feat: add lint for ignored format precision on non-float types

[nyurik]

Introduce a new Clippy lint, unused_format_precision, which warns when a precision specifier is used in formatting macros for types where it has no effect, such as integers, bool, char, and pointers. This helps catch misleading or potentially buggy formatting code.

What it does

Detects use of format precision for standard non-float types where it has no effect
(integers, bool, char, pointers). Precision is meaningful for floats, str (truncation), or user types.

Why is this bad?

Specifying precision for these types is misleading and may indicate a bug. The value is ignored by the formatter.

Example

println!("{:.5}", 42);   // precision ignored for integers
println!("{:.3}", true); // precision ignored for bool
println!("{:.2}", 'x');  // precision ignored for char

TODO

• I am not certain which of the lint groups applies best for this lint. All these seem somewhat appropriate:
  • correctness - code that is outright wrong or useless
  • suspicious - code that is most likely wrong or useless
  • complexity - code that does something simple but in a complex way

---

changelog: [unused_format_precision]: new lint to catch ignored format precision on non-float types

[rustbot]

r? @dswij

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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

[ada4a]

Not a reviewer, but LGTM:)

View changes since this review

[nyurik]

Adding @llogiq to this issue because it may make sense to combine this functionality into unused_format_specs as discussed in #16542

r? @llogiq

[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.