[`unused_enumerate_index`]: Keep explicit element type

Prior to this change, it might be that the lint would remove an explicit
type that was necessary for the type system to keep track of types.

This should be the last change that prevented this lint to be machine
applicable.

3 files changed, 67 insertions, 3 deletions

diff --git a/tests/ui/unused_enumerate_index.fixed b/tests/ui/unused_enumerate_index.fixed
index 1224eb54ba5..cffd02b0acc 100644
--- a/tests/ui/unused_enumerate_index.fixed
+++ b/tests/ui/unused_enumerate_index.fixed
@@ -1,4 +1,4 @@
-#![allow(unused)]
+#![allow(unused, clippy::map_identity)]
 #![warn(clippy::unused_enumerate_index)]
 
 use std::iter::Enumerate;
@@ -89,4 +89,18 @@ fn main() {
     #[allow(clippy::unused_enumerate_index)]
     let v = [1].iter().enumerate();
     v.map(|(_, _x)| {});
+
+    // This should keep the explicit type of `x`.
+    let v = [1, 2, 3].iter().copied();
+    let x = v.map(|x: i32| x).sum::<i32>();
+    assert_eq!(x, 6);
+
+    // This should keep the explicit type of `x`.
+    let v = [1, 2, 3].iter().copied();
+    let x = v.map(|x: i32| x).sum::<i32>();
+    assert_eq!(x, 6);
+
+    let v = [1, 2, 3].iter().copied();
+    let x = v.map(|x| x).sum::<i32>();
+    assert_eq!(x, 6);
 }
diff --git a/tests/ui/unused_enumerate_index.rs b/tests/ui/unused_enumerate_index.rs
index 66f98b690cb..f2b5f8b9124 100644
--- a/tests/ui/unused_enumerate_index.rs
+++ b/tests/ui/unused_enumerate_index.rs
@@ -1,4 +1,4 @@
-#![allow(unused)]
+#![allow(unused, clippy::map_identity)]
 #![warn(clippy::unused_enumerate_index)]
 
 use std::iter::Enumerate;
@@ -89,4 +89,18 @@ fn main() {
     #[allow(clippy::unused_enumerate_index)]
     let v = [1].iter().enumerate();
     v.map(|(_, _x)| {});
+
+    // This should keep the explicit type of `x`.
+    let v = [1, 2, 3].iter().copied().enumerate();
+    let x = v.map(|(_, x): (usize, i32)| x).sum::<i32>();
+    assert_eq!(x, 6);
+
+    // This should keep the explicit type of `x`.
+    let v = [1, 2, 3].iter().copied().enumerate();
+    let x = v.map(|(_, x): (_, i32)| x).sum::<i32>();
+    assert_eq!(x, 6);
+
+    let v = [1, 2, 3].iter().copied().enumerate();
+    let x = v.map(|(_, x)| x).sum::<i32>();
+    assert_eq!(x, 6);
 }
diff --git a/tests/ui/unused_enumerate_index.stderr b/tests/ui/unused_enumerate_index.stderr
index f8babf428c0..6ec07dcbff0 100644
--- a/tests/ui/unused_enumerate_index.stderr
+++ b/tests/ui/unused_enumerate_index.stderr
@@ -58,5 +58,41 @@ LL -     _ = mac2!().enumerate().map(|(_, _v)| {});
 LL +     _ = mac2!().map(|_v| {});
    |
 
-error: aborting due to 5 previous errors
+error: you seem to use `.enumerate()` and immediately discard the index
+  --> tests/ui/unused_enumerate_index.rs:94:39
+   |
+LL |     let v = [1, 2, 3].iter().copied().enumerate();
+   |                                       ^^^^^^^^^^^
+   |
+help: remove the `.enumerate()` call
+   |
+LL ~     let v = [1, 2, 3].iter().copied();
+LL ~     let x = v.map(|x: i32| x).sum::<i32>();
+   |
+
+error: you seem to use `.enumerate()` and immediately discard the index
+  --> tests/ui/unused_enumerate_index.rs:99:39
+   |
+LL |     let v = [1, 2, 3].iter().copied().enumerate();
+   |                                       ^^^^^^^^^^^
+   |
+help: remove the `.enumerate()` call
+   |
+LL ~     let v = [1, 2, 3].iter().copied();
+LL ~     let x = v.map(|x: i32| x).sum::<i32>();
+   |
+
+error: you seem to use `.enumerate()` and immediately discard the index
+  --> tests/ui/unused_enumerate_index.rs:103:39
+   |
+LL |     let v = [1, 2, 3].iter().copied().enumerate();
+   |                                       ^^^^^^^^^^^
+   |
+help: remove the `.enumerate()` call
+   |
+LL ~     let v = [1, 2, 3].iter().copied();
+LL ~     let x = v.map(|x| x).sum::<i32>();
+   |
+
+error: aborting due to 8 previous errors