give up when gurad has side effects

Author: tamaroning

clippy_lints/src/matches/needless_match.rs

@@ -8,7 +8,7 @@ use clippy_utils::{
 };
 use rustc_errors::Applicability;
 use rustc_hir::LangItem::OptionNone;
-use rustc_hir::{Arm, BindingAnnotation, Expr, ExprKind, FnRetTy, Node, Pat, PatKind, Path, QPath};
+use rustc_hir::{Arm, BindingAnnotation, Expr, ExprKind, FnRetTy, Guard, Node, Pat, PatKind, Path, QPath};
 use rustc_lint::LateContext;
 use rustc_span::sym;
 use rustc_typeck::hir_ty_to_ty;
@@ -65,6 +65,22 @@ pub(crate) fn check_if_let<'tcx>(cx: &LateContext<'tcx>, ex: &Expr<'_>, if_let:
 fn check_all_arms(cx: &LateContext<'_>, match_expr: &Expr<'_>, arms: &[Arm<'_>]) -> bool {
     for arm in arms {
         let arm_expr = peel_blocks_with_stmt(arm.body);
+
+        if let Some(guard_expr) = &arm.guard {
+            match guard_expr {
+                // gives up if `pat if expr` can have side effects
+                Guard::If(if_cond) => {
+                    if if_cond.can_have_side_effects() {
+                        return false;
+                    }
+                },
+                // gives up `pat if let ...` arm
+                Guard::IfLet(_) => {
+                    return false;
+                },
+            };
+        }
+
         if let PatKind::Wild = arm.pat.kind {
             if !eq_expr_value(cx, match_expr, strip_return(arm_expr)) {
                 return false;

tests/ui/needless_match.fixed

@@ -209,7 +209,7 @@ impl Tr for Result<i32, i32> {
 
 mod issue9084 {
     fn wildcard_if() {
-        let some_bool = true;
+        let mut some_bool = true;
         let e = Some(1);
 
         // should lint
@@ -230,6 +230,19 @@ mod issue9084 {
             _ if some_bool => e,
             _ => e,
         };
+
+        // should not lint (guard has side effects)
+        let _ = match e {
+            Some(i) => Some(i),
+            _ if {
+                some_bool = false;
+                some_bool
+            } =>
+            {
+                e
+            },
+            _ => e,
+        };
     }
 }
 

tests/ui/needless_match.rs

@@ -246,7 +246,7 @@ impl Tr for Result<i32, i32> {
 
 mod issue9084 {
     fn wildcard_if() {
-        let some_bool = true;
+        let mut some_bool = true;
         let e = Some(1);
 
         // should lint
@@ -274,6 +274,19 @@ mod issue9084 {
             _ if some_bool => e,
             _ => e,
         };
+
+        // should not lint (guard has side effects)
+        let _ = match e {
+            Some(i) => Some(i),
+            _ if {
+                some_bool = false;
+                some_bool
+            } =>
+            {
+                e
+            },
+            _ => e,
+        };
     }
 }