[single_match] fix suggestion when match irrefutable

Author: WeiTheShinobi

clippy_lints/src/matches/single_match.rs

@@ -91,6 +91,13 @@ fn report_single_pattern(cx: &LateContext<'_>, ex: &Expr<'_>, arm: &Arm<'_>, exp
         format!(" else {}", expr_block(cx, els, ctxt, "..", Some(expr.span), &mut app))
     });
 
+    if snippet(cx, ex.span, "..") == snippet(cx, arm.pat.span, "..") {
+        let msg = "this pattern will always match, `match` is useless";
+        let sugg = expr_block(cx, arm.body, ctxt, "..", Some(expr.span), &mut app);
+        span_lint_and_sugg(cx, lint, expr.span, msg, "try", sugg, app);
+        return;
+    }
+
     let (pat, pat_ref_count) = peel_hir_pat_refs(arm.pat);
     let (msg, sugg) = if let PatKind::Path(_) | PatKind::Lit(_) = pat.kind
         && let (ty, ty_ref_count) = peel_middle_ty_refs(cx.typeck_results().expr_ty(ex))

tests/ui/single_match.fixed

@@ -296,3 +296,21 @@ fn issue11365() {
 
     if let Some(A | B) = &Some(A) { println!() }
 }
+
+#[derive(Eq, PartialEq)]
+pub struct Data([u8; 4]);
+
+const DATA: Data = Data([1, 2, 3, 4]);
+const CONST_I32: i32 = 1;
+
+fn irrefutable_match() {
+    { println!() }
+
+    { println!() }
+
+    let i = 0;
+    {
+        let a = 1;
+        let b = 2;
+    }
+}

tests/ui/single_match.rs

@@ -360,3 +360,30 @@ fn issue11365() {
         None | Some(_) => {},
     }
 }
+
+#[derive(Eq, PartialEq)]
+pub struct Data([u8; 4]);
+
+const DATA: Data = Data([1, 2, 3, 4]);
+const CONST_I32: i32 = 1;
+
+fn irrefutable_match() {
+    match DATA {
+        DATA => println!(),
+        _ => {},
+    }
+
+    match CONST_I32 {
+        CONST_I32 => println!(),
+        _ => {},
+    }
+
+    let i = 0;
+    match i {
+        i => {
+            let a = 1;
+            let b = 2;
+        },
+        _ => {},
+    }
+}

tests/ui/single_match.stderr

@@ -216,5 +216,43 @@ LL | |         None | Some(_) => {},
 LL | |     }
    | |_____^ help: try: `if let Some(A | B) = &Some(A) { println!() }`
 
-error: aborting due to 20 previous errors
+error: this pattern will always match, `match` is useless
+  --> tests/ui/single_match.rs:371:5
+   |
+LL | /     match DATA {
+LL | |         DATA => println!(),
+LL | |         _ => {},
+LL | |     }
+   | |_____^ help: try: `{ println!() }`
+
+error: this pattern will always match, `match` is useless
+  --> tests/ui/single_match.rs:376:5
+   |
+LL | /     match CONST_I32 {
+LL | |         CONST_I32 => println!(),
+LL | |         _ => {},
+LL | |     }
+   | |_____^ help: try: `{ println!() }`
+
+error: this pattern will always match, `match` is useless
+  --> tests/ui/single_match.rs:382:5
+   |
+LL | /     match i {
+LL | |         i => {
+LL | |             let a = 1;
+LL | |             let b = 2;
+LL | |         },
+LL | |         _ => {},
+LL | |     }
+   | |_____^
+   |
+help: try
+   |
+LL ~     {
+LL +         let a = 1;
+LL +         let b = 2;
+LL +     }
+   |
+
+error: aborting due to 23 previous errors