Add test checking our behavior for assigning over a ConstIndex projection.

Author: pnkfelix

Diff for: src/test/ui/nll/issue-62007-assign-const-index.rs

@@ -0,0 +1,32 @@
+// Issue #62007: assigning over a const-index projection of an array
+// (in this case, `list[I] = n;`) should in theory be able to kill all borrows
+// of `list[0]`, so that `list[0]` could be borrowed on the next
+// iteration through the loop.
+//
+// Currently the compiler does not allow this. We may want to consider
+// loosening that restriction in the future. (However, doing so would
+// at *least* require T-lang team approval, and probably an RFC; e.g.
+// such loosening might make complicate the user's mental mode; it
+// also would make code more brittle in the face of refactorings that
+// replace constants with variables.
+
+#![allow(dead_code)]
+
+struct List<T> {
+    value: T,
+    next: Option<Box<List<T>>>,
+}
+
+fn to_refs<T>(mut list: [&mut List<T>; 2]) -> Vec<&mut T> {
+    let mut result = vec![];
+    loop {
+        result.push(&mut list[0].value); //~ ERROR cannot borrow `list[_].value` as mutable
+        if let Some(n) = list[0].next.as_mut() { //~ ERROR cannot borrow `list[_].next` as mutable
+            list[0] = n;
+        } else {
+            return result;
+        }
+    }
+}
+
+fn main() {}

Diff for: src/test/ui/nll/issue-62007-assign-const-index.stderr

@@ -0,0 +1,27 @@
+error[E0499]: cannot borrow `list[_].value` as mutable more than once at a time
+  --> $DIR/issue-62007-assign-const-index.rs:23:21
+   |
+LL | fn to_refs<T>(mut list: [&mut List<T>; 2]) -> Vec<&mut T> {
+   |                          - let's call the lifetime of this reference `'1`
+...
+LL |         result.push(&mut list[0].value);
+   |                     ^^^^^^^^^^^^^^^^^^ mutable borrow starts here in previous iteration of loop
+...
+LL |             return result;
+   |                    ------ returning this value requires that `list[_].value` is borrowed for `'1`
+
+error[E0499]: cannot borrow `list[_].next` as mutable more than once at a time
+  --> $DIR/issue-62007-assign-const-index.rs:24:26
+   |
+LL | fn to_refs<T>(mut list: [&mut List<T>; 2]) -> Vec<&mut T> {
+   |                          - let's call the lifetime of this reference `'1`
+...
+LL |         if let Some(n) = list[0].next.as_mut() {
+   |                          ^^^^^^^^^^^^---------
+   |                          |
+   |                          mutable borrow starts here in previous iteration of loop
+   |                          argument requires that `list[_].next` is borrowed for `'1`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0499`.