Add ui test to ensure that if 0 is returned from both if and else, it will not break clippy

Author: GuillaumeGomez

tests/ui/manual_arithmetic_check.fixed

@@ -1,8 +1,10 @@
 #![warn(clippy::implicit_saturating_sub)]
+#![allow(clippy::if_same_then_else)]
 
 fn main() {
     let a = 12u32;
     let b = 13u32;
+    let c = 8u32;
 
     let result = a.saturating_sub(b);
     //~^ ERROR: manual arithmetic check found
@@ -13,4 +15,10 @@ fn main() {
     //~^ ERROR: manual arithmetic check found
     let result = a.saturating_sub(b);
     //~^ ERROR: manual arithmetic check found
+
+    // Should not warn!
+    let result = if a > b { a - b } else { a - c };
+
+    // Just to check it won't break clippy.
+    let result = if b > a { 0 } else { 0 };
 }

tests/ui/manual_arithmetic_check.rs

@@ -1,4 +1,5 @@
 #![warn(clippy::implicit_saturating_sub)]
+#![allow(clippy::if_same_then_else)]
 
 fn main() {
     let a = 12u32;
@@ -17,4 +18,7 @@ fn main() {
 
     // Should not warn!
     let result = if a > b { a - b } else { a - c };
+
+    // Just to check it won't break clippy.
+    let result = if b > a { 0 } else { 0 };
 }

tests/ui/manual_arithmetic_check.stderr

@@ -1,5 +1,5 @@
 error: manual arithmetic check found
-  --> tests/ui/manual_arithmetic_check.rs:7:18
+  --> tests/ui/manual_arithmetic_check.rs:9:18
    |
 LL |     let result = if a > b { a - b } else { 0 };
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace it with: `a.saturating_sub(b)`
@@ -8,19 +8,19 @@ LL |     let result = if a > b { a - b } else { 0 };
    = help: to override `-D warnings` add `#[allow(clippy::implicit_saturating_sub)]`
 
 error: manual arithmetic check found
-  --> tests/ui/manual_arithmetic_check.rs:9:18
+  --> tests/ui/manual_arithmetic_check.rs:11:18
    |
 LL |     let result = if b < a { a - b } else { 0 };
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace it with: `a.saturating_sub(b)`
 
 error: manual arithmetic check found
-  --> tests/ui/manual_arithmetic_check.rs:12:18
+  --> tests/ui/manual_arithmetic_check.rs:14:18
    |
 LL |     let result = if a < b { 0 } else { a - b };
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace it with: `a.saturating_sub(b)`
 
 error: manual arithmetic check found
-  --> tests/ui/manual_arithmetic_check.rs:14:18
+  --> tests/ui/manual_arithmetic_check.rs:16:18
    |
 LL |     let result = if b > a { 0 } else { a - b };
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace it with: `a.saturating_sub(b)`