Add new lint: match with a single binding statement

- Lint name: MATCH_SINGLE_BINDING

clippy_lints/src/lib.rs

@@ -605,6 +605,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &matches::MATCH_OVERLAPPING_ARM,
         &matches::MATCH_REF_PATS,
         &matches::MATCH_WILD_ERR_ARM,
+        &matches::MATCH_SINGLE_BINDING,
         &matches::SINGLE_MATCH,
         &matches::SINGLE_MATCH_ELSE,
         &matches::WILDCARD_ENUM_MATCH_ARM,
@@ -1206,6 +1207,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&matches::MATCH_OVERLAPPING_ARM),
         LintId::of(&matches::MATCH_REF_PATS),
         LintId::of(&matches::MATCH_WILD_ERR_ARM),
+        LintId::of(&matches::MATCH_SINGLE_BINDING),
         LintId::of(&matches::SINGLE_MATCH),
         LintId::of(&matches::WILDCARD_IN_OR_PATTERNS),
         LintId::of(&mem_discriminant::MEM_DISCRIMINANT_NON_ENUM),
@@ -1483,6 +1485,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, 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::MATCH_SINGLE_BINDING),
         LintId::of(&matches::WILDCARD_IN_OR_PATTERNS),
         LintId::of(&methods::CHARS_NEXT_CMP),
         LintId::of(&methods::CLONE_ON_COPY),

clippy_lints/src/matches.rs

@@ -3,9 +3,9 @@ use crate::utils::paths;
 use crate::utils::sugg::Sugg;
 use crate::utils::usage::is_unused;
 use crate::utils::{
-    expr_block, is_allowed, is_expn_of, is_wild, match_qpath, match_type, multispan_sugg, remove_blocks, snippet,
-    snippet_with_applicability, span_lint_and_help, span_lint_and_note, span_lint_and_sugg, span_lint_and_then,
-    walk_ptrs_ty,
+    span_lint_and_help, span_lint_and_note, 
+    expr_block, in_macro, is_allowed, is_expn_of, is_wild, match_qpath, match_type, multispan_sugg, remove_blocks,
+    snippet, snippet_with_applicability, span_lint_and_sugg, span_lint_and_then,
 };
 use if_chain::if_chain;
 use rustc::lint::in_external_macro;
@@ -245,6 +245,33 @@ declare_clippy_lint! {
     "a wildcard pattern used with others patterns in same match arm"
 }
 
+declare_clippy_lint! {
+    /// **What it does:** Checks for useless match that binds to only one value.
+    ///
+    /// **Why is this bad?** Readability and needless complexity.
+    ///
+    /// **Known problems:** This situation frequently happen in macros, so can't lint there.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// # let a = 1;
+    /// # let b = 2;
+    ///
+    /// // Bad
+    /// match (a, b) {
+    ///     (c, d) => {
+    ///         // useless match
+    ///     }
+    /// }
+    ///
+    /// // Good
+    /// let (c, d) = (a, b);
+    /// ```
+    pub MATCH_SINGLE_BINDING,
+    complexity,
+    "a match with a single binding instead of using `let` statement"
+}
+
 declare_lint_pass!(Matches => [
     SINGLE_MATCH,
     MATCH_REF_PATS,
@@ -254,7 +281,8 @@ declare_lint_pass!(Matches => [
     MATCH_WILD_ERR_ARM,
     MATCH_AS_REF,
     WILDCARD_ENUM_MATCH_ARM,
-    WILDCARD_IN_OR_PATTERNS
+    WILDCARD_IN_OR_PATTERNS,
+    MATCH_SINGLE_BINDING,
 ]);
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Matches {
@@ -270,6 +298,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Matches {
             check_wild_enum_match(cx, ex, arms);
             check_match_as_ref(cx, ex, arms, expr);
             check_wild_in_or_pats(cx, arms);
+            check_match_single_binding(cx, ex, arms, expr);
         }
         if let ExprKind::Match(ref ex, ref arms, _) = expr.kind {
             check_match_ref_pats(cx, ex, arms, expr);
@@ -712,6 +741,29 @@ fn check_wild_in_or_pats(cx: &LateContext<'_, '_>, arms: &[Arm<'_>]) {
     }
 }
 
+fn check_match_single_binding(cx: &LateContext<'_, '_>, ex: &Expr<'_>, arms: &[Arm<'_>], expr: &Expr<'_>) {
+    if in_macro(expr.span) {
+        return;
+    }
+    if arms.len() == 1 {
+        let bind_names = arms[0].pat.span;
+        let matched_vars = ex.span;
+        span_lint_and_sugg(
+            cx,
+            MATCH_SINGLE_BINDING,
+            expr.span,
+            "this match could be written as a `let` statement",
+            "try this",
+            format!(
+                "let {} = {};",
+                snippet(cx, bind_names, ".."),
+                snippet(cx, matched_vars, "..")
+            ),
+            Applicability::HasPlaceholders,
+        );
+    }
+}
+
 /// Gets all arms that are unbounded `PatRange`s.
 fn all_ranges<'a, 'tcx>(
     cx: &LateContext<'a, 'tcx>,

tests/ui/match_single_binding.rs

@@ -0,0 +1,25 @@
+#![warn(clippy::match_single_binding)]
+#[allow(clippy::many_single_char_names)]
+
+fn main() {
+    let a = 1;
+    let b = 2;
+    let c = 3;
+    // Lint
+    match (a, b, c) {
+        (x, y, z) => {
+            println!("{} {} {}", x, y, z);
+        },
+    }
+    // Ok
+    match a {
+        2 => println!("2"),
+        _ => println!("Not 2"),
+    }
+    // Ok
+    let d = Some(5);
+    match d {
+        Some(d) => println!("5"),
+        _ => println!("None"),
+    }
+}

tests/ui/match_single_binding.stderr

@@ -0,0 +1,14 @@
+error: this match could be written as a `let` statement
+  --> $DIR/match_single_binding.rs:9:5
+   |
+LL | /     match (a, b, c) {
+LL | |         (x, y, z) => {
+LL | |             println!("{} {} {}", x, y, z);
+LL | |         },
+LL | |     }
+   | |_____^ help: try this: `let (x, y, z) = (a, b, c);`
+   |
+   = note: `-D clippy::match-single-binding` implied by `-D warnings`
+
+error: aborting due to previous error
+