Add ui test for unnecessary_result_map_or_else

tests/ui/unnecessary_result_map_or_else.fixed

@@ -0,0 +1,61 @@
+#![warn(clippy::unnecessary_result_map_or_else)]
+#![allow(clippy::unnecessary_literal_unwrap, clippy::let_and_return, clippy::let_unit_value)]
+
+fn main() {
+    let x: Result<(), ()> = Ok(());
+    x.unwrap_or_else(|err| err); //~ ERROR: unused "map closure" when calling
+
+    // Type ascribtion.
+    let x: Result<(), ()> = Ok(());
+    x.unwrap_or_else(|err: ()| err); //~ ERROR: unused "map closure" when calling
+
+    // Auto-deref.
+    let y = String::new();
+    let x: Result<&String, &String> = Ok(&y);
+    let y: &str = x.unwrap_or_else(|err| err); //~ ERROR: unused "map closure" when calling
+
+    // Temporary variable.
+    let x: Result<(), ()> = Ok(());
+    x.unwrap_or_else(|err| err);
+
+    // Should not warn.
+    let x: Result<usize, usize> = Ok(0);
+    x.map_or_else(|err| err, |n| n + 1);
+
+    // Should not warn.
+    let y = ();
+    let x: Result<(), ()> = Ok(());
+    x.map_or_else(|err| err, |_| y);
+
+    // Should not warn.
+    let y = ();
+    let x: Result<(), ()> = Ok(());
+    x.map_or_else(
+        |err| err,
+        |_| {
+            let tmp = y;
+            tmp
+        },
+    );
+
+    // Should not warn.
+    let x: Result<usize, usize> = Ok(1);
+    x.map_or_else(
+        |err| err,
+        |n| {
+            let tmp = n + 1;
+            tmp
+        },
+    );
+
+    // Should not warn.
+    let y = 0;
+    let x: Result<usize, usize> = Ok(1);
+    x.map_or_else(
+        |err| err,
+        |n| {
+            let tmp = n;
+            y
+        },
+    );
+}

tests/ui/unnecessary_result_map_or_else.rs

@@ -0,0 +1,69 @@
+#![warn(clippy::unnecessary_result_map_or_else)]
+#![allow(clippy::unnecessary_literal_unwrap, clippy::let_and_return, clippy::let_unit_value)]
+
+fn main() {
+    let x: Result<(), ()> = Ok(());
+    x.map_or_else(|err| err, |n| n); //~ ERROR: unused "map closure" when calling
+
+    // Type ascribtion.
+    let x: Result<(), ()> = Ok(());
+    x.map_or_else(|err: ()| err, |n: ()| n); //~ ERROR: unused "map closure" when calling
+
+    // Auto-deref.
+    let y = String::new();
+    let x: Result<&String, &String> = Ok(&y);
+    let y: &str = x.map_or_else(|err| err, |n| n); //~ ERROR: unused "map closure" when calling
+
+    // Temporary variable.
+    let x: Result<(), ()> = Ok(());
+    x.map_or_else(
+        //~^ ERROR: unused "map closure" when calling
+        |err| err,
+        |n| {
+            let tmp = n;
+            let tmp2 = tmp;
+            tmp2
+        },
+    );
+
+    // Should not warn.
+    let x: Result<usize, usize> = Ok(0);
+    x.map_or_else(|err| err, |n| n + 1);
+
+    // Should not warn.
+    let y = ();
+    let x: Result<(), ()> = Ok(());
+    x.map_or_else(|err| err, |_| y);
+
+    // Should not warn.
+    let y = ();
+    let x: Result<(), ()> = Ok(());
+    x.map_or_else(
+        |err| err,
+        |_| {
+            let tmp = y;
+            tmp
+        },
+    );
+
+    // Should not warn.
+    let x: Result<usize, usize> = Ok(1);
+    x.map_or_else(
+        |err| err,
+        |n| {
+            let tmp = n + 1;
+            tmp
+        },
+    );
+
+    // Should not warn.
+    let y = 0;
+    let x: Result<usize, usize> = Ok(1);
+    x.map_or_else(
+        |err| err,
+        |n| {
+            let tmp = n;
+            y
+        },
+    );
+}

tests/ui/unnecessary_result_map_or_else.stderr

@@ -0,0 +1,35 @@
+error: unused "map closure" when calling `Result::map_or_else` value
+  --> $DIR/unnecessary_result_map_or_else.rs:6:5
+   |
+LL |     x.map_or_else(|err| err, |n| n);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `unwrap_or_else`: `x.unwrap_or_else(|err| err)`
+   |
+   = note: `-D clippy::unnecessary-result-map-or-else` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::unnecessary_result_map_or_else)]`
+
+error: unused "map closure" when calling `Result::map_or_else` value
+  --> $DIR/unnecessary_result_map_or_else.rs:10:5
+   |
+LL |     x.map_or_else(|err: ()| err, |n: ()| n);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `unwrap_or_else`: `x.unwrap_or_else(|err: ()| err)`
+
+error: unused "map closure" when calling `Result::map_or_else` value
+  --> $DIR/unnecessary_result_map_or_else.rs:15:19
+   |
+LL |     let y: &str = x.map_or_else(|err| err, |n| n);
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `unwrap_or_else`: `x.unwrap_or_else(|err| err)`
+
+error: unused "map closure" when calling `Result::map_or_else` value
+  --> $DIR/unnecessary_result_map_or_else.rs:19:5
+   |
+LL | /     x.map_or_else(
+LL | |
+LL | |         |err| err,
+LL | |         |n| {
+...  |
+LL | |         },
+LL | |     );
+   | |_____^ help: consider using `unwrap_or_else`: `x.unwrap_or_else(|err| err)`
+
+error: aborting due to 4 previous errors
+