New lint: pats_with_wild_match_arm

 - Wildcard use with other pattern in same match arm

Author: ThibsG

CHANGELOG.md

@@ -1241,6 +1241,7 @@ Released 2018-09-13
 [`panicking_unwrap`]: https://rust-lang.github.io/rust-clippy/master/index.html#panicking_unwrap
 [`partialeq_ne_impl`]: https://rust-lang.github.io/rust-clippy/master/index.html#partialeq_ne_impl
 [`path_buf_push_overwrite`]: https://rust-lang.github.io/rust-clippy/master/index.html#path_buf_push_overwrite
+[`pats_with_wild_match_arm`]: https://rust-lang.github.io/rust-clippy/master/index.html#pats_with_wild_match_arm
 [`possible_missing_comma`]: https://rust-lang.github.io/rust-clippy/master/index.html#possible_missing_comma
 [`precedence`]: https://rust-lang.github.io/rust-clippy/master/index.html#precedence
 [`print_literal`]: https://rust-lang.github.io/rust-clippy/master/index.html#print_literal

clippy_lints/src/lib.rs

@@ -597,6 +597,7 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
         &matches::MATCH_OVERLAPPING_ARM,
         &matches::MATCH_REF_PATS,
         &matches::MATCH_WILD_ERR_ARM,
+        &matches::PATS_WITH_WILD_MATCH_ARM,
         &matches::SINGLE_MATCH,
         &matches::SINGLE_MATCH_ELSE,
         &matches::WILDCARD_ENUM_MATCH_ARM,
@@ -1001,6 +1002,7 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
         LintId::of(&integer_division::INTEGER_DIVISION),
         LintId::of(&let_underscore::LET_UNDERSCORE_MUST_USE),
         LintId::of(&literal_representation::DECIMAL_LITERAL_REPRESENTATION),
+        LintId::of(&matches::PATS_WITH_WILD_MATCH_ARM),
         LintId::of(&matches::WILDCARD_ENUM_MATCH_ARM),
         LintId::of(&mem_forget::MEM_FORGET),
         LintId::of(&methods::CLONE_ON_REF_PTR),

clippy_lints/src/matches.rs

@@ -223,6 +223,26 @@ declare_clippy_lint! {
     "a wildcard enum match arm using `_`"
 }
 
+declare_clippy_lint! {
+    /// **What it does:** Checks for wildcard pattern used with others patterns in same match arm.
+    ///
+    /// **Why is this bad?** Wildcard pattern already covers any other pattern as it will match anyway.
+    /// It makes the code less readable, especially to spot wildcard pattern use in match arm.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// match "foo" {
+    ///     "a" => {},
+    ///     "bar" | _ => {},
+    /// }
+    /// ```
+    pub PATS_WITH_WILD_MATCH_ARM,
+    restriction,
+    "a wildcard pattern used with others patterns in same match arm"
+}
+
 declare_lint_pass!(Matches => [
     SINGLE_MATCH,
     MATCH_REF_PATS,
@@ -231,7 +251,8 @@ declare_lint_pass!(Matches => [
     MATCH_OVERLAPPING_ARM,
     MATCH_WILD_ERR_ARM,
     MATCH_AS_REF,
-    WILDCARD_ENUM_MATCH_ARM
+    WILDCARD_ENUM_MATCH_ARM,
+    PATS_WITH_WILD_MATCH_ARM
 ]);
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Matches {
@@ -246,6 +267,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Matches {
             check_wild_err_arm(cx, ex, arms);
             check_wild_enum_match(cx, ex, arms);
             check_match_as_ref(cx, ex, arms, expr);
+            check_pats_wild_match(cx, ex, arms, expr);
         }
         if let ExprKind::Match(ref ex, ref arms, _) = expr.kind {
             check_match_ref_pats(cx, ex, arms, expr);
@@ -664,6 +686,25 @@ fn check_match_as_ref(cx: &LateContext<'_, '_>, ex: &Expr<'_>, arms: &[Arm<'_>],
     }
 }
 
+fn check_pats_wild_match(cx: &LateContext<'_, '_>, _ex: &Expr, arms: &[Arm], _expr: &Expr) {
+    for arm in arms {
+        if let PatKind::Or(ref fields) = arm.pat.kind {
+            // look for multiple fields where one at least matches Wild pattern
+            if fields.len() > 1 && fields.into_iter().any(|pat| is_wild(pat)) {
+                span_lint_and_sugg(
+                    cx,
+                    PATS_WITH_WILD_MATCH_ARM,
+                    arm.pat.span,
+                    "wildcard pattern covers any other pattern as it will match anyway. Consider replacing with wildcard pattern only",
+                    "try this",
+                    "_".to_string(),
+                    Applicability::MachineApplicable,
+                )
+            }
+        }
+    }
+}
+
 /// Gets all arms that are unbounded `PatRange`s.
 fn all_ranges<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, arms: &'tcx [Arm<'_>]) -> Vec<SpannedRange<Constant>> {
     arms.iter()

src/lintlist/mod.rs

@@ -1568,6 +1568,13 @@ pub const ALL_LINTS: [Lint; 345] = [
         deprecation: None,
         module: "path_buf_push_overwrite",
     },
+    Lint {
+        name: "pats_with_wild_match_arm",
+        group: "restriction",
+        desc: "a wildcard pattern used with others patterns in same match arm",
+        deprecation: None,
+        module: "matches",
+    },
     Lint {
         name: "possible_missing_comma",
         group: "correctness",

tests/ui/pats_with_wild_match_arm.fixed

@@ -0,0 +1,38 @@
+// run-rustfix
+
+#![warn(clippy::pats_with_wild_match_arm)]
+
+fn main() {
+    match "foo" {
+        "a" => {
+            dbg!("matched a");
+        },
+        _ => {
+            dbg!("matched (bar or) wild");
+        },
+    };
+    match "foo" {
+        "a" => {
+            dbg!("matched a");
+        },
+        _ => {
+            dbg!("matched (bar or bar2 or) wild");
+        },
+    };
+    match "foo" {
+        "a" => {
+            dbg!("matched a");
+        },
+        _ => {
+            dbg!("matched (bar or) wild");
+        },
+    };
+    match "foo" {
+        "a" => {
+            dbg!("matched a");
+        },
+        _ => {
+            dbg!("matched (bar or) wild");
+        },
+    };
+}

tests/ui/pats_with_wild_match_arm.rs

@@ -0,0 +1,38 @@
+// run-rustfix
+
+#![warn(clippy::pats_with_wild_match_arm)]
+
+fn main() {
+    match "foo" {
+        "a" => {
+            dbg!("matched a");
+        },
+        "bar" | _ => {
+            dbg!("matched (bar or) wild");
+        },
+    };
+    match "foo" {
+        "a" => {
+            dbg!("matched a");
+        },
+        "bar" | "bar2" | _ => {
+            dbg!("matched (bar or bar2 or) wild");
+        },
+    };
+    match "foo" {
+        "a" => {
+            dbg!("matched a");
+        },
+        _ | "bar" | _ => {
+            dbg!("matched (bar or) wild");
+        },
+    };
+    match "foo" {
+        "a" => {
+            dbg!("matched a");
+        },
+        _ | "bar" => {
+            dbg!("matched (bar or) wild");
+        },
+    };
+}

tests/ui/pats_with_wild_match_arm.stderr

@@ -0,0 +1,28 @@
+error: wildcard pattern covers any other pattern as it will match anyway. Consider replacing with wildcard pattern only
+  --> $DIR/pats_with_wild_match_arm.rs:10:9
+   |
+LL |         "bar" | _ => {
+   |         ^^^^^^^^^ help: try this: `_`
+   |
+   = note: `-D clippy::pats-with-wild-match-arm` implied by `-D warnings`
+
+error: wildcard pattern covers any other pattern as it will match anyway. Consider replacing with wildcard pattern only
+  --> $DIR/pats_with_wild_match_arm.rs:18:9
+   |
+LL |         "bar" | "bar2" | _ => {
+   |         ^^^^^^^^^^^^^^^^^^ help: try this: `_`
+
+error: wildcard pattern covers any other pattern as it will match anyway. Consider replacing with wildcard pattern only
+  --> $DIR/pats_with_wild_match_arm.rs:26:9
+   |
+LL |         _ | "bar" | _ => {
+   |         ^^^^^^^^^^^^^ help: try this: `_`
+
+error: wildcard pattern covers any other pattern as it will match anyway. Consider replacing with wildcard pattern only
+  --> $DIR/pats_with_wild_match_arm.rs:34:9
+   |
+LL |         _ | "bar" => {
+   |         ^^^^^^^^^ help: try this: `_`
+
+error: aborting due to 4 previous errors
+