add list trailing_comma config option

Turupawn · Turupawn · Dec 22, 2025 · Nov 9, 2025 · Nov 11, 2025 · Mar 7, 2025
commit 14cff9976d65279e35bcf9516db5d8412ba5d98f
diff --git a/crates/fmt/src/ast.rs b/crates/fmt/src/ast.rs
@@ -593,7 +593,6 @@ impl Rewrite for ast::VariantDefList {
     fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String> {
         let formatting = ListFormatting::new(shape)
             .tactic(ListTactic::Vertical)
-            .trailing_separator(false)
             .with_surround("{", "}");
         format_list(self, &formatting, context, shape)
     }

diff --git a/crates/fmt/src/config.rs b/crates/fmt/src/config.rs
@@ -1,3 +1,15 @@
+/// Controls when trailing commas are added to lists.
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
+pub enum TrailingComma {
+    /// Always add trailing commas.
+    Always,
+    /// Never add trailing commas.
+    Never,
+    /// Add trailing commas only for multiline (vertical) lists.
+    #[default]
+    Multiline,
+}
+
 /// Global configuration for the Fe formatter.
 #[derive(Clone, Debug)]
 pub struct Config {
@@ -7,6 +19,8 @@ pub struct Config {
     pub indent_width: usize,
     /// Indentation for `where` and `uses` clauses, in spaces.
     pub clause_indent: usize,
+    /// When to add trailing commas in lists.
+    pub trailing_comma: TrailingComma,
 }
 
 impl Default for Config {
@@ -15,6 +29,7 @@ impl Default for Config {
             max_width: 100,
             indent_width: 4,
             clause_indent: 2,
+            trailing_comma: TrailingComma::default(),
         }
     }
 }
diff --git a/crates/fmt/src/lib.rs b/crates/fmt/src/lib.rs
@@ -10,7 +10,7 @@ mod shape;
 #[cfg(test)]
 mod tests;
 
-pub use crate::config::Config;
+pub use crate::config::{Config, TrailingComma};
 pub use crate::context::RewriteContext;
 pub use crate::format::{FormatError, format_file, format_str};
 pub use crate::list::{ListFormatting, ListTactic, format_list};

diff --git a/crates/fmt/src/list.rs b/crates/fmt/src/list.rs
@@ -1,4 +1,4 @@
-use crate::{Indent, RewriteContext, Shape, rewrite::RewriteExt};
+use crate::{Indent, RewriteContext, Shape, config::TrailingComma, rewrite::RewriteExt};
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum ListTactic {
@@ -14,7 +14,8 @@ pub enum ListTactic {
 pub struct ListFormatting<'a> {
     pub tactic: ListTactic,
     pub separator: &'a str,
-    pub trailing_separator: bool,
+    /// Override trailing separator behavior. `None` means use config default.
+    pub trailing_separator: Option<bool>,
     pub surround: Option<(&'a str, &'a str)>,
     /// Add space padding inside surrounds for horizontal mode (e.g., `{ a, b }` instead of `{a, b}`).
     pub horizontal_padding: bool,
@@ -25,7 +26,7 @@ impl<'a> Default for ListFormatting<'a> {
         Self {
             tactic: ListTactic::Mixed,
             separator: ",",
-            trailing_separator: true,
+            trailing_separator: None,
             surround: None,
             horizontal_padding: false,
         }
@@ -45,7 +46,7 @@ impl<'a> ListFormatting<'a> {
     }
 
     pub fn trailing_separator(mut self, trailing: bool) -> Self {
-        self.trailing_separator = trailing;
+        self.trailing_separator = Some(trailing);
         self
     }
 
@@ -84,6 +85,14 @@ where
         return Some(format!("{}{}", open, close));
     }
 
+    // Resolve trailing separator behavior from override or config.
+    let trailing_for_horizontal = formatting.trailing_separator.unwrap_or_else(|| {
+        matches!(context.config.trailing_comma, TrailingComma::Always)
+    });
+    let trailing_for_vertical = formatting.trailing_separator.unwrap_or_else(|| {
+        !matches!(context.config.trailing_comma, TrailingComma::Never)
+    });
+
     // 1. Try Horizontal
     if matches!(formatting.tactic, ListTactic::Horizontal | ListTactic::Mixed) {
         let item_strings: Vec<String> = items
@@ -99,9 +108,7 @@ where
         let inline_len = joined.len();
 
         if inline_len <= available_width {
-            let trailing = if formatting.trailing_separator
-                && matches!(formatting.tactic, ListTactic::Horizontal)
-            {
+            let trailing = if trailing_for_horizontal {
                 formatting.separator
             } else {
                 ""
@@ -116,28 +123,28 @@ where
             return Some(result);
         }
     }
- 
+
     // 2. Vertical
     let indent_width = context.config.indent_width;
     let item_indent = shape.indent.block_indent + indent_width;
     let item_shape = Shape::with_width(shape.width, Indent::from_block(item_indent));
- 
+
     let mut out = String::new();
     out.push_str(open);
     out.push('\n');
- 
+
     for (idx, item) in items.iter().enumerate() {
         push_indent(&mut out, item_indent);
         out.push_str(&item.rewrite_or_original(context, item_shape));
-        if formatting.trailing_separator || idx + 1 != items.len() {
+        if trailing_for_vertical || idx + 1 != items.len() {
             out.push_str(formatting.separator);
         }
         out.push('\n');
     }
- 
+
     push_indent(&mut out, shape.indent.block_indent);
     out.push_str(close);
- 
+
     Some(out)
 }
 

diff --git a/crates/fmt/tests/fixtures/enums.snap b/crates/fmt/tests/fixtures/enums.snap
@@ -9,38 +9,38 @@ input_file: tests/fixtures/enums.fe
 enum Color {
     Red,
     Green,
-    Blue
+    Blue,
 }
 
 // Enum with tuple variants
 enum Option {
     None,
-    Some(i32)
+    Some(i32),
 }
 
 // Enum with record variants
 enum Result {
     Ok { value: i32 },
-    Err { code: i32 }
+    Err { code: i32 },
 }
 
 // Mixed variants
 enum Mixed {
     Unit,
     Tuple(i32, bool),
-    Record { x: i32, y: i32 }
+    Record { x: i32, y: i32 },
 }
 
 // Enum with generics
 enum Container<T> {
     Empty,
-    Value(T)
+    Value(T),
 }
 
 // Single line enum
 enum SingleLine {
     A,
     B,
     C { x: i32, y: u8 },
-    D(i8, i8)
+    D(i8, i8),
 }