Fix collapsible_match suggestion causing non-exhaustive pattern error

clippy_lints/src/matches/collapsible_match.rs

@@ -24,7 +24,17 @@ use super::{COLLAPSIBLE_MATCH, pat_contains_disallowed_or};
 pub(super) fn check_match<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, arms: &'tcx [Arm<'_>], msrv: Msrv) {
     if let Some(els_arm) = arms.iter().rfind(|arm| arm_is_wild_like(cx, arm)) {
         for arm in arms {
-            check_arm(cx, true, arm.pat, expr, arm.body, arm.guard, Some(els_arm.body), msrv);
+            check_arm(
+                cx,
+                true,
+                arm.pat,
+                expr,
+                arm.body,
+                arm.guard,
+                Some(els_arm.body),
+                Some(els_arm.pat),
+                msrv,
+            );
         }
     }
 }
@@ -37,7 +47,7 @@ pub(super) fn check_if_let<'tcx>(
     let_expr: &'tcx Expr<'_>,
     msrv: Msrv,
 ) {
-    check_arm(cx, false, pat, let_expr, body, None, else_expr, msrv);
+    check_arm(cx, false, pat, let_expr, body, None, else_expr, None, msrv);
 }
 
 #[expect(clippy::too_many_arguments, clippy::too_many_lines)]
@@ -49,6 +59,7 @@ fn check_arm<'tcx>(
     outer_then_body: &'tcx Expr<'tcx>,
     outer_guard: Option<&'tcx Expr<'tcx>>,
     outer_else_body: Option<&'tcx Expr<'tcx>>,
+    outer_else_pat: Option<&'tcx Pat<'tcx>>,
     msrv: Msrv,
 ) {
     let inner_expr = peel_blocks_with_stmt(outer_then_body);
@@ -187,6 +198,15 @@ fn check_arm<'tcx>(
                     sugg.push((else_inner_span, String::new()));
                 }
 
+                // When collapsing into a match guard, the guard doesn't count toward
+                // exhaustiveness. If the else arm's pattern is not a true wildcard
+                // (e.g. `None`), we must replace it with `_` to maintain exhaustiveness.
+                if let Some(els_pat) = outer_else_pat
+                    && !matches!(els_pat.kind, PatKind::Wild | PatKind::Binding(..))
+                {
+                    sugg.push((els_pat.span, "_".to_string()));
+                }
+
                 diag.multipart_suggestion("collapse nested if block", sugg, Applicability::MachineApplicable);
             },
         );

tests/ui/collapsible_match_fixable.fixed

@@ -1,6 +1,21 @@
 #![warn(clippy::collapsible_match)]
 #![allow(clippy::single_match, clippy::redundant_guards)]
 
+// issue #16860: collapsible_match suggestion causes non-exhaustive pattern error
+// when the else arm is `None` (not a true wildcard)
+fn issue16860() -> Option<i32> {
+    let x: Option<i32> = Some(1);
+    match x {
+        _ => {}
+        Some(v)
+            if v > 0 => {
+                //~^ collapsible_match
+                return Some(v);
+            }
+    }
+    None
+}
+
 fn issue16558() {
     let opt = Some(1);
     let _ = match opt {

tests/ui/collapsible_match_fixable.rs

@@ -1,6 +1,22 @@
 #![warn(clippy::collapsible_match)]
 #![allow(clippy::single_match, clippy::redundant_guards)]
 
+// issue #16860: collapsible_match suggestion causes non-exhaustive pattern error
+// when the else arm is `None` (not a true wildcard)
+fn issue16860() -> Option<i32> {
+    let x: Option<i32> = Some(1);
+    match x {
+        None => {},
+        Some(v) => {
+            if v > 0 {
+                //~^ collapsible_match
+                return Some(v);
+            }
+        },
+    }
+    None
+}
+
 fn issue16558() {
     let opt = Some(1);
     let _ = match opt {

tests/ui/collapsible_match_fixable.stderr

@@ -1,21 +1,40 @@
 error: this `if` can be collapsed into the outer `match`
-  --> tests/ui/collapsible_match_fixable.rs:8:13
+  --> tests/ui/collapsible_match_fixable.rs:11:13
    |
-LL |             if s == 1 { s } else { 1 }
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | /             if v > 0 {
+LL | |
+LL | |                 return Some(v);
+LL | |             }
+   | |_____________^
    |
    = note: `-D clippy::collapsible-match` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::collapsible_match)]`
 help: collapse nested if block
    |
+LL ~         _ => {}
+LL ~         Some(v)
+LL ~             if v > 0 => {
+LL |
+LL |                 return Some(v);
+LL ~             }
+   |
+
+error: this `if` can be collapsed into the outer `match`
+  --> tests/ui/collapsible_match_fixable.rs:24:13
+   |
+LL |             if s == 1 { s } else { 1 }
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: collapse nested if block
+   |
 LL ~         Some(s)
 LL ~             if s == 1 => { s }
 LL |
 LL ~         ,
    |
 
 error: this `if` can be collapsed into the outer `match`
-  --> tests/ui/collapsible_match_fixable.rs:16:13
+  --> tests/ui/collapsible_match_fixable.rs:32:13
    |
 LL | /             (if s == 1 {
 LL | |
@@ -33,7 +52,7 @@ LL ~             },
    |
 
 error: this `if` can be collapsed into the outer `match`
-  --> tests/ui/collapsible_match_fixable.rs:26:13
+  --> tests/ui/collapsible_match_fixable.rs:42:13
    |
 LL |             if s == 1 { s } else { 1 }
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -46,5 +65,5 @@ LL |
 LL ~         ,
    |
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors