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Author: ada4a

clippy_lints/src/duplicate_match_guard.rs

@@ -1,5 +1,7 @@
-use clippy_utils::diagnostics::span_lint;
+use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::eq_expr_value;
+use clippy_utils::source::snippet_with_applicability;
+use rustc_errors::Applicability;
 use rustc_hir::{Arm, ExprKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::declare_lint_pass;
@@ -54,10 +56,29 @@ impl<'tcx> LateLintPass<'tcx> for DuplicateMatchGuard {
             && let ExprKind::Block(block, _) = arm.body.kind
             && block.stmts.is_empty()
             && let Some(trailing_expr) = block.expr
-            && let ExprKind::If(cond, _, None) = trailing_expr.kind
+            && let ExprKind::If(cond, then, None) = trailing_expr.kind
             && eq_expr_value(cx, guard, cond.peel_drop_temps())
         {
-            span_lint(cx, DUPLICATE_MATCH_GUARD, cond.span, "condition duplicates match guard");
+            let ExprKind::Block(then, _) = then.kind else {
+                unreachable!("the `then` expr in `ExprKind::If` is always `ExprKind::Block`")
+            };
+
+            // the two expressions may be syntactically different, even if identical
+            // semantically -- the user might want to replace the condition in the guard
+            // with the one in the body
+            let mut applicability = Applicability::MaybeIncorrect;
+
+            let sugg = snippet_with_applicability(cx, then.span, "..", &mut applicability);
+
+            span_lint_and_sugg(
+                cx,
+                DUPLICATE_MATCH_GUARD,
+                trailing_expr.span,
+                "condition duplicates match guard",
+                "remove the condition",
+                sugg.to_string(),
+                applicability,
+            );
         }
     }
 }

tests/ui/duplicate_match_guard.fixed

@@ -0,0 +1,65 @@
+#![allow(clippy::needless_return, clippy::needless_else)]
+#![warn(clippy::duplicate_match_guard)]
+
+fn main() {
+    let mut a = 5;
+    let b = 4;
+    let mut v: Vec<u32> = vec![];
+
+    match 0u32 {
+        0 if true => {
+            {
+                //~^ duplicate_match_guard
+                return;
+            }
+        },
+        0 if a > b => {
+            {
+                //~^ duplicate_match_guard
+                return;
+            }
+        },
+        // not _identical_, but the meaning is the same
+        0 if a > b => {
+            {
+                //~^ duplicate_match_guard
+                return;
+            }
+        },
+        // a bit more complicated
+        0 if a > 0 && b > 0 => {
+            {
+                //~^ duplicate_match_guard
+                return;
+            }
+        },
+
+        // no warnings
+        0 if true => {
+            if false {
+                return;
+            }
+        },
+        // has side effects, so don't lint
+        0 if v.pop().is_some() => {
+            if v.pop().is_some() {
+                return;
+            }
+        },
+        // there's _something_ in the else branch
+        0 if a > b => {
+            if a > b {
+                a += b;
+            } else {
+            }
+        },
+        // body has statements before the if
+        0 if a > b => {
+            a += b;
+            if a > b {
+                return;
+            }
+        },
+        _ => todo!(),
+    };
+}

tests/ui/duplicate_match_guard.stderr

@@ -1,29 +1,72 @@
 error: condition duplicates match guard
-  --> tests/ui/duplicate_match_guard.rs:11:16
+  --> tests/ui/duplicate_match_guard.rs:11:13
    |
-LL |             if true {
-   |                ^^^^
+LL | /             if true {
+LL | |
+LL | |                 return;
+LL | |             }
+   | |_____________^
    |
    = note: `-D clippy::duplicate-match-guard` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::duplicate_match_guard)]`
+help: remove the condition
+   |
+LL ~             {
+LL +
+LL +                 return;
+LL +             }
+   |
 
 error: condition duplicates match guard
-  --> tests/ui/duplicate_match_guard.rs:17:16
+  --> tests/ui/duplicate_match_guard.rs:17:13
+   |
+LL | /             if a > b {
+LL | |
+LL | |                 return;
+LL | |             }
+   | |_____________^
+   |
+help: remove the condition
+   |
+LL ~             {
+LL +
+LL +                 return;
+LL +             }
    |
-LL |             if a > b {
-   |                ^^^^^
 
 error: condition duplicates match guard
-  --> tests/ui/duplicate_match_guard.rs:24:16
+  --> tests/ui/duplicate_match_guard.rs:24:13
+   |
+LL | /             if b < a {
+LL | |
+LL | |                 return;
+LL | |             }
+   | |_____________^
+   |
+help: remove the condition
+   |
+LL ~             {
+LL +
+LL +                 return;
+LL +             }
    |
-LL |             if b < a {
-   |                ^^^^^
 
 error: condition duplicates match guard
-  --> tests/ui/duplicate_match_guard.rs:31:16
+  --> tests/ui/duplicate_match_guard.rs:31:13
+   |
+LL | /             if a > 0 && b > 0 {
+LL | |
+LL | |                 return;
+LL | |             }
+   | |_____________^
+   |
+help: remove the condition
+   |
+LL ~             {
+LL +
+LL +                 return;
+LL +             }
    |
-LL |             if a > 0 && b > 0 {
-   |                ^^^^^^^^^^^^^^
 
 error: aborting due to 4 previous errors