Put empty trait braces on same line if possible

Author: mujpao

src/items.rs

@@ -1122,12 +1122,24 @@ pub(crate) fn format_trait(
             }
         }
 
+        let block_span = mk_sp(generics.where_clause.span.hi(), item.span.hi());
+        let snippet = context.snippet(block_span);
+        let open_pos = snippet.find_uncommented("{")? + 1;
+
         match context.config.brace_style() {
             _ if last_line_contains_single_line_comment(&result)
                 || last_line_width(&result) + 2 > context.budget(offset.width()) =>
             {
                 result.push_str(&offset.to_string_with_newline(context.config));
             }
+            _ if context.config.empty_item_single_line()
+                && trait_items.is_empty()
+                && !result.contains('\n')
+                && !contains_comment(&snippet[open_pos..]) =>
+            {
+                result.push_str(" {}");
+                return Some(result);
+            }
             BraceStyle::AlwaysNextLine => {
                 result.push_str(&offset.to_string_with_newline(context.config));
             }
@@ -1144,9 +1156,6 @@ pub(crate) fn format_trait(
         }
         result.push('{');
 
-        let block_span = mk_sp(generics.where_clause.span.hi(), item.span.hi());
-        let snippet = context.snippet(block_span);
-        let open_pos = snippet.find_uncommented("{")? + 1;
         let outer_indent_str = offset.block_only().to_string_with_newline(context.config);
 
         if !trait_items.is_empty() || contains_comment(&snippet[open_pos..]) {

tests/source/empty-item-single-line-false.rs

@@ -0,0 +1,46 @@
+// rustfmt-brace_style: AlwaysNextLine
+// rustfmt-empty_item_single_line: false
+
+fn function()
+{
+
+}
+
+struct Struct
+{
+
+}
+
+enum Enum
+{
+
+}
+
+trait Trait
+{
+
+}
+
+impl<T> Trait for T
+{
+
+}
+
+trait Trait2<T>
+where
+    T: Copy + Display + Write + Read + FromStr, {}
+
+trait Trait3<T>
+where
+    T: Something
+        + SomethingElse
+        + Sync
+        + Send
+        + Display
+        + Debug
+        + Copy
+        + Hash
+        + Debug
+        + Display
+        + Write
+        + Read, {}

tests/source/item-brace-style-always-next-line.rs

@@ -27,3 +27,38 @@ mod M {
 
     struct D<T> where T: Copy {}
 }
+
+
+fn function()
+{
+
+}
+
+trait Trait
+{
+
+}
+
+impl<T> Trait for T
+{
+
+}
+
+trait Trait2<T>
+where
+    T: Copy + Display + Write + Read + FromStr, {}
+
+trait Trait3<T>
+where
+    T: Something
+        + SomethingElse
+        + Sync
+        + Send
+        + Display
+        + Debug
+        + Copy
+        + Hash
+        + Debug
+        + Display
+        + Write
+        + Read, {}

tests/target/empty-item-single-line-false.rs

@@ -0,0 +1,41 @@
+// rustfmt-brace_style: AlwaysNextLine
+// rustfmt-empty_item_single_line: false
+
+fn function()
+{
+}
+
+struct Struct {}
+
+enum Enum {}
+
+trait Trait
+{
+}
+
+impl<T> Trait for T
+{
+}
+
+trait Trait2<T>
+where
+    T: Copy + Display + Write + Read + FromStr,
+{
+}
+
+trait Trait3<T>
+where
+    T: Something
+        + SomethingElse
+        + Sync
+        + Send
+        + Display
+        + Debug
+        + Copy
+        + Hash
+        + Debug
+        + Display
+        + Write
+        + Read,
+{
+}

tests/target/item-brace-style-always-next-line.rs

@@ -40,3 +40,32 @@ mod M
     where
         T: Copy, {}
 }
+
+fn function() {}
+
+trait Trait {}
+
+impl<T> Trait for T {}
+
+trait Trait2<T>
+where
+    T: Copy + Display + Write + Read + FromStr,
+{
+}
+
+trait Trait3<T>
+where
+    T: Something
+        + SomethingElse
+        + Sync
+        + Send
+        + Display
+        + Debug
+        + Copy
+        + Hash
+        + Debug
+        + Display
+        + Write
+        + Read,
+{
+}