better help for mixed_case_hex_literals (#14235)

It can be error-prone for developers to manually change literals with
mixed uppercase and lowercase letters into consistently all-lowercase or
all-uppercase literals. Therefore, this lint rule should suggest
alternative literals.

changelog: [`mixed_case_hex_literals`]: add alternative suggestions

clippy_lints/src/misc_early/mixed_case_hex_literals.rs

@@ -1,32 +1,51 @@
-use clippy_utils::diagnostics::span_lint;
+use clippy_utils::diagnostics::span_lint_and_help;
 use rustc_lint::EarlyContext;
 use rustc_span::Span;
 
 use super::MIXED_CASE_HEX_LITERALS;
 
 pub(super) fn check(cx: &EarlyContext<'_>, lit_span: Span, suffix: &str, lit_snip: &str) {
-    let Some(maybe_last_sep_idx) = lit_snip.len().checked_sub(suffix.len() + 1) else {
-        return; // It's useless so shouldn't lint.
+    let num_end_idx = match lit_snip.strip_suffix(suffix) {
+        Some(p) if p.ends_with('_') => lit_snip.len() - (suffix.len() + 1),
+        Some(_) => lit_snip.len() - suffix.len(),
+        None => lit_snip.len(),
     };
-    if maybe_last_sep_idx <= 2 {
+
+    if num_end_idx <= 2 {
         // It's meaningless or causes range error.
         return;
     }
+
     let mut seen = (false, false);
-    for ch in &lit_snip.as_bytes()[2..=maybe_last_sep_idx] {
+    for ch in &lit_snip.as_bytes()[2..num_end_idx] {
         match ch {
             b'a'..=b'f' => seen.0 = true,
             b'A'..=b'F' => seen.1 = true,
             _ => {},
         }
         if seen.0 && seen.1 {
-            span_lint(
+            let raw_digits = &lit_snip[2..num_end_idx];
+            let (sugg_lower, sugg_upper) = if suffix.is_empty() {
+                (
+                    format!("0x{}", raw_digits.to_lowercase()),
+                    format!("0x{}", raw_digits.to_uppercase()),
+                )
+            } else {
+                (
+                    format!("0x{}_{}", raw_digits.to_lowercase(), suffix),
+                    format!("0x{}_{}", raw_digits.to_uppercase(), suffix),
+                )
+            };
+
+            span_lint_and_help(
                 cx,
                 MIXED_CASE_HEX_LITERALS,
                 lit_span,
                 "inconsistent casing in hexadecimal literal",
+                None,
+                format!("consider using `{sugg_lower}` or `{sugg_upper}`"),
             );
-            break;
+            return;
         }
     }
 }