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.

CHANGELOG.md

@@ -976,6 +976,7 @@ Released 2018-09-13
 [`diverging_sub_expression`]: https://rust-lang.github.io/rust-clippy/master/index.html#diverging_sub_expression
 [`doc_markdown`]: https://rust-lang.github.io/rust-clippy/master/index.html#doc_markdown
 [`double_comparisons`]: https://rust-lang.github.io/rust-clippy/master/index.html#double_comparisons
+[`double_must_use`]: https://rust-lang.github.io/rust-clippy/master/index.html#double_must_use
 [`double_neg`]: https://rust-lang.github.io/rust-clippy/master/index.html#double_neg
 [`double_parens`]: https://rust-lang.github.io/rust-clippy/master/index.html#double_parens
 [`drop_bounds`]: https://rust-lang.github.io/rust-clippy/master/index.html#drop_bounds
@@ -1095,6 +1096,8 @@ Released 2018-09-13
 [`modulo_one`]: https://rust-lang.github.io/rust-clippy/master/index.html#modulo_one
 [`multiple_crate_versions`]: https://rust-lang.github.io/rust-clippy/master/index.html#multiple_crate_versions
 [`multiple_inherent_impl`]: https://rust-lang.github.io/rust-clippy/master/index.html#multiple_inherent_impl
+[`must_use_candidate`]: https://rust-lang.github.io/rust-clippy/master/index.html#must_use_candidate
+[`must_use_unit`]: https://rust-lang.github.io/rust-clippy/master/index.html#must_use_unit
 [`mut_from_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#mut_from_ref
 [`mut_mut`]: https://rust-lang.github.io/rust-clippy/master/index.html#mut_mut
 [`mut_range_bound`]: https://rust-lang.github.io/rust-clippy/master/index.html#mut_range_bound

README.md

@@ -6,7 +6,7 @@
 
 A collection of lints to catch common mistakes and improve your [Rust](https://github.com/rust-lang/rust) code.
 
-[There are 321 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 324 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
 
 We have a bunch of lint categories to allow you to choose how much Clippy is supposed to ~~annoy~~ help you:
 

clippy_dev/src/lib.rs

@@ -42,6 +42,7 @@ pub struct Lint {
 }
 
 impl Lint {
+    #[must_use]
     pub fn new(name: &str, group: &str, desc: &str, deprecation: Option<&str>, module: &str) -> Self {
         Self {
             name: name.to_lowercase(),
@@ -58,19 +59,22 @@ impl Lint {
     }
 
     /// Returns the lints in a `HashMap`, grouped by the different lint groups
+    #[must_use]
     pub fn by_lint_group(lints: &[Self]) -> HashMap<String, Vec<Self>> {
         lints
             .iter()
             .map(|lint| (lint.group.to_string(), lint.clone()))
             .into_group_map()
     }
 
+    #[must_use]
     pub fn is_internal(&self) -> bool {
         self.group.starts_with("internal")
     }
 }
 
 /// Generates the Vec items for `register_lint_group` calls in `clippy_lints/src/lib.rs`.
+#[must_use]
 pub fn gen_lint_group_list(lints: Vec<Lint>) -> Vec<String> {
     lints
         .into_iter()
@@ -86,6 +90,7 @@ pub fn gen_lint_group_list(lints: Vec<Lint>) -> Vec<String> {
 }
 
 /// Generates the `pub mod module_name` list in `clippy_lints/src/lib.rs`.
+#[must_use]
 pub fn gen_modules_list(lints: Vec<Lint>) -> Vec<String> {
     lints
         .into_iter()
@@ -103,6 +108,7 @@ pub fn gen_modules_list(lints: Vec<Lint>) -> Vec<String> {
 }
 
 /// Generates the list of lint links at the bottom of the README
+#[must_use]
 pub fn gen_changelog_lint_list(lints: Vec<Lint>) -> Vec<String> {
     let mut lint_list_sorted: Vec<Lint> = lints;
     lint_list_sorted.sort_by_key(|l| l.name.clone());
@@ -119,6 +125,7 @@ pub fn gen_changelog_lint_list(lints: Vec<Lint>) -> Vec<String> {
 }
 
 /// Generates the `register_removed` code in `./clippy_lints/src/lib.rs`.
+#[must_use]
 pub fn gen_deprecated(lints: &[Lint]) -> Vec<String> {
     lints
         .iter()

clippy_dev/src/stderr_length_check.rs

@@ -42,6 +42,7 @@ fn stderr_files() -> impl Iterator<Item = walkdir::DirEntry> {
         .filter(|f| f.path().extension() == Some(OsStr::new("stderr")))
 }
 
+#[must_use]
 fn count_linenumbers(filepath: &str) -> usize {
     if let Ok(mut file) = File::open(filepath) {
         let mut content = String::new();

clippy_lints/src/approx_const.rs

@@ -93,6 +93,7 @@ fn check_known_consts(cx: &LateContext<'_, '_>, e: &Expr, s: symbol::Symbol, mod
 /// Returns `false` if the number of significant figures in `value` are
 /// less than `min_digits`; otherwise, returns true if `value` is equal
 /// to `constant`, rounded to the number of digits present in `value`.
+#[must_use]
 fn is_approx_const(constant: f64, value: &str, min_digits: usize) -> bool {
     if value.len() <= min_digits {
         false

clippy_lints/src/assign_ops.rs

@@ -229,6 +229,7 @@ fn lint_misrefactored_assign_op(
     );
 }
 
+#[must_use]
 fn is_commutative(op: hir::BinOpKind) -> bool {
     use rustc::hir::BinOpKind::*;
     match op {

clippy_lints/src/bit_mask.rs

@@ -100,6 +100,7 @@ pub struct BitMask {
 }
 
 impl BitMask {
+    #[must_use]
     pub fn new(verbose_bit_mask_threshold: u64) -> Self {
         Self {
             verbose_bit_mask_threshold,
@@ -150,6 +151,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for BitMask {
     }
 }
 
+#[must_use]
 fn invert_cmp(cmp: BinOpKind) -> BinOpKind {
     match cmp {
         BinOpKind::Eq => BinOpKind::Eq,

clippy_lints/src/checked_conversions.rs

@@ -160,6 +160,7 @@ impl<'a> Conversion<'a> {
 
 impl ConversionType {
     /// Creates a conversion type if the type is allowed & conversion is valid
+    #[must_use]
     fn try_new(from: &str, to: &str) -> Option<Self> {
         if UINTS.contains(&from) {
             Some(Self::FromUnsigned)

clippy_lints/src/cognitive_complexity.rs

@@ -29,6 +29,7 @@ pub struct CognitiveComplexity {
 }
 
 impl CognitiveComplexity {
+    #[must_use]
     pub fn new(limit: u64) -> Self {
         Self {
             limit: LimitStack::new(limit),

clippy_lints/src/doc.rs

@@ -191,6 +191,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for DocMarkdown {
 /// need to keep track of
 /// the spans but this function is inspired from the later.
 #[allow(clippy::cast_possible_truncation)]
+#[must_use]
 pub fn strip_doc_comment_decoration(comment: &str, span: Span) -> (String, Vec<(usize, Span)>) {
     // one-line comments lose their prefix
     const ONELINERS: &[&str] = &["///!", "///", "//!", "//"];

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(),
@@ -123,13 +124,15 @@ 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
     pre.chars().zip(name_iter).take_while(|&(l, r)| l == r).count()
 }
 
 /// 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;

clippy_lints/src/excessive_precision.rs

@@ -62,6 +62,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ExcessivePrecision {
 
 impl ExcessivePrecision {
     // None if nothing to lint, Some(suggestion) if lint necessary
+    #[must_use]
     fn check(self, sym: Symbol, fty: FloatTy) -> Option<String> {
         let max = max_digits(fty);
         let sym_str = sym.as_str();
@@ -97,6 +98,7 @@ impl ExcessivePrecision {
 /// Should we exclude the float because it has a `.0` or `.` suffix
 /// Ex `1_000_000_000.0`
 /// Ex `1_000_000_000.`
+#[must_use]
 fn dot_zero_exclusion(s: &str) -> bool {
     s.split('.').nth(1).map_or(false, |after_dec| {
         let mut decpart = after_dec.chars().take_while(|c| *c != 'e' || *c != 'E');
@@ -109,6 +111,7 @@ fn dot_zero_exclusion(s: &str) -> bool {
     })
 }
 
+#[must_use]
 fn max_digits(fty: FloatTy) -> u32 {
     match fty {
         FloatTy::F32 => f32::DIGITS,
@@ -117,6 +120,7 @@ fn max_digits(fty: FloatTy) -> u32 {
 }
 
 /// Counts the digits excluding leading zeros
+#[must_use]
 fn count_digits(s: &str) -> usize {
     // Note that s does not contain the f32/64 suffix, and underscores have been stripped
     s.chars()
@@ -138,6 +142,7 @@ enum FloatFormat {
     Normal,
 }
 impl FloatFormat {
+    #[must_use]
     fn new(s: &str) -> Self {
         s.chars()
             .find_map(|x| match x {

clippy_lints/src/formatting.rs

@@ -235,6 +235,7 @@ fn check_else(cx: &EarlyContext<'_>, expr: &Expr) {
     }
 }
 
+#[must_use]
 fn has_unary_equivalent(bin_op: BinOpKind) -> bool {
     // &, *, -
     bin_op == BinOpKind::And || bin_op == BinOpKind::Mul || bin_op == BinOpKind::Sub