Add option space_before_fn_sig_paren

Configurations.md

@@ -2095,6 +2095,52 @@ fn lorem<T : Eq>(t : T) {
 
 See also: [`space_after_colon`](#space_after_colon).
 
+## `space_before_fn_sig_paren`
+
+Whether to put a space before the opening paren in function signatures
+
+- **Default value**: `false`
+- **Possible values**: `true`, `false`
+- **Stable**: No
+
+#### `false` (default):
+
+```rust
+fn lorem() {
+    // body
+}
+
+fn lorem(ipsum: usize) {
+    // body
+}
+
+fn lorem<T>(ipsum: T)
+where
+    T: Add + Sub + Mul + Div,
+{
+    // body
+}
+```
+
+#### `true`:
+
+```rust
+fn lorem () {
+    // body
+}
+
+fn lorem (ipsum: usize) {
+    // body
+}
+
+fn lorem<T> (ipsum: T)
+where
+    T: Add + Sub + Mul + Div,
+{
+    // body
+}
+```
+
 ## `spaces_around_ranges`
 
 Put spaces around the .., ..=, and ... range operators

src/config.rs

@@ -94,6 +94,8 @@ create_config! {
     spaces_around_ranges: bool, false, false, "Put spaces around the  .. and ..= range operators";
     binop_separator: SeparatorPlace, SeparatorPlace::Front, true,
         "Where to put a binary operator when a binary expression goes multiline";
+    space_before_fn_sig_paren: bool, false, false,
+        "Whether to put a space before the opening paren in function signatures";
 
     // Misc.
     remove_nested_parens: bool, true, true, "Remove nested parens";
@@ -597,6 +599,7 @@ space_after_colon = true
 space_around_attr_eq = true
 spaces_around_ranges = false
 binop_separator = "Front"
+space_before_fn_sig_paren = false
 remove_nested_parens = true
 combine_control_expr = true
 overflow_delimited_expr = false

src/formatting/items.rs

@@ -2228,6 +2228,10 @@ fn rewrite_fn_base(
         .last()
         .map_or(false, |l| l.trim_start().len() == 1);
 
+    if context.config.space_before_fn_sig_paren() {
+        result.push(' ');
+    }
+
     // Note that the width and indent don't really matter, we'll re-layout the
     // return type later anyway.
     let ret_str = fd

tests/source/configs/space_before_fn_sig_paren/example.rs

@@ -0,0 +1,19 @@
+// rustfmt-space_before_fn_sig_paren: true
+// rustfmt-max_width: 30
+// Function space before function paren
+
+fn foo() {
+    // ...
+}
+fn foo_with_multi_lined(a: u32, b: u32, c: u32) {
+    // ...
+}
+fn foo<T>(bar: T) {
+    // ...
+}
+fn foo<T>(a: T, b: u32, c: u32) {
+    // ...
+}
+fn foo<T: Foo + Bar, F: FooBar>(a: T, b: u32, c: u32) {
+    // ...
+}

tests/source/configs/space_before_fn_sig_paren/fn.rs

@@ -0,0 +1,17 @@
+// rustfmt-space_before_fn_sig_paren: true
+// Function space before function paren
+
+fn lorem() {
+    // body
+}
+
+fn lorem(ipsum: usize) {
+    // body
+}
+
+fn lorem<T>(ipsum: T)
+where
+    T: Add + Sub + Mul + Div,
+{
+    // body
+}

tests/source/configs/space_before_fn_sig_paren/indent_style.rs

@@ -0,0 +1,20 @@
+// rustfmt-space_before_fn_sig_paren: true
+// rustfmt-indent_style: Visual
+// rustfmt-max_width: 30
+// Function space before function paren
+
+fn foo() {
+    // ...
+}
+fn foo_with_multi_lined(a: u32, b: u32, c: u32) {
+    // ...
+}
+fn foo<T>(bar: T) {
+    // ...
+}
+fn foo<T>(a: T, b: u32, c: u32) {
+    // ...
+}
+fn foo<T: Foo + Bar, F: FooBar>(a: T, b: u32, c: u32) {
+    // ...
+}

tests/source/configs/space_before_fn_sig_paren/max_width.rs

@@ -0,0 +1,13 @@
+// rustfmt-space_before_fn_sig_paren: true
+// rustfmt-max_width: 118
+// Trait space before function paren
+
+trait Story {
+    fn swap_context<T: 'static + Context + Send + Sync>(&mut self, context: T) -> Option<Box<Context + Send + Sync>>;
+}
+
+impl Story for () {
+    fn swap_context<T: 'static + Context + Send + Sync>(&mut self, context: T) -> Option<Box<Context + Send + Sync>> {
+        // ...
+    }
+}

tests/source/configs/space_before_fn_sig_paren/trait.rs

@@ -0,0 +1,17 @@
+// rustfmt-space_before_fn_sig_paren: true
+// Trait space before function paren
+
+trait Story {
+    fn swap_context<T>(&mut self, context: T) -> Option<Box<Context>>
+    where
+        T: Context;
+}
+
+impl Story {
+    fn swap_context<T>(&mut self, context: T) -> Option<Box<Context>>
+    where
+        T: Context,
+    {
+        // ...
+    }
+}

tests/target/configs/space_before_fn_sig_paren/example.rs

@@ -0,0 +1,34 @@
+// rustfmt-space_before_fn_sig_paren: true
+// rustfmt-max_width: 30
+// Function space before function paren
+
+fn foo () {
+    // ...
+}
+fn foo_with_multi_lined (
+    a: u32,
+    b: u32,
+    c: u32,
+) {
+    // ...
+}
+fn foo<T> (bar: T) {
+    // ...
+}
+fn foo<T> (
+    a: T,
+    b: u32,
+    c: u32,
+) {
+    // ...
+}
+fn foo<
+    T: Foo + Bar,
+    F: FooBar,
+> (
+    a: T,
+    b: u32,
+    c: u32,
+) {
+    // ...
+}

tests/target/configs/space_before_fn_sig_paren/fn.rs

@@ -0,0 +1,17 @@
+// rustfmt-space_before_fn_sig_paren: true
+// Function space before function paren
+
+fn lorem () {
+    // body
+}
+
+fn lorem (ipsum: usize) {
+    // body
+}
+
+fn lorem<T> (ipsum: T)
+where
+    T: Add + Sub + Mul + Div,
+{
+    // body
+}

tests/target/configs/space_before_fn_sig_paren/indent_style.rs

@@ -0,0 +1,29 @@
+// rustfmt-space_before_fn_sig_paren: true
+// rustfmt-indent_style: Visual
+// rustfmt-max_width: 30
+// Function space before function paren
+
+fn foo () {
+    // ...
+}
+fn foo_with_multi_lined (a: u32,
+                         b: u32,
+                         c: u32)
+{
+    // ...
+}
+fn foo<T> (bar: T) {
+    // ...
+}
+fn foo<T> (a: T,
+           b: u32,
+           c: u32) {
+    // ...
+}
+fn foo<T: Foo + Bar,
+       F: FooBar> (
+    a: T,
+    b: u32,
+    c: u32) {
+    // ...
+}

tests/target/configs/space_before_fn_sig_paren/max_width.rs

@@ -0,0 +1,17 @@
+// rustfmt-space_before_fn_sig_paren: true
+// rustfmt-max_width: 118
+// Trait space before function paren
+
+trait Story {
+    fn swap_context<T: 'static + Context + Send + Sync> (&mut self, context: T)
+    -> Option<Box<Context + Send + Sync>>;
+}
+
+impl Story for () {
+    fn swap_context<T: 'static + Context + Send + Sync> (
+        &mut self,
+        context: T,
+    ) -> Option<Box<Context + Send + Sync>> {
+        // ...
+    }
+}

tests/target/configs/space_before_fn_sig_paren/trait.rs

@@ -0,0 +1,17 @@
+// rustfmt-space_before_fn_sig_paren: true
+// Trait space before function paren
+
+trait Story {
+    fn swap_context<T> (&mut self, context: T) -> Option<Box<Context>>
+    where
+        T: Context;
+}
+
+impl Story {
+    fn swap_context<T> (&mut self, context: T) -> Option<Box<Context>>
+    where
+        T: Context,
+    {
+        // ...
+    }
+}