new lints around #[must_use] fns

`must_use_unit` lints unit-returning functions with a `#[must_use]`
attribute, suggesting to remove it.

`double_must_use` lints functions with a plain `#[must_use]`
attribute, but which return a type which is already `#[must_use]`,
so the attribute has no benefit.

`must_use_candidate` is a pedantic lint that lints functions and
methods that return some non-unit type that is not already
`#[must_use]` and suggests to add the annotation.

clippy_lints/src/regex.rs

@@ -129,6 +129,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Regex {
 }
 
 #[allow(clippy::cast_possible_truncation)] // truncation very unlikely here
+#[must_use]
 fn str_span(base: Span, c: regex_syntax::ast::Span, offset: u16) -> Span {
     let offset = u32::from(offset);
     let end = base.lo() + BytePos(u32::try_from(c.end.offset).expect("offset too large") + offset);