5834: 0. ..1.
While it looks a bit awkward, I think it's the least awkward option of all. IMO 0.0..1. feels unbalanced and 0.0..1.0 feels excessive; also there is the questions of what to do with things like 1.0/2.0..1.0. With (0.)..1. I don't like the fact that switching from Always to Never and then back to Always would produce irreversible changes (or we would need to remove the parens, which is something rustfmt currently doesn't do).
Also there was already a precedent of adding a space between a float and .. in rustfmt, so it's consistent with the current formatting practice.