feat: make unused_format_specs catch width issues

[nyurik]

implements #15039

Improve [unused_format_specs] lint to detect format width that is below the minimum output size for certain format traits (e.g. {:#02x} outputs "0x1", so width 2 is ignored). The lint suggests removing the width or increasing it above the minimum.

error: format width has no effect on the output for this format trait
  --> tests/ui/unused_format_specs_width.rs:9:15
   |
LL |     println!("{:#02X}", 1u8);
   |               ^^^^^^^
   |
   = help: consider removing the width or increasing it to at least 4
   = note: `-D clippy::unused-format-specs` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::unused_format_specs)]`

---

changelog: [unused_format_specs]: detect format width below minimum output size for #x/#o/#b, exponent, and pointer

[rustbot]

r? @llogiq

rustbot has assigned @llogiq.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: 7 candidates
• 7 candidates expanded to 7 candidates
• Random selection from Jarcho, dswij, llogiq, samueltardieu

[oscargus]

increasing it above 3 so increasing it to at least 4? Maybe the latter is more clear? (Or maybe it should be at least 3?)

[nyurik]

Thanks, fixed

[llogiq]

I think adding this to the unused_format_specs lint would be acceptable. I doubt that anyone wants one but not the other.

[nyurik]

@llogiq i modified it to be part of unused_format_specs. Note that I also made #16540 - which catches similar issue, and it may make sense to also make part of this lint. What do you think?

[nyurik]

r? @samueltardieu

[rustbot]

Error: Parsing assign command in comment failed: ...'' | error: specify user to assign to at >| ''...

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #triagebot on Zulip.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[nyurik]

@llogiq friendly ping, or do you want someone else to review it?
cc: @samueltardieu

[samueltardieu]

r? @samueltardieu

Please don't target me specifically unless I have manifested an explicit and clear interest in a particular PR. Moreover, I am out of rotation for around two weeks.

r? clippy

[llogiq]

Looks good to me. Thank you!

View changes since this review