fix need-mut false positive in closure capture of match scrutinee

crates/hir-def/src/body.rs

@@ -227,9 +227,8 @@ impl Body {
         });
     }
 
-    pub fn walk_pats(&self, pat_id: PatId, f: &mut impl FnMut(PatId)) {
+    pub fn walk_pats_shallow(&self, pat_id: PatId, mut f: impl FnMut(PatId)) {
         let pat = &self[pat_id];
-        f(pat_id);
         match pat {
             Pat::Range { .. }
             | Pat::Lit(..)
@@ -239,23 +238,28 @@ impl Body {
             | Pat::Missing => {}
             &Pat::Bind { subpat, .. } => {
                 if let Some(subpat) = subpat {
-                    self.walk_pats(subpat, f);
+                    f(subpat);
                 }
             }
             Pat::Or(args) | Pat::Tuple { args, .. } | Pat::TupleStruct { args, .. } => {
-                args.iter().copied().for_each(|p| self.walk_pats(p, f));
+                args.iter().copied().for_each(|p| f(p));
             }
-            Pat::Ref { pat, .. } => self.walk_pats(*pat, f),
+            Pat::Ref { pat, .. } => f(*pat),
             Pat::Slice { prefix, slice, suffix } => {
                 let total_iter = prefix.iter().chain(slice.iter()).chain(suffix.iter());
-                total_iter.copied().for_each(|p| self.walk_pats(p, f));
+                total_iter.copied().for_each(|p| f(p));
             }
             Pat::Record { args, .. } => {
-                args.iter().for_each(|RecordFieldPat { pat, .. }| self.walk_pats(*pat, f));
+                args.iter().for_each(|RecordFieldPat { pat, .. }| f(*pat));
             }
-            Pat::Box { inner } => self.walk_pats(*inner, f),
+            Pat::Box { inner } => f(*inner),
         }
     }
+
+    pub fn walk_pats(&self, pat_id: PatId, f: &mut impl FnMut(PatId)) {
+        f(pat_id);
+        self.walk_pats_shallow(pat_id, |p| self.walk_pats(p, f));
+    }
 }
 
 impl Default for Body {

crates/hir-ty/src/infer/closure.rs

@@ -643,7 +643,21 @@ impl InferenceContext<'_> {
             }
             None => *result = Some(ck),
         };
-        self.body.walk_pats(pat, &mut |p| match &self.body[p] {
+
+        self.walk_pat_inner(
+            pat,
+            &mut update_result,
+            BorrowKind::Mut { allow_two_phase_borrow: false },
+        );
+    }
+
+    fn walk_pat_inner(
+        &mut self,
+        p: PatId,
+        update_result: &mut impl FnMut(CaptureKind),
+        mut for_mut: BorrowKind,
+    ) {
+        match &self.body[p] {
             Pat::Ref { .. }
             | Pat::Box { .. }
             | Pat::Missing
@@ -678,13 +692,15 @@ impl InferenceContext<'_> {
                     }
                 }
                 crate::BindingMode::Ref(r) => match r {
-                    Mutability::Mut => update_result(CaptureKind::ByRef(BorrowKind::Mut {
+                    Mutability::Mut => update_result(CaptureKind::ByRef(for_mut)),
-                        allow_two_phase_borrow: false,
-                    })),
                     Mutability::Not => update_result(CaptureKind::ByRef(BorrowKind::Shared)),
                 },
             },
-        });
+        }
+        if self.result.pat_adjustments.get(&p).map_or(false, |x| !x.is_empty()) {
+            for_mut = BorrowKind::Unique;
+        }
+        self.body.walk_pats_shallow(p, |p| self.walk_pat_inner(p, update_result, for_mut));
     }
 
     fn expr_ty(&self, expr: ExprId) -> Ty {

crates/hir-ty/src/layout/tests/closure.rs

@@ -201,7 +201,7 @@ fn match_pattern() {
         ]
         |x: i64| {
             match y {
-                X(_a, _b, _c) => x,
+                X(_a, _, _c) => x,
             }
         }
     }
@@ -217,6 +217,18 @@ fn match_pattern() {
             }
         }
     }
+    size_and_align_expr! {
+        minicore: copy;
+        stmts: [
+            struct X(i64, i32, (u8, i128));
+            let y: X = X(2, 5, (7, 3));
+        ]
+        |x: i64| {
+            match y {
+                ref _y => x,
+            }
+        }
+    }
 }
 
 #[test]

crates/ide-diagnostics/src/handlers/mutability_errors.rs

@@ -352,6 +352,32 @@ fn main() {
         );
     }
 
+    #[test]
+    fn match_closure_capture() {
+        check_diagnostics(
+            r#"
+//- minicore: option
+fn main() {
+    let mut v = &mut Some(2);
+      //^^^^^ 💡 weak: variable does not need to be mutable
+    let _ = || match v {
+        Some(k) => {
+            *k = 5;
+        }
+        None => {}
+    };
+    let v = &mut Some(2);
+    let _ = || match v {
+                   //^ 💡 error: cannot mutate immutable variable `v`
+        ref mut k => {
+            *k = &mut Some(5);
+        }
+    };
+}
+"#,
+        );
+    }
+
     #[test]
     fn match_bindings() {
         check_diagnostics(