Fix subslice capture in closure

Author: clubby789

compiler/rustc_hir_typeck/src/upvar.rs

@@ -1893,14 +1893,13 @@ fn restrict_capture_precision(
 
     for (i, proj) in place.projections.iter().enumerate() {
         match proj.kind {
-            ProjectionKind::Index => {
-                // Arrays are completely captured, so we drop Index projections
+            ProjectionKind::Index | ProjectionKind::Subslice => {
+                // Arrays are completely captured, so we drop Index and Subslice projections
                 truncate_place_to_len_and_update_capture_kind(&mut place, &mut curr_mode, i);
                 return (place, curr_mode);
             }
             ProjectionKind::Deref => {}
             ProjectionKind::Field(..) => {} // ignore
-            ProjectionKind::Subslice => {}  // We never capture this
         }
     }
 

tests/ui/closures/2229_closure_analysis/array_subslice.rs

@@ -0,0 +1,13 @@
+// regression test for #109298
+// edition: 2021
+
+pub fn subslice_array(x: [u8; 3]) {
+    let f = || {
+        let [_x @ ..] = x;
+        let [ref y, ref mut z @ ..] = x; //~ ERROR cannot borrow `x[..]` as mutable
+    };
+
+    f(); //~ ERROR cannot borrow `f` as mutable
+}
+
+fn main() {}

tests/ui/closures/2229_closure_analysis/array_subslice.stderr

@@ -0,0 +1,26 @@
+error[E0596]: cannot borrow `x[..]` as mutable, as `x` is not declared as mutable
+  --> $DIR/array_subslice.rs:7:21
+   |
+LL | pub fn subslice_array(x: [u8; 3]) {
+   |                       - help: consider changing this to be mutable: `mut x`
+...
+LL |         let [ref y, ref mut z @ ..] = x;
+   |                     ^^^^^^^^^ cannot borrow as mutable
+
+error[E0596]: cannot borrow `f` as mutable, as it is not declared as mutable
+  --> $DIR/array_subslice.rs:10:5
+   |
+LL |         let [ref y, ref mut z @ ..] = x;
+   |                                       - calling `f` requires mutable binding due to mutable borrow of `x`
+...
+LL |     f();
+   |     ^ cannot borrow as mutable
+   |
+help: consider changing this to be mutable
+   |
+LL |     let mut f = || {
+   |         +++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0596`.