Fix FormattingOptions instantiation with Default

[nyurik]

The fill value by default should be set to ' ' (space), but the current implementation uses #[derive(Default)] which sets it to \0.

Note that FormattingOptions is being released as part of 1.85 (unstable) - so this might warrant a backport to that branch.

Tracking issue: #118117

Follow up from #118159

CC: @EliasHolzmann @programmerjake

r? @m-ou-se

[programmerjake]

Note that FormattingOptions is being released as part of 1.85 (unstable) - so this might warrant a backport to that branch.

afaik we generally don't backport changes that only affect unstable APIs, since they're intended to only work on nightly, not beta or stable. If this fixes some stable API then that's a different story.

[joboet]

FormattingOptions::default isn't yet used internally, so this didn't affect public behaviour and hence doesn't need backporting. Good catch, though!

@bors r+ rollup
r? joboet

[bors]

📌 Commit c9ae0bb has been approved by joboet

It is now in the queue for this repository.

[EliasHolzmann]

@nyurik Thanks, nice catch!

[nyurik]

"There is (now) a lint for that" (tm)

rust-lang/rust-clippy#14234