[unnecessary_fold]: suggest turbofish if necessary

tests/ui/unnecessary_fold.fixed

@@ -49,4 +49,28 @@ fn unnecessary_fold_over_multiple_lines() {
         .any(|x| x > 2);
 }
 
+fn issue10000() {
+    use std::collections::HashMap;
+    use std::hash::BuildHasher;
+
+    fn anything<T>(_: T) {}
+    fn num(_: i32) {}
+    fn smoketest_map<S: BuildHasher>(mut map: HashMap<i32, i32, S>) {
+        map.insert(0, 0);
+        assert_eq!(map.values().sum::<i32>(), 0);
+
+        // more cases:
+        let _ = map.values().sum::<i32>();
+        let _ = map.values().product::<i32>();
+        let _: i32 = map.values().sum();
+        let _: i32 = map.values().product();
+        anything(map.values().sum::<i32>());
+        anything(map.values().product::<i32>());
+        num(map.values().sum());
+        num(map.values().product());
+    }
+
+    smoketest_map(HashMap::new());
+}
+
 fn main() {}

tests/ui/unnecessary_fold.rs

@@ -49,4 +49,28 @@ fn unnecessary_fold_over_multiple_lines() {
         .fold(false, |acc, x| acc || x > 2);
 }
 
+fn issue10000() {
+    use std::collections::HashMap;
+    use std::hash::BuildHasher;
+
+    fn anything<T>(_: T) {}
+    fn num(_: i32) {}
+    fn smoketest_map<S: BuildHasher>(mut map: HashMap<i32, i32, S>) {
+        map.insert(0, 0);
+        assert_eq!(map.values().fold(0, |x, y| x + y), 0);
+
+        // more cases:
+        let _ = map.values().fold(0, |x, y| x + y);
+        let _ = map.values().fold(1, |x, y| x * y);
+        let _: i32 = map.values().fold(0, |x, y| x + y);
+        let _: i32 = map.values().fold(1, |x, y| x * y);
+        anything(map.values().fold(0, |x, y| x + y));
+        anything(map.values().fold(1, |x, y| x * y));
+        num(map.values().fold(0, |x, y| x + y));
+        num(map.values().fold(1, |x, y| x * y));
+    }
+
+    smoketest_map(HashMap::new());
+}
+
 fn main() {}

tests/ui/unnecessary_fold.stderr

@@ -36,5 +36,59 @@ error: this `.fold` can be written more succinctly using another method
 LL |         .fold(false, |acc, x| acc || x > 2);
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `any(|x| x > 2)`
 
-error: aborting due to 6 previous errors
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:60:33
+   |
+LL |         assert_eq!(map.values().fold(0, |x, y| x + y), 0);
+   |                                 ^^^^^^^^^^^^^^^^^^^^^ help: try: `sum::<i32>()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:63:30
+   |
+LL |         let _ = map.values().fold(0, |x, y| x + y);
+   |                              ^^^^^^^^^^^^^^^^^^^^^ help: try: `sum::<i32>()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:64:30
+   |
+LL |         let _ = map.values().fold(1, |x, y| x * y);
+   |                              ^^^^^^^^^^^^^^^^^^^^^ help: try: `product::<i32>()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:65:35
+   |
+LL |         let _: i32 = map.values().fold(0, |x, y| x + y);
+   |                                   ^^^^^^^^^^^^^^^^^^^^^ help: try: `sum()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:66:35
+   |
+LL |         let _: i32 = map.values().fold(1, |x, y| x * y);
+   |                                   ^^^^^^^^^^^^^^^^^^^^^ help: try: `product()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:67:31
+   |
+LL |         anything(map.values().fold(0, |x, y| x + y));
+   |                               ^^^^^^^^^^^^^^^^^^^^^ help: try: `sum::<i32>()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:68:31
+   |
+LL |         anything(map.values().fold(1, |x, y| x * y));
+   |                               ^^^^^^^^^^^^^^^^^^^^^ help: try: `product::<i32>()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:69:26
+   |
+LL |         num(map.values().fold(0, |x, y| x + y));
+   |                          ^^^^^^^^^^^^^^^^^^^^^ help: try: `sum()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> $DIR/unnecessary_fold.rs:70:26
+   |
+LL |         num(map.values().fold(1, |x, y| x * y));
+   |                          ^^^^^^^^^^^^^^^^^^^^^ help: try: `product()`
+
+error: aborting due to 15 previous errors