Fix empty trim_newline panic, add impl macro test

src/utils.rs

@@ -146,7 +146,11 @@ pub fn semicolon_for_stmt(stmt: &ast::Stmt) -> bool {
 pub fn trim_newlines(input: &str) -> &str {
     let start = input.find(|c| c != '\n' && c != '\r').unwrap_or(0);
     let end = input.rfind(|c| c != '\n' && c != '\r').unwrap_or(0) + 1;
-    &input[start..end]
+    if start == 0 && end == 1 {
+        input
+    } else {
+        &input[start..end]
+    }
 }
 
 #[inline]

tests/source/impls.rs

@@ -51,3 +51,10 @@ mod b {
         }
     }
 }
+
+impl Foo { add_fun!(); }
+
+impl Blah {
+    fn boop() {}
+    add_fun!();
+}

tests/target/impls.rs

@@ -63,3 +63,12 @@ mod b {
         }
     }
 }
+
+impl Foo {
+    add_fun!();
+}
+
+impl Blah {
+    fn boop() {}
+    add_fun!();
+}