Support where clauses on the same line as the function decl where it all fits on one line.

src/config.rs

@@ -91,6 +91,8 @@ create_config! {
     fn_args_paren_newline: bool,
     fn_args_layout: Density,
     fn_arg_indent: BlockIndentStyle,
+    where_density: Density, // Should we at least try to put the where clause on the same line as
+                            // the rest of the function decl?
     where_indent: BlockIndentStyle, // Visual will be treated like Tabbed
     where_layout: ListTactic,
     where_pred_indent: BlockIndentStyle,
@@ -121,6 +123,7 @@ impl Default for Config {
             fn_args_paren_newline: true,
             fn_args_layout: Density::Tall,
             fn_arg_indent: BlockIndentStyle::Visual,
+            where_density: Density::Tall,
             where_indent: BlockIndentStyle::Tabbed,
             where_layout: ListTactic::Vertical,
             where_pred_indent: BlockIndentStyle::Visual,

src/items.rs

@@ -18,7 +18,7 @@ use expr::rewrite_assign_rhs;
 use comment::FindUncommented;
 use visitor::FmtVisitor;
 use rewrite::Rewrite;
-use config::{Config, BlockIndentStyle};
+use config::{Config, BlockIndentStyle, Density};
 
 use syntax::{ast, abi};
 use syntax::codemap::{self, Span, BytePos};
@@ -99,7 +99,7 @@ impl<'a> FmtVisitor<'a> {
                       vis: ast::Visibility,
                       span: Span)
                       -> String {
-        let newline_brace = self.newline_for_brace(&generics.where_clause);
+        let mut newline_brace = self.newline_for_brace(&generics.where_clause);
 
         let mut result = self.rewrite_fn_base(indent,
                                               ident,
@@ -113,6 +113,10 @@ impl<'a> FmtVisitor<'a> {
                                               span,
                                               newline_brace);
 
+        if self.config.fn_brace_style != BraceStyle::AlwaysNextLine && !result.contains('\n') {
+            newline_brace = false;
+        }
+
         // Prepare for the function body by possibly adding a newline and
         // indent.
         // FIXME we'll miss anything between the end of the signature and the
@@ -281,10 +285,18 @@ impl<'a> FmtVisitor<'a> {
             }
         }
 
+        let where_density = if self.config.where_density == Density::Compressed &&
+                               !result.contains('\n') {
+            Density::Compressed
+        } else {
+            Density::Tall
+        };
+
         // Where clause.
         result.push_str(&self.rewrite_where_clause(where_clause,
                                                    self.config,
                                                    indent,
+                                                   where_density,
                                                    span.hi));
 
         result
@@ -682,6 +694,7 @@ impl<'a> FmtVisitor<'a> {
             result.push_str(&self.rewrite_where_clause(&generics.where_clause,
                                                        self.config,
                                                        self.block_indent,
+                                                       Density::Tall,
                                                        span.hi));
             result.push_str(&make_indent(self.block_indent));
             result.push('\n');
@@ -794,6 +807,7 @@ impl<'a> FmtVisitor<'a> {
                             where_clause: &ast::WhereClause,
                             config: &Config,
                             indent: usize,
+                            density: Density,
                             span_end: BytePos)
                             -> String {
         if where_clause.predicates.is_empty() {
@@ -839,10 +853,17 @@ impl<'a> FmtVisitor<'a> {
             v_width: budget,
             ends_with_newline: true,
         };
+        let preds_str = write_list(&items.collect::<Vec<_>>(), &fmt);
 
-        format!("\n{}where {}",
-                make_indent(indent + extra_indent),
-                write_list(&items.collect::<Vec<_>>(), &fmt))
+        // 9 = " where ".len() + " {".len()
+        if density == Density::Tall || preds_str.contains('\n') ||
+           indent + 9 + preds_str.len() > self.config.max_width {
+            format!("\n{}where {}",
+                    make_indent(indent + extra_indent),
+                    preds_str)
+        } else {
+            format!(" where {}", preds_str)
+        }
     }
 
     fn rewrite_return(&self, ret: &ast::FunctionRetTy) -> String {

tests/config/small_tabs.toml

@@ -8,6 +8,7 @@ fn_return_indent = "WithArgs"
 fn_args_paren_newline = true
 fn_args_layout = "Tall"
 fn_arg_indent = "Visual"
+where_density = "Tall"
 where_indent = "Tabbed"
 where_layout = "Vertical"
 where_pred_indent = "Visual"

tests/source/fn-custom-4.rs

@@ -1,6 +1,15 @@
 // rustfmt-where_pred_indent: Tabbed
+// rustfmt-where_density: Compressed
 // Test different indents.
 
 fn qux() where X: TTTTTTTTTTTTTTTTTTTTTTTTTTTT, X: TTTTTTTTTTTTTTTTTTTTTTTTTTTT, X: TTTTTTTTTTTTTTTTTTTTTTTTTTTT, X: TTTTTTTTTTTTTTTTTTTTTTTTTTTT {
     baz();
 }
+
+fn qux() where X: TTTTTTTTTTTTTTTTTTTTTTTTTTTT {
+    baz();
+}
+
+fn qux(a: Aaaaaaaaaaaaaaaaa) where X: TTTTTTTTTTTTTTTTTTTTTTTTTTTT, X: TTTTTTTTTTTTTTTTTTTTTTTTTTTT {
+    baz();
+}

tests/target/fn-custom-4.rs

@@ -1,4 +1,5 @@
 // rustfmt-where_pred_indent: Tabbed
+// rustfmt-where_density: Compressed
 // Test different indents.
 
 fn qux()
@@ -9,3 +10,14 @@ fn qux()
 {
     baz();
 }
+
+fn qux() where X: TTTTTTTTTTTTTTTTTTTTTTTTTTTT {
+    baz();
+}
+
+fn qux(a: Aaaaaaaaaaaaaaaaa)
+    where X: TTTTTTTTTTTTTTTTTTTTTTTTTTTT,
+        X: TTTTTTTTTTTTTTTTTTTTTTTTTTTT
+{
+    baz();
+}