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

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

Author: xizheyin

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

@@ -0,0 +1,19 @@
+// We don't suggest change `&` to `&mut`
+// instead we suggest using .get_mut() instead of &mut, see issue #143732
+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]
+}

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

@@ -0,0 +1,36 @@
+error[E0596]: cannot borrow `*string` as mutable, as it is behind a `&` reference
+  --> $DIR/suggest-use-as-mut-for-map.rs:8:5
+   |
+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:14:5
+   |
+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:18:5
+   |
+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: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0596`.