add tests for collapsible_match: no lint when intermediate non-wildcard arm present

Author: Souradip121

tests/ui/collapsible_match.rs

@@ -390,6 +390,26 @@ fn take<T>(t: T) {}
 
 fn main() {}
 
+// https://github.com/rust-lang/rust-clippy/issues/16875
+// Adding a match guard allows fall-through to subsequent arms, which changes semantics
+// when non-wildcard arms follow the arm being collapsed.
+fn issue16875(a: Option<&str>, b: i32) -> i32 {
+    let mut res = 0;
+    // should NOT lint: `_ if b == 1` is not wild-like (has a guard), so collapsing
+    // `Some(_)` into `Some(_) if b == 0` would let `_ if b == 1` match Some values
+    // that previously fell through to the no-op arm body.
+    match a {
+        Some(_) => {
+            if b == 0 {
+                res = 1;
+            }
+        },
+        _ if b == 1 => res = 2,
+        _ => {},
+    }
+    res
+}
+
 fn issue16705(x: Option<String>) {
     fn takes_ownership(s: String) -> bool {
         true

tests/ui/collapsible_match_fixable.fixed

@@ -28,3 +28,18 @@ fn issue16558() {
         _ => 1,
     };
 }
+
+// https://github.com/rust-lang/rust-clippy/issues/16875
+// lint still fires when only wildcard-like arms follow (fall-through is harmless)
+fn issue16875(a: Option<&str>, b: i32) -> i32 {
+    let mut res = 0;
+    match a {
+        Some(_)
+            if b == 0 => {
+                //~^ collapsible_match
+                res = 1;
+            },
+        _ => {},
+    }
+    res
+}

tests/ui/collapsible_match_fixable.rs

@@ -29,3 +29,19 @@ fn issue16558() {
         _ => 1,
     };
 }
+
+// https://github.com/rust-lang/rust-clippy/issues/16875
+// lint still fires when only wildcard-like arms follow (fall-through is harmless)
+fn issue16875(a: Option<&str>, b: i32) -> i32 {
+    let mut res = 0;
+    match a {
+        Some(_) => {
+            if b == 0 {
+                //~^ collapsible_match
+                res = 1;
+            }
+        },
+        _ => {},
+    }
+    res
+}

tests/ui/collapsible_match_fixable.stderr

@@ -46,5 +46,23 @@ LL |
 LL ~         ,
    |
 
-error: aborting due to 3 previous errors
+error: this `if` can be collapsed into the outer `match`
+  --> tests/ui/collapsible_match_fixable.rs:39:13
+   |
+LL | /             if b == 0 {
+LL | |
+LL | |                 res = 1;
+LL | |             }
+   | |_____________^
+   |
+help: collapse nested if block
+   |
+LL ~         Some(_)
+LL ~             if b == 0 => {
+LL |
+LL |                 res = 1;
+LL ~             },
+   |
+
+error: aborting due to 4 previous errors