Add ui test for string_to_string in map calls

Author: GuillaumeGomez

tests/ui/string_to_string_in_map.rs

@@ -0,0 +1,25 @@
+#![deny(clippy::string_to_string)]
+#![allow(clippy::unnecessary_literal_unwrap, clippy::useless_vec)]
+
+fn main() {
+    let variable1 = String::new();
+    let v = &variable1;
+    let variable2 = Some(v);
+    let _ = variable2.map(String::to_string);
+    //~^ string_to_string
+    let _ = variable2.map(|x| {
+        println!();
+        x.to_string()
+    });
+    //~^^ string_to_string
+    let _ = variable2.map(|x| x.to_string());
+    //~^ string_to_string
+    let x = Some(String::new());
+    let _ = x.unwrap_or_else(|| v.to_string());
+    //~^ string_to_string
+
+    let _ = vec![String::new()].iter().map(String::to_string).collect::<Vec<_>>();
+    //~^ string_to_string
+    let _ = vec![String::new()].iter().map(|x| x.to_string()).collect::<Vec<_>>();
+    //~^ string_to_string
+}

tests/ui/string_to_string_in_map.stderr

@@ -0,0 +1,55 @@
+error: `to_string()` called on a `String`
+  --> tests/ui/string_to_string_in_map.rs:8:13
+   |
+LL |     let _ = variable2.map(String::to_string);
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using `.cloned()`
+note: the lint level is defined here
+  --> tests/ui/string_to_string_in_map.rs:1:9
+   |
+LL | #![deny(clippy::string_to_string)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: `to_string()` called on a `String`
+  --> tests/ui/string_to_string_in_map.rs:12:9
+   |
+LL |         x.to_string()
+   |         ^^^^^^^^^^^^^
+   |
+   = help: consider using `.clone()`
+
+error: `to_string()` called on a `String`
+  --> tests/ui/string_to_string_in_map.rs:15:13
+   |
+LL |     let _ = variable2.map(|x| x.to_string());
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using `.cloned()`
+
+error: `to_string()` called on a `String`
+  --> tests/ui/string_to_string_in_map.rs:18:33
+   |
+LL |     let _ = x.unwrap_or_else(|| v.to_string());
+   |                                 ^^^^^^^^^^^^^
+   |
+   = help: consider using `.clone()`
+
+error: `to_string()` called on a `String`
+  --> tests/ui/string_to_string_in_map.rs:21:13
+   |
+LL |     let _ = vec![String::new()].iter().map(String::to_string).collect::<Vec<_>>();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using `.cloned()`
+
+error: `to_string()` called on a `String`
+  --> tests/ui/string_to_string_in_map.rs:23:13
+   |
+LL |     let _ = vec![String::new()].iter().map(|x| x.to_string()).collect::<Vec<_>>();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using `.cloned()`
+
+error: aborting due to 6 previous errors
+