Add chain_split_single_child option

Configurations.md

@@ -112,6 +112,28 @@ let lorem = ipsum.dolor().sit().amet().consectetur().adipiscing().elit();
 #### Lines longer than `chain_one_line_max`:
 See [`chain_indent`](#chain_indent).
 
+## `chain_split_single_child`
+
+Split a chain with a single child if its length exceeds [`chain_one_line_max`](#chain_one_line_max).
+
+- **Default value**: `false`
+- **Possible values**: `false`, `true`
+
+#### `false`
+
+```rust
+let files = fs::read_dir("tests/coverage/source").expect("Couldn't read source dir");
+```
+
+#### `true`
+
+```rust
+let files = fs::read_dir("tests/coverage/source")
+    .expect("Couldn't read source dir");
+```
+
+See also [`chain_one_line_max`](#chain_one_line_max).
+
 ## `closure_block_indent_threshold`
 
 How many lines a closure must have before it is block indented. -1 means never use block indent.

src/chains.rs

@@ -163,10 +163,10 @@ pub fn rewrite_chain(expr: &ast::Expr, context: &RewriteContext, shape: Shape) -
            first_child_shape,
            other_child_shape);
 
-    let child_shape_iter = Some(first_child_shape)
-        .into_iter()
-        .chain(::std::iter::repeat(other_child_shape)
-                   .take(subexpr_list.len() - 1));
+    let child_shape_iter =
+        Some(first_child_shape)
+            .into_iter()
+            .chain(::std::iter::repeat(other_child_shape).take(subexpr_list.len() - 1));
     let iter = subexpr_list.iter().rev().zip(child_shape_iter);
     let mut rewrites = try_opt!(iter.map(|(e, shape)| {
                                              rewrite_chain_subexpr(e, total_span, context, shape)
@@ -186,7 +186,7 @@ pub fn rewrite_chain(expr: &ast::Expr, context: &RewriteContext, shape: Shape) -
         if rewrites.len() > 1 {
             true
         } else if rewrites.len() == 1 {
-            parent_rewrite.len() > context.config.chain_one_line_max() / 2
+            context.config.chain_split_single_child() || one_line_len > shape.width
         } else {
             false
         }

tests/target/configs-chain_split_single_child-false.rs

@@ -0,0 +1,5 @@
+// rustfmt-chain_split_single_child: false
+
+fn main() {
+    let files = fs::read_dir("tests/source").expect("Couldn't read source dir");
+}

tests/target/configs-chain_split_single_child-true.rs

@@ -0,0 +1,6 @@
+// rustfmt-chain_split_single_child: true
+
+fn main() {
+    let files = fs::read_dir("tests/source")
+        .expect("Couldn't read source dir");
+}