Auto merge of #119650 - chenyukang:yukang-fix-118596-ref-mut, r=wesleywiser

Suggest ref mut for pattern matching assignment

Fixes #118596

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -3743,7 +3743,6 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             | None => (self.describe_any_place(place.as_ref()), assigned_span),
             Some(decl) => (self.describe_any_place(err_place.as_ref()), decl.source_info.span),
         };
-
         let mut err = self.cannot_reassign_immutable(span, &place_description, from_arg);
         let msg = if from_arg {
             "cannot assign to immutable argument"
@@ -3763,6 +3762,22 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 format!("mut {name}"),
                 Applicability::MachineApplicable,
             );
+            if !from_arg
+                && matches!(
+                    decl.local_info(),
+                    LocalInfo::User(BindingForm::Var(VarBindingForm {
+                        opt_match_place: Some((Some(_), _)),
+                        ..
+                    }))
+                )
+            {
+                err.span_suggestion(
+                    decl.source_info.span,
+                    "to modify the original value, take a borrow instead",
+                    format!("ref mut {name}"),
+                    Applicability::MaybeIncorrect,
+                );
+            }
         }
         err.span_label(span, msg);
         self.buffer_error(err);

tests/ui/borrowck/borrowck-match-binding-is-assignment.stderr

@@ -2,56 +2,86 @@ error[E0384]: cannot assign twice to immutable variable `x`
   --> $DIR/borrowck-match-binding-is-assignment.rs:14:13
    |
 LL |         x => {
-   |         -
+   |         - first assignment to `x`
-   |         |
-   |         first assignment to `x`
-   |         help: consider making this binding mutable: `mut x`
 LL |             x += 1;
    |             ^^^^^^ cannot assign twice to immutable variable
+   |
+help: consider making this binding mutable
+   |
+LL |         mut x => {
+   |         ~~~~~
+help: to modify the original value, take a borrow instead
+   |
+LL |         ref mut x => {
+   |         ~~~~~~~~~
 
 error[E0384]: cannot assign twice to immutable variable `x`
   --> $DIR/borrowck-match-binding-is-assignment.rs:20:13
    |
 LL |         E::Foo(x) => {
-   |                -
+   |                - first assignment to `x`
-   |                |
-   |                first assignment to `x`
-   |                help: consider making this binding mutable: `mut x`
 LL |             x += 1;
    |             ^^^^^^ cannot assign twice to immutable variable
+   |
+help: consider making this binding mutable
+   |
+LL |         E::Foo(mut x) => {
+   |                ~~~~~
+help: to modify the original value, take a borrow instead
+   |
+LL |         E::Foo(ref mut x) => {
+   |                ~~~~~~~~~
 
 error[E0384]: cannot assign twice to immutable variable `x`
   --> $DIR/borrowck-match-binding-is-assignment.rs:26:13
    |
 LL |         S { bar: x } => {
-   |                  -
+   |                  - first assignment to `x`
-   |                  |
-   |                  first assignment to `x`
-   |                  help: consider making this binding mutable: `mut x`
 LL |             x += 1;
    |             ^^^^^^ cannot assign twice to immutable variable
+   |
+help: consider making this binding mutable
+   |
+LL |         S { bar: mut x } => {
+   |                  ~~~~~
+help: to modify the original value, take a borrow instead
+   |
+LL |         S { bar: ref mut x } => {
+   |                  ~~~~~~~~~
 
 error[E0384]: cannot assign twice to immutable variable `x`
   --> $DIR/borrowck-match-binding-is-assignment.rs:32:13
    |
 LL |         (x,) => {
-   |          -
+   |          - first assignment to `x`
-   |          |
-   |          first assignment to `x`
-   |          help: consider making this binding mutable: `mut x`
 LL |             x += 1;
    |             ^^^^^^ cannot assign twice to immutable variable
+   |
+help: consider making this binding mutable
+   |
+LL |         (mut x,) => {
+   |          ~~~~~
+help: to modify the original value, take a borrow instead
+   |
+LL |         (ref mut x,) => {
+   |          ~~~~~~~~~
 
 error[E0384]: cannot assign twice to immutable variable `x`
   --> $DIR/borrowck-match-binding-is-assignment.rs:38:13
    |
 LL |         [x,_,_] => {
-   |          -
+   |          - first assignment to `x`
-   |          |
-   |          first assignment to `x`
-   |          help: consider making this binding mutable: `mut x`
 LL |             x += 1;
    |             ^^^^^^ cannot assign twice to immutable variable
+   |
+help: consider making this binding mutable
+   |
+LL |         [mut x,_,_] => {
+   |          ~~~~~
+help: to modify the original value, take a borrow instead
+   |
+LL |         [ref mut x,_,_] => {
+   |          ~~~~~~~~~
 
 error: aborting due to 5 previous errors
 

tests/ui/borrowck/suggest-ref-mut-issue-118596.rs

@@ -0,0 +1,11 @@
+fn main() {
+    let y = Some(0);
+    if let Some(x) = y {
+        x = 2; //~ ERROR cannot assign twice to immutable variable `x`
+    }
+
+    let mut arr = [1, 2, 3];
+    let [x, ref xs_hold @ ..] = arr;
+    x = 0; //~ ERROR cannot assign twice to immutable variable `x`
+    eprintln!("{:?}", arr);
+}

tests/ui/borrowck/suggest-ref-mut-issue-118596.stderr

@@ -0,0 +1,37 @@
+error[E0384]: cannot assign twice to immutable variable `x`
+  --> $DIR/suggest-ref-mut-issue-118596.rs:4:9
+   |
+LL |     if let Some(x) = y {
+   |                 - first assignment to `x`
+LL |         x = 2;
+   |         ^^^^^ cannot assign twice to immutable variable
+   |
+help: consider making this binding mutable
+   |
+LL |     if let Some(mut x) = y {
+   |                 ~~~~~
+help: to modify the original value, take a borrow instead
+   |
+LL |     if let Some(ref mut x) = y {
+   |                 ~~~~~~~~~
+
+error[E0384]: cannot assign twice to immutable variable `x`
+  --> $DIR/suggest-ref-mut-issue-118596.rs:9:5
+   |
+LL |     let [x, ref xs_hold @ ..] = arr;
+   |          - first assignment to `x`
+LL |     x = 0;
+   |     ^^^^^ cannot assign twice to immutable variable
+   |
+help: consider making this binding mutable
+   |
+LL |     let [mut x, ref xs_hold @ ..] = arr;
+   |          ~~~~~
+help: to modify the original value, take a borrow instead
+   |
+LL |     let [ref mut x, ref xs_hold @ ..] = arr;
+   |          ~~~~~~~~~
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0384`.

tests/ui/mut/mut-pattern-internal-mutability.stderr

@@ -2,12 +2,18 @@ error[E0384]: cannot assign twice to immutable variable `x`
   --> $DIR/mut-pattern-internal-mutability.rs:5:5
    |
 LL |     let &mut x = foo;
-   |              -
+   |              - first assignment to `x`
-   |              |
-   |              first assignment to `x`
-   |              help: consider making this binding mutable: `mut x`
 LL |     x += 1;
    |     ^^^^^^ cannot assign twice to immutable variable
+   |
+help: consider making this binding mutable
+   |
+LL |     let &mut mut x = foo;
+   |              ~~~~~
+help: to modify the original value, take a borrow instead
+   |
+LL |     let &mut ref mut x = foo;
+   |              ~~~~~~~~~
 
 error[E0506]: cannot assign to `*foo` because it is borrowed
   --> $DIR/mut-pattern-internal-mutability.rs:13:5

tests/ui/pattern/bindings-after-at/pat-at-same-name-both.stderr

@@ -70,13 +70,19 @@ error[E0384]: cannot assign twice to immutable variable `a`
   --> $DIR/pat-at-same-name-both.rs:13:15
    |
 LL |         Ok(a @ b @ a)
-   |                    -
+   |                    - first assignment to `a`
-   |                    |
-   |                    first assignment to `a`
-   |                    help: consider making this binding mutable: `mut a`
 LL |
 LL |         | Err(a @ b @ a)
    |               ^ cannot assign twice to immutable variable
+   |
+help: consider making this binding mutable
+   |
+LL |         Ok(a @ b @ mut a)
+   |                    ~~~~~
+help: to modify the original value, take a borrow instead
+   |
+LL |         Ok(a @ b @ ref mut a)
+   |                    ~~~~~~~~~
 
 error: aborting due to 12 previous errors
 

tests/ui/pattern/move-ref-patterns/borrowck-move-ref-pattern.stderr

@@ -15,12 +15,18 @@ error[E0384]: cannot assign twice to immutable variable `_x1`
   --> $DIR/borrowck-move-ref-pattern.rs:9:5
    |
 LL |     let [ref _x0_hold, _x1, ref xs_hold @ ..] = arr;
-   |                        ---
+   |                        --- first assignment to `_x1`
-   |                        |
-   |                        first assignment to `_x1`
-   |                        help: consider making this binding mutable: `mut _x1`
 LL |     _x1 = U;
    |     ^^^^^^^ cannot assign twice to immutable variable
+   |
+help: consider making this binding mutable
+   |
+LL |     let [ref _x0_hold, mut _x1, ref xs_hold @ ..] = arr;
+   |                        ~~~~~~~
+help: to modify the original value, take a borrow instead
+   |
+LL |     let [ref _x0_hold, ref mut _x1, ref xs_hold @ ..] = arr;
+   |                        ~~~~~~~~~~~
 
 error[E0505]: cannot move out of `arr[..]` because it is borrowed
   --> $DIR/borrowck-move-ref-pattern.rs:11:10
@@ -73,12 +79,18 @@ error[E0384]: cannot assign twice to immutable variable `_x1`
   --> $DIR/borrowck-move-ref-pattern.rs:23:5
    |
 LL |     let (ref _x0, _x1, ref _x2, ..) = tup;
-   |                   ---
+   |                   --- first assignment to `_x1`
-   |                   |
-   |                   first assignment to `_x1`
-   |                   help: consider making this binding mutable: `mut _x1`
 LL |     _x1 = U;
    |     ^^^^^^^ cannot assign twice to immutable variable
+   |
+help: consider making this binding mutable
+   |
+LL |     let (ref _x0, mut _x1, ref _x2, ..) = tup;
+   |                   ~~~~~~~
+help: to modify the original value, take a borrow instead
+   |
+LL |     let (ref _x0, ref mut _x1, ref _x2, ..) = tup;
+   |                   ~~~~~~~~~~~
 
 error[E0502]: cannot borrow `tup.0` as mutable because it is also borrowed as immutable
   --> $DIR/borrowck-move-ref-pattern.rs:24:20

tests/ui/pattern/mut-ref-mut-2021.stderr

@@ -2,12 +2,18 @@ error[E0384]: cannot assign twice to immutable variable `a`
   --> $DIR/mut-ref-mut-2021.rs:9:5
    |
 LL |     let Foo(a) = Foo(0);
-   |             -
+   |             - first assignment to `a`
-   |             |
-   |             first assignment to `a`
-   |             help: consider making this binding mutable: `mut a`
 LL |     a = 42;
    |     ^^^^^^ cannot assign twice to immutable variable
+   |
+help: consider making this binding mutable
+   |
+LL |     let Foo(mut a) = Foo(0);
+   |             ~~~~~
+help: to modify the original value, take a borrow instead
+   |
+LL |     let Foo(ref mut a) = Foo(0);
+   |             ~~~~~~~~~
 
 error[E0384]: cannot assign twice to immutable variable `a`
   --> $DIR/mut-ref-mut-2021.rs:15:5