it works

Author: ada4a

CHANGELOG.md

@@ -5748,6 +5748,7 @@ Released 2018-09-13
 [`drop_copy`]: https://rust-lang.github.io/rust-clippy/master/index.html#drop_copy
 [`drop_non_drop`]: https://rust-lang.github.io/rust-clippy/master/index.html#drop_non_drop
 [`drop_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#drop_ref
+[`duplicate_match_guard`]: https://rust-lang.github.io/rust-clippy/master/index.html#duplicate_match_guard
 [`duplicate_mod`]: https://rust-lang.github.io/rust-clippy/master/index.html#duplicate_mod
 [`duplicate_underscore_argument`]: https://rust-lang.github.io/rust-clippy/master/index.html#duplicate_underscore_argument
 [`duplicated_attributes`]: https://rust-lang.github.io/rust-clippy/master/index.html#duplicated_attributes

clippy_lints/src/declared_lints.rs

@@ -134,6 +134,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::drop_forget_ref::DROP_NON_DROP_INFO,
     crate::drop_forget_ref::FORGET_NON_DROP_INFO,
     crate::drop_forget_ref::MEM_FORGET_INFO,
+    crate::duplicate_match_guard::DUPLICATE_MATCH_GUARD_INFO,
     crate::duplicate_mod::DUPLICATE_MOD_INFO,
     crate::else_if_without_else::ELSE_IF_WITHOUT_ELSE_INFO,
     crate::empty_drop::EMPTY_DROP_INFO,

clippy_lints/src/duplicate_match_guard.rs

@@ -0,0 +1,63 @@
+use clippy_utils::diagnostics::span_lint;
+use clippy_utils::eq_expr_value;
+use rustc_hir::{Arm, ExprKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_lint_pass;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for the same condition being checked in a match guard and in the match body
+    ///
+    /// ### Why is this bad?
+    /// This is usually just a typo or a copy and paste error.
+    ///
+    /// ### Known problems
+    /// False negatives: if the condition is an impure function, it could've been called twice on
+    /// purpose for its side effects
+    ///
+    /// ### Example
+    /// ```no_run
+    /// # let n = 0;
+    /// # let a = 3;
+    /// # let b = 4;
+    /// match n {
+    ///     0 if a > b => {
+    ///         if a > b {
+    ///             return;
+    ///         }
+    ///     }
+    ///     _ => {}
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// # let n = 0;
+    /// # let a = 3;
+    /// # let b = 4;
+    /// match n {
+    ///     0 if a > b => {
+    ///         return;
+    ///     }
+    ///     _ => {}
+    /// }
+    /// ```
+    #[clippy::version = "1.89.0"]
+    pub DUPLICATE_MATCH_GUARD,
+    nursery,
+    "a condition in match body duplicating the match guard"
+}
+declare_lint_pass!(DuplicateMatchGuard => [DUPLICATE_MATCH_GUARD]);
+
+impl<'tcx> LateLintPass<'tcx> for DuplicateMatchGuard {
+    fn check_arm(&mut self, cx: &LateContext<'tcx>, arm: &'tcx Arm<'tcx>) {
+        if let Some(guard) = arm.guard
+            && 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
+            && eq_expr_value(cx, guard, cond.peel_drop_temps())
+        {
+            span_lint(cx, DUPLICATE_MATCH_GUARD, cond.span, "condition duplicates match guard");
+        }
+    }
+}

clippy_lints/src/lib.rs

@@ -122,6 +122,7 @@ mod disallowed_types;
 mod doc;
 mod double_parens;
 mod drop_forget_ref;
+mod duplicate_match_guard;
 mod duplicate_mod;
 mod else_if_without_else;
 mod empty_drop;
@@ -951,5 +952,6 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(|_| Box::new(cloned_ref_to_slice_refs::ClonedRefToSliceRefs::new(conf)));
     store.register_late_pass(|_| Box::new(infallible_try_from::InfallibleTryFrom));
     store.register_late_pass(|_| Box::new(coerce_container_to_any::CoerceContainerToAny));
+    store.register_late_pass(|_| Box::new(duplicate_match_guard::DuplicateMatchGuard));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }

tests/ui/duplicate_match_guard.rs

@@ -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 => {
+            if true {
+                //~^ duplicate_match_guard
+                return;
+            }
+        },
+        0 if a > b => {
+            if a > b {
+                //~^ duplicate_match_guard
+                return;
+            }
+        },
+        // not _identical_, but the meaning is the same
+        0 if a > b => {
+            if b < a {
+                //~^ duplicate_match_guard
+                return;
+            }
+        },
+        // a bit more complicated
+        0 if a > 0 && b > 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

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