New lint: preferred case for hex digit literals

What it does

Warns if {upper,lower}case hex digits are used in a literal (i.e. forces a particular case). That is, it is stronger than the mixed_case_hex_literals lint.

This is a "preference lint pair", like the {un,}separated_literal_suffix and inline_asm_x86_{att,intel}_syntax ones.

Categories (optional)

• Kind: style.

Drawbacks

In general, it is arguable whether this sort of "formatting-like" lints like {un,}separated_literal_suffix or the current one should be in Clippy vs. rustfmt (e.g. rustfmt already has a hex_literal_case option).

One advantage of having them as lints is that projects may allow them locally in certain cases.

Example

0xabc

Could be written as:

0xABC

or viceversa.

Start by examining the existing mixed_case_hex_literals lint and the preference-lint pairs named in the issue to understand their implementation and tests. Add the new lint so it consistently warns about the non-preferred case of hexadecimal digits, with coverage for the example and its alternative spelling.