Add test suggest-use-as-mut-for-map.rs

Signed-off-by: xizheyin <[email protected]>

Author: xizheyin

tests/ui/borrowck/suggest-use-as-mut-for-map.rs

@@ -0,0 +1,44 @@
+// We don't suggest change `&` to `&mut`
+// instead we suggest using .get_mut() instead of &mut, see issue #143732
+
+// Custom type that implements Index<usize> but not IndexMut
+struct ReadOnlyVec<T> {
+    data: Vec<T>,
+}
+
+impl<T> ReadOnlyVec<T> {
+    fn new(data: Vec<T>) -> Self {
+        ReadOnlyVec { data }
+    }
+}
+
+impl<T> std::ops::Index<usize> for ReadOnlyVec<T> {
+    type Output = T;
+    
+    fn index(&self, index: usize) -> &Self::Output {
+        &self.data[index]
+    }
+}
+
+fn main() {
+    let mut map = std::collections::BTreeMap::new();
+    map.insert(0, "string".to_owned());
+
+    let string = &map[&0];
+    string.push_str("test"); //~ ERROR cannot borrow `*string` as mutable, as it is behind a `&` reference [E0596]
+
+    let mut map = std::collections::HashMap::new();
+    map.insert(0, "string".to_owned());
+
+    let string = &map[&0];
+    string.push_str("test"); //~ ERROR cannot borrow `*string` as mutable, as it is behind a `&` reference [E0596]
+
+    let mut vec = vec![String::new(), String::new()];
+    let string = &vec[0];
+    string.push_str("test"); //~ ERROR cannot borrow `*string` as mutable, as it is behind a `&` reference [E0596]
+
+    // Example with our custom ReadOnlyVec type
+    let read_only_vec = ReadOnlyVec::new(vec![String::new(), String::new()]);
+    let string_ref = &read_only_vec[0];
+    string_ref.push_str("test"); //~ ERROR cannot borrow `*string_ref` as mutable, as it is behind a `&` reference [E0596]
+}

tests/ui/borrowck/suggest-use-as-mut-for-map.stderr

@@ -0,0 +1,47 @@
+error[E0596]: cannot borrow `*string` as mutable, as it is behind a `&` reference
+  --> $DIR/suggest-use-as-mut-for-map.rs:28:5
+   |
+LL |     string.push_str("test");
+   |     ^^^^^^ `string` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |
+help: consider changing this to be a mutable reference
+   |
+LL |     let string = &mut map[&0];
+   |                   +++
+
+error[E0596]: cannot borrow `*string` as mutable, as it is behind a `&` reference
+  --> $DIR/suggest-use-as-mut-for-map.rs:34:5
+   |
+LL |     string.push_str("test");
+   |     ^^^^^^ `string` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |
+help: consider changing this to be a mutable reference
+   |
+LL |     let string = &mut map[&0];
+   |                   +++
+
+error[E0596]: cannot borrow `*string` as mutable, as it is behind a `&` reference
+  --> $DIR/suggest-use-as-mut-for-map.rs:38:5
+   |
+LL |     string.push_str("test");
+   |     ^^^^^^ `string` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |
+help: consider changing this to be a mutable reference
+   |
+LL |     let string = &mut vec[0];
+   |                   +++
+
+error[E0596]: cannot borrow `*string_ref` as mutable, as it is behind a `&` reference
+  --> $DIR/suggest-use-as-mut-for-map.rs:43:5
+   |
+LL |     string_ref.push_str("test");
+   |     ^^^^^^^^^^ `string_ref` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |
+help: consider changing this to be a mutable reference
+   |
+LL |     let string_ref = &mut read_only_vec[0];
+   |                       +++
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0596`.