test: add case for mutating iterator

Note that this incorrectly suggests a shared borrow,
but at least we know it's happening.

Author: notriddle

src/test/ui/suggestions/slice-issue-87994.rs

@@ -4,4 +4,13 @@ fn main() {
     //~^ ERROR [i32]` is not an iterator [E0277]
     //~^^ ERROR known at compilation time
   }
+  struct K {
+    n: i32,
+  }
+  let mut v2 = vec![K { n: 1 }, K { n: 1 }, K { n: 1 }];
+  for i2 in v2[1..] {
+    //~^ ERROR [K]` is not an iterator [E0277]
+    //~^^ ERROR known at compilation time
+    i2.n = 2;
+  }
 }

src/test/ui/suggestions/slice-issue-87994.stderr

@@ -32,6 +32,40 @@ note: required by `into_iter`
 LL |     fn into_iter(self) -> Self::IntoIter;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error[E0277]: the size for values of type `[K]` cannot be known at compilation time
+  --> $DIR/slice-issue-87994.rs:11:13
+   |
+LL |   for i2 in v2[1..] {
+   |             ^^^^^^^
+   |             |
+   |             expected an implementor of trait `IntoIterator`
+   |             help: consider borrowing here: `&v2[1..]`
+   |
+   = note: the trait bound `[K]: IntoIterator` is not satisfied
+   = note: required because of the requirements on the impl of `IntoIterator` for `[K]`
+note: required by `into_iter`
+  --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
+   |
+LL |     fn into_iter(self) -> Self::IntoIter;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0277]: `[K]` is not an iterator
+  --> $DIR/slice-issue-87994.rs:11:13
+   |
+LL |   for i2 in v2[1..] {
+   |             ^^^^^^^
+   |             |
+   |             expected an implementor of trait `IntoIterator`
+   |             help: consider borrowing here: `&v2[1..]`
+   |
+   = note: the trait bound `[K]: IntoIterator` is not satisfied
+   = note: required because of the requirements on the impl of `IntoIterator` for `[K]`
+note: required by `into_iter`
+  --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
+   |
+LL |     fn into_iter(self) -> Self::IntoIter;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0277`.