Auto merge of #11242 - samueltardieu:issue-11238, r=Centri3,giraffate

New lint `ignored_unit_patterns`

This idea comes from #11238. I've put the lint in `pedantic` as it might trigger numerous positives (three in Clippy itself).

changelog: [`ignored_unit_patterns`]: new lint

clippy_lints/src/declared_lints.rs

@@ -203,6 +203,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::if_let_mutex::IF_LET_MUTEX_INFO,
     crate::if_not_else::IF_NOT_ELSE_INFO,
     crate::if_then_some_else_none::IF_THEN_SOME_ELSE_NONE_INFO,
+    crate::ignored_unit_patterns::IGNORED_UNIT_PATTERNS_INFO,
     crate::implicit_hasher::IMPLICIT_HASHER_INFO,
     crate::implicit_return::IMPLICIT_RETURN_INFO,
     crate::implicit_saturating_add::IMPLICIT_SATURATING_ADD_INFO,

clippy_lints/src/ignored_unit_patterns.rs

@@ -0,0 +1,52 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use hir::PatKind;
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for usage of `_` in patterns of type `()`.
+    ///
+    /// ### Why is this bad?
+    /// Matching with `()` explicitly instead of `_` outlines
+    /// the fact that the pattern contains no data. Also it
+    /// would detect a type change that `_` would ignore.
+    ///
+    /// ### Example
+    /// ```rust
+    /// match std::fs::create_dir("tmp-work-dir") {
+    ///    Ok(_) => println!("Working directory created"),
+    ///    Err(s) => eprintln!("Could not create directory: {s}"),
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// match std::fs::create_dir("tmp-work-dir") {
+    ///    Ok(()) => println!("Working directory created"),
+    ///    Err(s) => eprintln!("Could not create directory: {s}"),
+    /// }
+    /// ```
+    #[clippy::version = "1.73.0"]
+    pub IGNORED_UNIT_PATTERNS,
+    pedantic,
+    "suggest replacing `_` by `()` in patterns where appropriate"
+}
+declare_lint_pass!(IgnoredUnitPatterns => [IGNORED_UNIT_PATTERNS]);
+
+impl<'tcx> LateLintPass<'tcx> for IgnoredUnitPatterns {
+    fn check_pat(&mut self, cx: &LateContext<'tcx>, pat: &'tcx hir::Pat<'tcx>) {
+        if matches!(pat.kind, PatKind::Wild) && cx.typeck_results().pat_ty(pat).is_unit() {
+            span_lint_and_sugg(
+                cx,
+                IGNORED_UNIT_PATTERNS,
+                pat.span,
+                "matching over `()` is more explicit",
+                "use `()` instead of `_`",
+                String::from("()"),
+                Applicability::MachineApplicable,
+            );
+        }
+    }
+}

clippy_lints/src/lib.rs

@@ -147,6 +147,7 @@ mod future_not_send;
 mod if_let_mutex;
 mod if_not_else;
 mod if_then_some_else_none;
+mod ignored_unit_patterns;
 mod implicit_hasher;
 mod implicit_return;
 mod implicit_saturating_add;
@@ -1093,6 +1094,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         })
     });
     store.register_late_pass(|_| Box::new(redundant_locals::RedundantLocals));
+    store.register_late_pass(|_| Box::new(ignored_unit_patterns::IgnoredUnitPatterns));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/option_if_let_else.rs

@@ -155,7 +155,7 @@ fn try_get_option_occurrence<'tcx>(
                 });
                 if let ExprKind::Path(QPath::Resolved(None, Path { res: Res::Local(local_id), .. })) = e.kind {
                     match some_captures.get(local_id)
-                        .or_else(|| (method_sugg == "map_or_else").then_some(()).and_then(|_| none_captures.get(local_id)))
+                        .or_else(|| (method_sugg == "map_or_else").then_some(()).and_then(|()| none_captures.get(local_id)))
                     {
                         Some(CaptureKind::Value | CaptureKind::Ref(Mutability::Mut)) => return None,
                         Some(CaptureKind::Ref(Mutability::Not)) if as_mut => return None,