Add fn_arg_one_line option

If set, arguments will be kept on one line if they fit. Currently only
applies when `fn_args_layout` is set to `Block`.

This commit also fixes a bug where newlines were inserted inbetween
argument brackets when there were no arguments and `fn_args_layout` was
set to `Block`.

src/config.rs

@@ -310,6 +310,7 @@ create_config! {
     fn_args_density: Density, Density::Tall, "Argument density in functions";
     fn_args_layout: StructLitStyle, StructLitStyle::Visual, "Layout of function arguments";
     fn_arg_indent: BlockIndentStyle, BlockIndentStyle::Visual, "Indent on function arguments";
+    fn_arg_one_line: bool, false, "Keep arguments on one line if they fit";
     type_punctuation_density: TypeDensity, TypeDensity::Wide,
         "Determines if '+' or '=' are wrapped in spaces in the punctuation of types";
     // Should we at least try to put the where clause on the same line as the rest of the

src/items.rs

@@ -1293,6 +1293,10 @@ fn rewrite_fn_base(context: &RewriteContext,
     let (mut one_line_budget, multi_line_budget, mut arg_indent) =
         compute_budgets_for_args(context, &result, indent, ret_str_len, newline_brace);
 
+    if context.config.fn_args_layout == StructLitStyle::Block {
+        arg_indent = indent.block_indent(context.config);
+    }
+
     debug!("rewrite_fn: one_line_budget: {}, multi_line_budget: {}, arg_indent: {:?}",
            one_line_budget,
            multi_line_budget,
@@ -1309,10 +1313,6 @@ fn rewrite_fn_base(context: &RewriteContext,
             result.push_str("(\n");
             result.push_str(&arg_indent.to_string(context.config));
         }
-    } else if context.config.fn_args_layout == StructLitStyle::Block {
-        arg_indent = indent.block_indent(context.config);
-        result.push_str("(\n");
-        result.push_str(&arg_indent.to_string(context.config));
     } else {
         result.push('(');
     }
@@ -1336,12 +1336,25 @@ fn rewrite_fn_base(context: &RewriteContext,
                                         arg_indent,
                                         args_span,
                                         fd.variadic));
-    result.push_str(&arg_str);
-    if context.config.fn_args_layout == StructLitStyle::Block {
+
+    let multi_line_arg_str = arg_str.contains('\n');
+
+    let should_put_args_in_block = context.config.fn_args_layout == StructLitStyle::Block &&
+                                   (multi_line_arg_str || !context.config.fn_arg_one_line) &&
+                                   fd.inputs.len() > 0;
+
+    if should_put_args_in_block {
+        arg_indent = indent.block_indent(context.config);
+        result.push('\n');
+        result.push_str(&arg_indent.to_string(context.config));
+        result.push_str(&arg_str);
         result.push('\n');
         result.push_str(&indent.to_string(context.config));
+        result.push(')');
+    } else {
+        result.push_str(&arg_str);
+        result.push(')');
     }
-    result.push(')');
 
     // Return type.
     if !ret_str.is_empty() {

tests/source/fn_arg_one_line.rs

@@ -0,0 +1,48 @@
+// rustfmt-fn_args_layout: Block
+// rustfmt-fn_arg_one_line: true
+
+fn foo() {
+    foo();
+}
+
+fn foo(a: Aaaaaaaaaaaaaa, b: Bbbbbbbbbbbbbb) {
+    foo();
+}
+
+fn bar(a: Aaaaaaaaaaaaaa, b: Bbbbbbbbbbbbbb, c: Cccccccccccccccccc, d: Dddddddddddddddd, e: Eeeeeeeeeeeeeee) {
+    bar();
+}
+
+fn foo(a: Aaaaaaaaaaaaaa, b: Bbbbbbbbbbbbbb) -> String {
+    foo();
+}
+
+fn bar(a: Aaaaaaaaaaaaaa, b: Bbbbbbbbbbbbbb, c: Cccccccccccccccccc, d: Dddddddddddddddd, e: Eeeeeeeeeeeeeee) -> String {
+    bar();
+}
+
+fn foo(a: u8 /* Comment 1 */, b: u8 /* Comment 2 */) -> u8 {
+    bar()
+}
+
+fn foo(a: u8 /* Comment 1 */, b: Bbbbbbbbbbbbbb, c: Cccccccccccccccccc, d: Dddddddddddddddd, e: Eeeeeeeeeeeeeee /* Comment 2 */) -> u8 {
+    bar()
+}
+
+fn bar(a: Aaaaaaaaaaaaaa, b: Bbbbbbbbbbbbbb, c: Cccccccccccccccccc, d: Dddddddddddddddd, e: Eeeeeeeeeeeeeee) -> String where X: Fooooo, Y: Baaar  {
+    bar();
+}
+
+fn foo() -> T {
+    foo();
+}
+
+fn foo() -> T where X: Foooo, Y: Baaar {
+    foo();
+}
+
+trait Test {
+    fn foo(a: u8) {}
+
+    fn bar(a: Aaaaaaaaaaaaaa, b: Bbbbbbbbbbbbbb, c: Cccccccccccccccccc, d: Dddddddddddddddd, e: Eeeeeeeeeeeeeee) -> String {}
+}

tests/source/fn_args_layout-block.rs

@@ -1,5 +1,8 @@
 // rustfmt-fn_args_layout: Block
 
+fn foo() {
+    foo();
+}
 
 fn foo(a: Aaaaaaaaaaaaaa, b: Bbbbbbbbbbbbbb) {
     foo();

tests/target/fn_arg_one_line.rs

@@ -0,0 +1,85 @@
+// rustfmt-fn_args_layout: Block
+// rustfmt-fn_arg_one_line: true
+
+fn foo() {
+    foo();
+}
+
+fn foo(a: Aaaaaaaaaaaaaa, b: Bbbbbbbbbbbbbb) {
+    foo();
+}
+
+fn bar(
+    a: Aaaaaaaaaaaaaa,
+    b: Bbbbbbbbbbbbbb,
+    c: Cccccccccccccccccc,
+    d: Dddddddddddddddd,
+    e: Eeeeeeeeeeeeeee
+) {
+    bar();
+}
+
+fn foo(a: Aaaaaaaaaaaaaa, b: Bbbbbbbbbbbbbb) -> String {
+    foo();
+}
+
+fn bar(
+    a: Aaaaaaaaaaaaaa,
+    b: Bbbbbbbbbbbbbb,
+    c: Cccccccccccccccccc,
+    d: Dddddddddddddddd,
+    e: Eeeeeeeeeeeeeee
+) -> String {
+    bar();
+}
+
+fn foo(a: u8 /* Comment 1 */, b: u8 /* Comment 2 */) -> u8 {
+    bar()
+}
+
+fn foo(
+    a: u8, // Comment 1
+    b: Bbbbbbbbbbbbbb,
+    c: Cccccccccccccccccc,
+    d: Dddddddddddddddd,
+    e: Eeeeeeeeeeeeeee // Comment 2
+) -> u8 {
+    bar()
+}
+
+fn bar(
+    a: Aaaaaaaaaaaaaa,
+    b: Bbbbbbbbbbbbbb,
+    c: Cccccccccccccccccc,
+    d: Dddddddddddddddd,
+    e: Eeeeeeeeeeeeeee
+) -> String
+    where X: Fooooo,
+          Y: Baaar
+{
+    bar();
+}
+
+fn foo() -> T {
+    foo();
+}
+
+fn foo() -> T
+    where X: Foooo,
+          Y: Baaar
+{
+    foo();
+}
+
+trait Test {
+    fn foo(a: u8) {}
+
+    fn bar(
+        a: Aaaaaaaaaaaaaa,
+        b: Bbbbbbbbbbbbbb,
+        c: Cccccccccccccccccc,
+        d: Dddddddddddddddd,
+        e: Eeeeeeeeeeeeeee
+    ) -> String {
+    }
+}

tests/target/fn_args_layout-block.rs

@@ -1,5 +1,8 @@
 // rustfmt-fn_args_layout: Block
 
+fn foo() {
+    foo();
+}
 
 fn foo(
     a: Aaaaaaaaaaaaaa, b: Bbbbbbbbbbbbbb