Add option space_after_function_name

Author: dylni

Configurations.md

@@ -2063,6 +2063,52 @@ fn lorem<T:Eq>(t:T) {
 
 See also: [`space_before_colon`](#space_before_colon).
 
+## `space_after_function_name`
+
+Leave a space after the function name.
+
+- **Default value**: `"Never"`
+- **Possible values**: `"AfterGenerics"`, `"Never"`
+- **Stable**: No (tracking issue: [#3564](https://github.com/rust-lang/rustfmt/issues/3564))
+
+#### `"Never"` (default):
+
+```rust
+fn lorem() {
+    // body
+}
+
+fn lorem(ipsum: usize) {
+    // body
+}
+
+fn lorem<T>(ipsum: T)
+where
+    T: Add + Sub + Mul + Div,
+{
+    // body
+}
+```
+
+#### `"AfterGenerics"`:
+
+```rust
+fn lorem () {
+    // body
+}
+
+fn lorem (ipsum: usize) {
+    // body
+}
+
+fn lorem<T> (ipsum: T)
+where
+    T: Add + Sub + Mul + Div,
+{
+    // body
+}
+```
+
 ## `space_before_colon`
 
 Leave a space before the colon.

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_after_function_name: SpaceAfterFunctionName, SpaceAfterFunctionName::Never, false,
+        "Leave a space after the function name.";
 
     // 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_after_function_name = "Never"
 remove_nested_parens = true
 combine_control_expr = true
 overflow_delimited_expr = false

src/config/options.rs

@@ -359,6 +359,15 @@ pub enum MatchArmLeadingPipe {
     KeepExisting,
 }
 
+#[config_type]
+/// Where to add spaces after the function name.
+pub enum SpaceAfterFunctionName {
+    /// Never add spaces.
+    Never,
+    /// Add a space, and put generics before the space.
+    AfterGenerics,
+}
+
 #[cfg(test)]
 mod test {
     use std::path::PathBuf;

src/formatting/items.rs

@@ -9,7 +9,7 @@ use rustc_ast::{ast, ptr};
 use rustc_span::{source_map, symbol, BytePos, Span, DUMMY_SP};
 
 use crate::config::lists::*;
-use crate::config::{BraceStyle, Config, IndentStyle};
+use crate::config::{BraceStyle, Config, IndentStyle, SpaceAfterFunctionName};
 use crate::formatting::{
     attr::filter_inline_attrs,
     comment::{
@@ -2228,6 +2228,10 @@ fn rewrite_fn_base(
         .last()
         .map_or(false, |l| l.trim_start().len() == 1);
 
+    if let SpaceAfterFunctionName::AfterGenerics = context.config.space_after_function_name() {
+        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_after_function_name/fn_after_generics.rs

@@ -0,0 +1,17 @@
+// rustfmt-space_after_function_name: AfterGenerics
+// Function space after function name
+
+fn lorem() {
+    // body
+}
+
+fn lorem(ipsum: usize) {
+    // body
+}
+
+fn lorem<T>(ipsum: T)
+where
+    T: Add + Sub + Mul + Div,
+{
+    // body
+}

tests/source/configs/space_after_function_name/trait_after_generics.rs

@@ -0,0 +1,17 @@
+// rustfmt-space_after_function_name: AfterGenerics
+// Trait space after function name
+
+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/source/configs/space_after_function_name/width_heuristics.rs

@@ -0,0 +1,13 @@
+// rustfmt-space_after_function_name: AfterGenerics
+// rustfmt-max_width: 118
+// Trait space after function name
+
+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_after_function_name/fn_after_generics.rs

@@ -0,0 +1,17 @@
+// rustfmt-space_after_function_name: AfterGenerics
+// Function space after function name
+
+fn lorem () {
+    // body
+}
+
+fn lorem (ipsum: usize) {
+    // body
+}
+
+fn lorem<T> (ipsum: T)
+where
+    T: Add + Sub + Mul + Div,
+{
+    // body
+}

tests/target/configs/space_after_function_name/trait_after_generics.rs

@@ -0,0 +1,17 @@
+// rustfmt-space_after_function_name: AfterGenerics
+// Trait space after function name
+
+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_after_function_name/width_heuristics.rs

@@ -0,0 +1,17 @@
+// rustfmt-space_after_function_name: AfterGenerics
+// rustfmt-max_width: 118
+// Trait space after function name
+
+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>> {
+        // ...
+    }
+}