fn_width config option

Configurations.md

@@ -768,6 +768,29 @@ 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)
 
+
+## `enable_fn_param_limit`
+
+Switch to enable the unstable feature `fn_param_limit`.
+
+- **Default value**: `false`
+- **Possible values**: `true`, `false`
+- **Stable**: No
+
+See also [`fn_param_limit`](#fn_param_limit) 
+
+
+## `fn_param_limit`
+
+Maximum parameters in the declaration of a function signature before falling back to formatting chosen with `fn_param_layout`.
+
+- **Default value**: `4`
+- **Possible values**: any non-negative integer
+- **Stable**: No
+
+See also [`enable_fn_param_limit`](#enable_fn_param_limit) and [`fn_params_layout`](#fn_params_layout)
+
+
 ## `fn_params_layout`
 
 Control the layout of parameters in function signatures.

src/config/mod.rs

@@ -87,6 +87,9 @@ create_config! {
         "Put small struct literals on a single line";
     fn_single_line: bool, false, false, "Put single-expression functions on a single line";
     where_single_line: bool, false, false, "Force where-clauses to be on a single line";
+    enable_fn_param_limit: bool, false, false, "Switch to enable `fn_param_limit`";
+    fn_param_limit: usize, 4, false, "How many parameters in a function declaration before \
+        falling back to formatting chosen with `fn_param_layout`";
 
     // Imports
     imports_indent: IndentStyle, IndentStyle::Block, false, "Indent of imports";
@@ -649,6 +652,8 @@ empty_item_single_line = true
 struct_lit_single_line = true
 fn_single_line = false
 where_single_line = false
+enable_fn_param_limit = false
+fn_param_limit = 4
 imports_indent = "Block"
 imports_layout = "Mixed"
 imports_granularity = "Preserve"

src/items.rs

@@ -2390,6 +2390,7 @@ fn rewrite_fn_base(
         ret_str_len,
         fn_brace_style,
         multi_line_ret_str,
+        fd.inputs.len(),
     )?;
 
     debug!(
@@ -2779,6 +2780,7 @@ fn compute_budgets_for_params(
     ret_str_len: usize,
     fn_brace_style: FnBraceStyle,
     force_vertical_layout: bool,
+    param_count: usize,
 ) -> Option<(usize, usize, Indent)> {
     debug!(
         "compute_budgets_for_params {} {:?}, {}, {:?}",
@@ -2787,8 +2789,10 @@ fn compute_budgets_for_params(
         ret_str_len,
         fn_brace_style,
     );
+    let over_param_limit =
+        context.config.enable_fn_param_limit() && param_count > context.config.fn_param_limit();
     // Try keeping everything on the same line.
-    if !result.contains('\n') && !force_vertical_layout {
+    if !result.contains('\n') && !force_vertical_layout && !over_param_limit {
         // 2 = `()`, 3 = `() `, space is before ret_string.
         let overhead = if ret_str_len == 0 { 2 } else { 3 };
         let mut used_space = indent.width() + result.len() + ret_str_len + overhead;

tests/source/fn_param_limit/0.rs

@@ -0,0 +1,95 @@
+// rustfmt-enable_fn_param_limit: true
+// rustfmt-fn_param_limit: 0
+// rustfmt-edition: 2018
+
+fn lorem(mut commands: Commands) {
+    // block
+}
+fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+fn lorem(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>) {
+    // block
+}
+fn lorem(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>, mut materials: ResMut<Assets<StandardMaterial>>) {
+    // block
+}
+
+fn comments(mut commands: Commands, /**/ icons: Res<Icons>) {
+    // block
+}
+
+fn comments(mut commands: Commands, /* really loooooong intermission */ icons: Res<Icons>) {
+    // block
+}
+
+fn generic<C>(query: Query<C>, query2: Query<C>) {
+    // block
+}
+
+fn lorem<C>(query: Query<C>) where C: Add + Sub + Mul + Div {
+    // body
+}
+
+fn lorem<C>(query: Query<C>, mut commands: Commands, icons: Res<Icons>) where C: Add + Sub + Mul + Div {
+    // body
+}
+
+fn lorem(mut commands: Commands, icons: Res<Icons>) -> bool {
+    // block
+}
+
+pub fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+
+pub(crate) fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+
+pub(super) fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+
+async fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+
+unsafe fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+
+extern "C" {
+    fn lorem(mut commands: Commands, icons: Res<Icons>) {
+        // block
+    }
+}
+
+impl Trait {
+    fn lorem(mut commands: Commands);
+    fn lorem(mut commands: Commands, icons: Res<Icons>);
+    fn lorem(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>);
+    fn lorem(
+        mut commands: Commands,
+        icons: Res<Icons>,
+        mut meshes: ResMut<Assets<Mesh>>,
+        mut materials: ResMut<Assets<StandardMaterial>>,
+    );
+}
+
+fn outer(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>) {
+    // block
+
+    fn inner(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>) {
+        // block
+    }
+}
+
+mod example {
+    fn mod_func(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>) {
+        // block
+        fn nested(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>) {
+            // block
+        }
+    }
+}

tests/source/fn_param_limit/1.rs

@@ -0,0 +1,95 @@
+// rustfmt-enable_fn_param_limit: true
+// rustfmt-fn_param_limit: 1
+// rustfmt-edition: 2018
+
+fn lorem(mut commands: Commands) {
+    // block
+}
+fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+fn lorem(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>) {
+    // block
+}
+fn lorem(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>, mut materials: ResMut<Assets<StandardMaterial>>) {
+    // block
+}
+
+fn comments(mut commands: Commands, /**/ icons: Res<Icons>) {
+    // block
+}
+
+fn comments(mut commands: Commands, /* really loooooong intermission */ icons: Res<Icons>) {
+    // block
+}
+
+fn generic<C>(query: Query<C>, query2: Query<C>) {
+    // block
+}
+
+fn lorem<C>(query: Query<C>) where C: Add + Sub + Mul + Div {
+    // body
+}
+
+fn lorem<C>(query: Query<C>, mut commands: Commands, icons: Res<Icons>) where C: Add + Sub + Mul + Div {
+    // body
+}
+
+fn lorem(mut commands: Commands, icons: Res<Icons>) -> bool {
+    // block
+}
+
+pub fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+
+pub(crate) fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+
+pub(super) fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+
+async fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+
+unsafe fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+
+extern "C" {
+    fn lorem(mut commands: Commands, icons: Res<Icons>) {
+        // block
+    }
+}
+
+impl Trait {
+    fn lorem(mut commands: Commands);
+    fn lorem(mut commands: Commands, icons: Res<Icons>);
+    fn lorem(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>);
+    fn lorem(
+        mut commands: Commands,
+        icons: Res<Icons>,
+        mut meshes: ResMut<Assets<Mesh>>,
+        mut materials: ResMut<Assets<StandardMaterial>>,
+    );
+}
+
+fn outer(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>) {
+    // block
+
+    fn inner(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>) {
+        // block
+    }
+}
+
+mod example {
+    fn mod_func(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>) {
+        // block
+        fn nested(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>) {
+            // block
+        }
+    }
+}

tests/source/fn_param_limit/2.rs

@@ -0,0 +1,95 @@
+// rustfmt-enable_fn_param_limit: true
+// rustfmt-fn_param_limit: 2
+// rustfmt-edition: 2018
+
+fn lorem(mut commands: Commands) {
+    // block
+}
+fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+fn lorem(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>) {
+    // block
+}
+fn lorem(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>, mut materials: ResMut<Assets<StandardMaterial>>) {
+    // block
+}
+
+fn comments(mut commands: Commands, /**/ icons: Res<Icons>) {
+    // block
+}
+
+fn comments(mut commands: Commands, /* really really loooooooong intermission */ icons: Res<Icons>) {
+    // block
+}
+
+fn generic<C>(query: Query<C>, query2: Query<C>) {
+    // block
+}
+
+fn lorem<C>(query: Query<C>) where C: Add + Sub + Mul + Div {
+    // body
+}
+
+fn lorem<C>(query: Query<C>, mut commands: Commands, icons: Res<Icons>) where C: Add + Sub + Mul + Div {
+    // body
+}
+
+fn lorem(mut commands: Commands, icons: Res<Icons>) -> bool {
+    // block
+}
+
+pub fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+
+pub(crate) fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+
+pub(super) fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+
+async fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+
+unsafe fn lorem(mut commands: Commands, icons: Res<Icons>) {
+    // block
+}
+
+extern "C" {
+    fn lorem(mut commands: Commands, icons: Res<Icons>) {
+        // block
+    }
+}
+
+impl Trait {
+    fn lorem(mut commands: Commands);
+    fn lorem(mut commands: Commands, icons: Res<Icons>);
+    fn lorem(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>);
+    fn lorem(
+        mut commands: Commands,
+        icons: Res<Icons>,
+        mut meshes: ResMut<Assets<Mesh>>,
+        mut materials: ResMut<Assets<StandardMaterial>>,
+    );
+}
+
+fn outer(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>) {
+    // block
+
+    fn inner(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>) {
+        // block
+    }
+}
+
+mod example {
+    fn mod_func(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>) {
+        // block
+        fn nested(mut commands: Commands, icons: Res<Icons>, mut meshes: ResMut<Assets<Mesh>>) {
+            // block
+        }
+    }
+}