Auto merge of #4960 - ThibsG:patterns_with_wildcard_#4640, r=flip1995

New lint: pats_with_wild_match_arm

Wildcard use with other pattern in same match arm.

The wildcard covers other(s) pattern(s) as it will match anyway.

changelog: add new lint when multiple patterns (including wildcard) are used in a match arm.

Fixes #4640.

clippy_lints/src/lib.rs

@@ -600,6 +600,7 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
         &matches::SINGLE_MATCH,
         &matches::SINGLE_MATCH_ELSE,
         &matches::WILDCARD_ENUM_MATCH_ARM,
+        &matches::WILDCARD_IN_OR_PATTERNS,
         &mem_discriminant::MEM_DISCRIMINANT_NON_ENUM,
         &mem_forget::MEM_FORGET,
         &mem_replace::MEM_REPLACE_OPTION_WITH_NONE,
@@ -1188,6 +1189,7 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
         LintId::of(&matches::MATCH_REF_PATS),
         LintId::of(&matches::MATCH_WILD_ERR_ARM),
         LintId::of(&matches::SINGLE_MATCH),
+        LintId::of(&matches::WILDCARD_IN_OR_PATTERNS),
         LintId::of(&mem_discriminant::MEM_DISCRIMINANT_NON_ENUM),
         LintId::of(&mem_replace::MEM_REPLACE_OPTION_WITH_NONE),
         LintId::of(&mem_replace::MEM_REPLACE_WITH_DEFAULT),
@@ -1462,6 +1464,7 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
         LintId::of(&map_unit_fn::OPTION_MAP_UNIT_FN),
         LintId::of(&map_unit_fn::RESULT_MAP_UNIT_FN),
         LintId::of(&matches::MATCH_AS_REF),
+        LintId::of(&matches::WILDCARD_IN_OR_PATTERNS),
         LintId::of(&methods::CHARS_NEXT_CMP),
         LintId::of(&methods::CLONE_ON_COPY),
         LintId::of(&methods::FILTER_NEXT),

clippy_lints/src/matches.rs

@@ -3,7 +3,8 @@ use crate::utils::paths;
 use crate::utils::sugg::Sugg;
 use crate::utils::{
     expr_block, is_allowed, is_expn_of, match_qpath, match_type, multispan_sugg, remove_blocks, snippet,
-    snippet_with_applicability, span_lint_and_sugg, span_lint_and_then, span_note_and_lint, walk_ptrs_ty,
+    snippet_with_applicability, span_help_and_lint, span_lint_and_sugg, span_lint_and_then, span_note_and_lint,
+    walk_ptrs_ty,
 };
 use if_chain::if_chain;
 use rustc::declare_lint_pass;
@@ -223,6 +224,26 @@ 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 WILDCARD_IN_OR_PATTERNS,
+    complexity,
+    "a wildcard pattern used with others patterns in same match arm"
+}
+
 declare_lint_pass!(Matches => [
     SINGLE_MATCH,
     MATCH_REF_PATS,
@@ -231,7 +252,8 @@ declare_lint_pass!(Matches => [
     MATCH_OVERLAPPING_ARM,
     MATCH_WILD_ERR_ARM,
     MATCH_AS_REF,
-    WILDCARD_ENUM_MATCH_ARM
+    WILDCARD_ENUM_MATCH_ARM,
+    WILDCARD_IN_OR_PATTERNS
 ]);
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Matches {
@@ -246,6 +268,7 @@ 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_wild_in_or_pats(cx, arms);
         }
         if let ExprKind::Match(ref ex, ref arms, _) = expr.kind {
             check_match_ref_pats(cx, ex, arms, expr);
@@ -664,6 +687,23 @@ fn check_match_as_ref(cx: &LateContext<'_, '_>, ex: &Expr<'_>, arms: &[Arm<'_>],
     }
 }
 
+fn check_wild_in_or_pats(cx: &LateContext<'_, '_>, arms: &[Arm<'_>]) {
+    for arm in arms {
+        if let PatKind::Or(ref fields) = arm.pat.kind {
+            // look for multiple fields in this arm that contains at least one Wild pattern
+            if fields.len() > 1 && fields.iter().any(is_wild) {
+                span_help_and_lint(
+                    cx,
+                    WILDCARD_IN_OR_PATTERNS,
+                    arm.pat.span,
+                    "wildcard pattern covers any other pattern as it will match anyway.",
+                    "Consider handling `_` separately.",
+                );
+            }
+        }
+    }
+}
+
 /// 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()