2 files changed, 25 insertions, 0 deletions
diff --git a/src/test/ui/btreemap/btreemap-index-mut.rs b/src/test/ui/btreemap/btreemap-index-mut.rs
new file mode 100644
index 000000000..62972acab
--- /dev/null
+++ b/src/test/ui/btreemap/btreemap-index-mut.rs
@@ -0,0 +1,6 @@
+use std::collections::BTreeMap;
+
+fn main() {
+    let mut map = BTreeMap::<u32, u32>::new();
+    map[&0] = 1; //~ ERROR cannot assign
+}
diff --git a/src/test/ui/btreemap/btreemap-index-mut.stderr b/src/test/ui/btreemap/btreemap-index-mut.stderr
new file mode 100644
index 000000000..26f2a4c4b
--- /dev/null
+++ b/src/test/ui/btreemap/btreemap-index-mut.stderr
@@ -0,0 +1,19 @@
+error[E0594]: cannot assign to data in an index of `BTreeMap<u32, u32>`
+  --> $DIR/btreemap-index-mut.rs:5:5
+   |
+LL |     map[&0] = 1;
+   |     ^^^^^^^^^^^ cannot assign
+   |
+   = help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `BTreeMap<u32, u32>`
+help: to modify a `BTreeMap<u32, u32>`, use `.get_mut()`, `.insert()` or the entry API
+   |
+LL |     map.insert(&0, 1);
+   |        ~~~~~~~~  ~  +
+LL |     map.get_mut(&0).map(|val| { *val = 1; });
+   |        ~~~~~~~~~  ~~~~~~~~~~~~~~~~~~    ++++
+LL |     let val = map.entry(&0).or_insert(1);
+   |     +++++++++    ~~~~~~~  ~~~~~~~~~~~~ +
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0594`.