Fix wrong question_mark suggestion when match arm body is a destructuring assignment (#16863)

* Added `&& sugg.starts_with('{')` to the block-insertion branch. When
the snippet doesn't start with `{` (i.e., destructuring assignment), it
falls through to the else branch which correctly wraps the suggestion in
an explicit block.
* Added a new test case `issue_destructuring_assignment` in
`tests/ui/question_mark.rs` to verify that the lint provides correct
suggestion for match expressions with destructuring assignments that can
be replaced by the `?` operator.

fixes #16862

changelog: [`question_mark`]: Fix wrong suggestion when match arm body
is a destructuring assignment

Author: llogiq

clippy_lints/src/question_mark.rs

@@ -495,7 +495,7 @@ fn check_if_try_match<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'tcx>) {
                         let mut sugg = reindent_multiline(&arm_body_snippet, true, Some(indent));
                         let binding_snippet = snippet_with_applicability(cx, binding_span, "..", &mut applicability);
                         let inner_indent = " ".repeat(indent + 4);
-                        if matches!(arm_body.kind, ExprKind::Block(..)) {
+                        if matches!(arm_body.kind, ExprKind::Block(..)) && sugg.starts_with('{') {
                             sugg.insert_str(
                                 1,
                                 &format!("\n{inner_indent}let {binding_snippet} = {scrutinee_snippet}?;"),

tests/ui/question_mark.fixed

@@ -564,3 +564,14 @@ fn issue16751(mut v: Option<usize>) -> Option<usize> {
         if n > 10 { Some(42) } else { None }
     }
 }
+
+fn issue_destructuring_assignment() -> Option<(i32, i32)> {
+    let mut a = 0i32;
+    let mut b = 0i32;
+    let opt: Option<(i32, i32)> = Some((1, 2));
+    {
+        let x = opt?;
+        (a, b) = x
+    }
+    Some((a, b))
+}

tests/ui/question_mark.rs

@@ -710,3 +710,15 @@ fn issue16751(mut v: Option<usize>) -> Option<usize> {
         None => return None,
     }
 }
+
+fn issue_destructuring_assignment() -> Option<(i32, i32)> {
+    let mut a = 0i32;
+    let mut b = 0i32;
+    let opt: Option<(i32, i32)> = Some((1, 2));
+    match opt {
+        //~^ question_mark
+        Some(x) => (a, b) = x,
+        None => return None,
+    }
+    Some((a, b))
+}

tests/ui/question_mark.stderr

@@ -498,5 +498,23 @@ LL +         if n > 10 { Some(42) } else { None }
 LL +     }
    |
 
-error: aborting due to 46 previous errors
+error: this `match` expression can be replaced with `?`
+  --> tests/ui/question_mark.rs:718:5
+   |
+LL | /     match opt {
+LL | |
+LL | |         Some(x) => (a, b) = x,
+LL | |         None => return None,
+LL | |     }
+   | |_____^
+   |
+help: try instead
+   |
+LL ~     {
+LL +         let x = opt?;
+LL +         (a, b) = x
+LL +     }
+   |
+
+error: aborting due to 47 previous errors