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/enum_variants.rs

@@ -107,6 +107,7 @@ pub struct EnumVariantNames {
 }
 
 impl EnumVariantNames {
+    #[must_use]
     pub fn new(threshold: u64) -> Self {
         Self {
             modules: Vec::new(),
@@ -123,6 +124,7 @@ impl_lint_pass!(EnumVariantNames => [
 ]);
 
 /// Returns the number of chars that match from the start
+#[must_use]
 fn partial_match(pre: &str, name: &str) -> usize {
     let mut name_iter = name.chars();
     let _ = name_iter.next_back(); // make sure the name is never fully matched
@@ -130,6 +132,7 @@ fn partial_match(pre: &str, name: &str) -> usize {
 }
 
 /// Returns the number of chars that match from the end
+#[must_use]
 fn partial_rmatch(post: &str, name: &str) -> usize {
     let mut name_iter = name.chars();
     let _ = name_iter.next(); // make sure the name is never fully matched
@@ -211,6 +214,7 @@ fn check_variant(
     );
 }
 
+#[must_use]
 fn to_camel_case(item_name: &str) -> String {
     let mut s = String::new();
     let mut up = true;