Added new option "chain_count" to limit number of chained functions per line.

Configurations.md

@@ -307,6 +307,58 @@ By default this option is set as a percentage of [`max_width`](#max_width) provi
 
 See also [`max_width`](#max_width) and [`use_small_heuristics`](#use_small_heuristics)
 
+## `chain_count`
+
+Maximum number of chained function calls to fit on one line.
+
+- **Default value**: `0`
+- **Possible values**: any positive integer less than `4294967296`, although anything larger than one quarter the value of [`chain_width`](#chain_width) will have no effect as the minimum size of a function chain is 4 (e.g. `.a()`).
+- **Stable**: No (tracking issue: [#2263](https://github.com/rust-lang/rustfmt/issues/2263))
+
+This option co-exists with [`chain_width`](#chain_width) and chained
+method calls will be wrapped if either option is triggered. Any line
+with more method chains than defined in this option will have every
+chained call put on a new line.
+
+Setting this option to `0` disables this rule, and `1` puts every single
+chained call onto a newline.
+
+#### `0` (default):
+
+```rust
+fn main() {
+    a.foo().bar().baz();
+}
+```
+
+#### `1`:
+
+```rust
+fn main() {
+    a.foo()
+        .bar()
+        .baz();
+}
+```
+
+#### `2`:
+
+```rust
+fn main() {
+    a.foo()
+        .bar()
+        .baz();
+}
+```
+
+#### `3`:
+
+```rust
+fn main() {
+    a.foo().bar().baz();
+}
+```
+
 ## `color`
 
 Whether to use colored output or not.

src/chains.rs

@@ -717,7 +717,10 @@ impl<'a> ChainFormatterShared<'a> {
     }
 
     fn join_rewrites(&self, context: &RewriteContext<'_>, child_shape: Shape) -> Option<String> {
-        let connector = if self.fits_single_line {
+        let force_multiline =
+            self.child_count > context.config.chain_count() && context.config.chain_count() != 0;
+
+        let connector = if self.fits_single_line && !force_multiline {
             // Yay, we can put everything on one line.
             Cow::from("")
         } else {

src/config/mod.rs

@@ -56,6 +56,7 @@ create_config! {
     array_width: usize, 60, true,  "Maximum width of an array literal before falling \
         back to vertical formatting.";
     chain_width: usize, 60, true, "Maximum length of a chain to fit on a single line.";
+    chain_count: usize, 0, false, "Maximum number of chained method calls to fit on a single line.";
     single_line_if_else_max_width: usize, 50, true, "Maximum line length for single line if-else \
         expressions. A value of zero means always break if-else expressions.";
 
@@ -618,6 +619,7 @@ struct_lit_width = 18
 struct_variant_width = 35
 array_width = 60
 chain_width = 60
+chain_count = 0
 single_line_if_else_max_width = 50
 wrap_comments = false
 format_code_in_doc_comments = false

tests/config/issue-2263.toml

@@ -0,0 +1 @@
+chain_count = 3

tests/source/chains.rs

@@ -214,6 +214,14 @@ impl Foo {
     }
 }
 
+// #2263
+// Limit chain functions per line.
+fn issue2263() {
+    let a = "test";
+
+    a.to_string().to_string().to_string();
+}
+
 // #2415
 // Avoid orphan in chain
 fn issue2415() {

tests/source/issue-2263.rs

@@ -0,0 +1,10 @@
+fn main() {
+    let foo = "foo";
+
+    foo.to_string().to_string();
+    foo.to_string().to_string().to_string();
+    foo.to_string()
+        .to_string()
+        .to_string();
+    foo.to_string().to_string().to_string().to_string();
+}

tests/target/chains.rs

@@ -252,6 +252,14 @@ impl Foo {
     }
 }
 
+// #2263
+// Limit chain functions per line.
+fn issue2263() {
+    let a = "test";
+
+    a.to_string().to_string().to_string();
+}
+
 // #2415
 // Avoid orphan in chain
 fn issue2415() {

tests/target/issue-2263.rs

@@ -0,0 +1,11 @@
+fn main() {
+    let foo = "foo";
+
+    foo.to_string().to_string();
+    foo.to_string().to_string().to_string();
+    foo.to_string().to_string().to_string();
+    foo.to_string()
+        .to_string()
+        .to_string()
+        .to_string();
+}