Fix turbofish tests to do what they say they do, and add negative variants to make sure the logic is correct

Author: viliml

tests/ui/unnecessary_fold.fixed

@@ -87,11 +87,17 @@ fn issue10000() {
 
     smoketest_map(HashMap::new());
 
-    fn add_no_turbofish_necessary() -> i32 {
+    fn add_turbofish_not_necessary() -> i32 {
         (0..3).sum()
     }
-    fn mul_no_turbofish_necessary() -> i32 {
-        (0..3).sum()
+    fn mul_turbofish_not_necessary() -> i32 {
+        (0..3).product()
+    }
+    fn add_turbofish_necessary() -> impl Add {
+        (0..3).sum::<i32>()
+    }
+    fn mul_turbofish_necessary() -> impl Mul {
+        (0..3).product::<i32>()
     }
 }
 

tests/ui/unnecessary_fold.rs

@@ -87,12 +87,18 @@ fn issue10000() {
 
     smoketest_map(HashMap::new());
 
-    fn add_no_turbofish_necessary() -> i32 {
+    fn add_turbofish_not_necessary() -> i32 {
         (0..3).fold(0, |acc, x| acc + x)
     }
-    fn mul_no_turbofish_necessary() -> i32 {
+    fn mul_turbofish_not_necessary() -> i32 {
+        (0..3).fold(1, |acc, x| acc * x)
+    }
+    fn add_turbofish_necessary() -> impl Add {
         (0..3).fold(0, |acc, x| acc + x)
     }
+    fn mul_turbofish_necessary() -> impl Mul {
+        (0..3).fold(1, |acc, x| acc * x)
+    }
 }
 
 fn main() {}

tests/ui/unnecessary_fold.stderr

@@ -169,8 +169,20 @@ LL |         (0..3).fold(0, |acc, x| acc + x)
 error: this `.fold` can be written more succinctly using another method
   --> tests/ui/unnecessary_fold.rs:94:16
    |
+LL |         (0..3).fold(1, |acc, x| acc * x)
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `product()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> tests/ui/unnecessary_fold.rs:97:16
+   |
 LL |         (0..3).fold(0, |acc, x| acc + x)
-   |                ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `sum()`
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `sum::<i32>()`
+
+error: this `.fold` can be written more succinctly using another method
+  --> tests/ui/unnecessary_fold.rs:100:16
+   |
+LL |         (0..3).fold(1, |acc, x| acc * x)
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `product::<i32>()`
 
-error: aborting due to 28 previous errors
+error: aborting due to 30 previous errors