Fix unnecessary_fold bug

clippy_lints/src/methods.rs

@@ -1141,6 +1141,13 @@ fn lint_unnecessary_fold(cx: &LateContext, expr: &hir::Expr, fold_args: &[hir::E
     assert!(fold_args.len() == 3,
         "Expected fold_args to have three entries - the receiver, the initial value and the closure");
 
+    fn is_exactly_closure_param(expr: &hir::Expr, closure_param: ast::Name) -> bool {
+        if let hir::ExprPath(hir::QPath::Resolved(None, ref path)) = expr.node {
+            return path.segments.len() == 1 && &path.segments[0].name == &closure_param;
+        }
+        false
+    }
+
     fn check_fold_with_op(
         cx: &LateContext,
         fold_args: &[hir::Expr],
@@ -1162,8 +1169,8 @@ fn lint_unnecessary_fold(cx: &LateContext, expr: &hir::Expr, fold_args: &[hir::E
             if let Some(first_arg_ident) = get_arg_name(&closure_body.arguments[0].pat);
             if let Some(second_arg_ident) = get_arg_name(&closure_body.arguments[1].pat);
 
-            if let hir::ExprPath(hir::QPath::Resolved(None, ref path)) = left_expr.node;
-            if path.segments.len() == 1 && &path.segments[0].name == &first_arg_ident;
+            if is_exactly_closure_param(&*left_expr, first_arg_ident);
+            if replacement_has_args || is_exactly_closure_param(&*right_expr, second_arg_ident);
 
             then {
                 // Span containing `.fold(...)`

tests/ui/methods.rs

@@ -419,6 +419,9 @@ fn unnecessary_fold_should_ignore() {
     let _ = (0..3).fold(true, |acc, x| x > 2 && acc);
     let _ = (0..3).fold(0, |acc, x| x + acc);
     let _ = (0..3).fold(1, |acc, x| x * acc);
+
+    let _ = [(0..2), (0..3)].iter().fold(0, |a, b| a + b.len());
+    let _ = [(0..2), (0..3)].iter().fold(1, |a, b| a * b.len());
 }
 
 #[allow(similar_names)]

tests/ui/methods.stderr

@@ -526,9 +526,9 @@ error: this `.fold` can be written more succinctly using another method
     |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `.any(|x| x > 2)`
 
 error: used unwrap() on an Option value. If you don't want to handle the None case gracefully, consider using expect() to provide a better panic message
-   --> $DIR/methods.rs:427:13
+   --> $DIR/methods.rs:430:13
     |
-427 |     let _ = opt.unwrap();
+430 |     let _ = opt.unwrap();
     |             ^^^^^^^^^^^^
     |
     = note: `-D option-unwrap-used` implied by `-D warnings`