Handle index out of bound errors during const eval without panic

Author: estebank

src/librustc_mir/interpret/place.rs

@@ -361,8 +361,12 @@ where
                 offsets[usize::try_from(field).unwrap()],
             layout::FieldPlacement::Array { stride, .. } => {
                 let len = base.len(self)?;
-                assert!(field < len, "Tried to access element {} of array/slice with length {}",
-                    field, len);
+                if field >= len {
+                    // This can be violated because this runs during promotion on code where the
+                    // type system has not yet ensured that such things don't happen.
+                    debug!("Tried to access element {} of array/slice with length {}", field, len);
+                    return err!(BoundsCheck { len, index: field });
+                }
                 stride * field
             }
             layout::FieldPlacement::Union(count) => {

src/test/ui/consts/array-literal-index-oob.rs

@@ -0,0 +1,6 @@
+fn main() {
+    &{[1, 2, 3][4]};
+    //~^ ERROR index out of bounds
+    //~| ERROR reaching this expression at runtime will panic or abort
+    //~| ERROR this expression will panic at runtime
+}

src/test/ui/consts/array-literal-index-oob.stderr

@@ -0,0 +1,24 @@
+error: index out of bounds: the len is 3 but the index is 4
+  --> $DIR/array-literal-index-oob.rs:2:7
+   |
+LL |     &{[1, 2, 3][4]};
+   |       ^^^^^^^^^^^^
+   |
+   = note: #[deny(const_err)] on by default
+
+error: this expression will panic at runtime
+  --> $DIR/array-literal-index-oob.rs:2:5
+   |
+LL |     &{[1, 2, 3][4]};
+   |     ^^^^^^^^^^^^^^^ index out of bounds: the len is 3 but the index is 4
+
+error: reaching this expression at runtime will panic or abort
+  --> $DIR/array-literal-index-oob.rs:2:7
+   |
+LL |     &{[1, 2, 3][4]};
+   |     --^^^^^^^^^^^^-
+   |       |
+   |       index out of bounds: the len is 3 but the index is 4
+
+error: aborting due to 3 previous errors
+