Auto merge of #4560 - rust-lang:must-use-pure, r=phansch

new lints around`#[must_use]`

changelog: Add `must_use_candidate` lint,  add `must-use-unit` lint, add `double_must_use` lint

The first one checks if an public function or method has no mutable argument and mutates no non-local data and lints if it has no `#[must_use]` attribute. It will skip inner functions, because those are usually highly local and the attribute doesn't have as much benefit there.

The second lints `#[must_use]` attributes on functions and methods that return unit. Those attributes are likely a remnant from a refactoring that removed the return value.

The last one lints for `#[must_use]` attributrs without text on functions that return a type which is already marked `#[must_use]`. This has no auto-suggestion, because while it would be easy to do, there may be value in writing a detailed text for the attribute instead.

This fixes #4526

2 files changed, 8 insertions, 0 deletions

diff --git a/clippy_dev/src/lib.rs b/clippy_dev/src/lib.rs
index 7df7109c75f..84b2814a7ce 100644
--- a/clippy_dev/src/lib.rs
+++ b/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(),
@@ -58,6 +59,7 @@ 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()
@@ -65,12 +67,14 @@ impl Lint {
             .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()
diff --git a/clippy_dev/src/stderr_length_check.rs b/clippy_dev/src/stderr_length_check.rs
index 3049c45ddc8..2d7d119f3ff 100644
--- a/clippy_dev/src/stderr_length_check.rs
+++ b/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();