block indent large closures

somewhat rough around the edges

src/config.rs

@@ -188,6 +188,12 @@ impl ConfigType for usize {
     }
 }
 
+impl ConfigType for isize {
+    fn doc_hint() -> String {
+        String::from("<signed integer>")
+    }
+}
+
 impl ConfigType for String {
     fn doc_hint() -> String {
         String::from("<string>")
@@ -384,6 +390,8 @@ create_config! {
     match_block_trailing_comma: bool, false,
         "Put a trailing comma after a block based match arm (non-block arms are not affected)";
     match_wildcard_trailing_comma: bool, true, "Put a trailing comma after a wildcard arm";
+    closure_block_indent_threshold: isize, 4, "How many lines a closure must have before it is \
+                                                block indented. -1 means never use block indent.";
     write_mode: WriteMode, WriteMode::Replace,
         "What Write Mode to use when none is supplied: Replace, Overwrite, Display, Diff, Coverage";
 }

src/expr.rs

@@ -455,9 +455,19 @@ fn rewrite_closure(capture: ast::CaptureBy,
 
     // We couldn't format the closure body as a single line expression; fall
     // back to block formatting.
-    let body_rewrite = inner_block.rewrite(&context, budget, Indent::empty());
+    let body_rewrite = try_opt!(inner_block.rewrite(&context, budget, Indent::empty()));
 
-    Some(format!("{} {}", prefix, try_opt!(body_rewrite)))
+    let block_threshold = context.config.closure_block_indent_threshold;
+    if block_threshold < 0 || body_rewrite.matches('\n').count() <= block_threshold as usize {
+        return Some(format!("{} {}", prefix, body_rewrite));
+    }
+
+    // The body of the closure is big enough to be block indented, that means we
+    // must re-format.
+    let mut context = context.clone();
+    context.block_indent.alignment = 0;
+    let body_rewrite = try_opt!(inner_block.rewrite(&context, budget, Indent::empty()));
+    Some(format!("{} {}", prefix, body_rewrite))
 }
 
 fn and_one_line(x: Option<String>) -> Option<String> {

src/rewrite.rs

@@ -27,6 +27,7 @@ pub trait Rewrite {
     fn rewrite(&self, context: &RewriteContext, width: usize, offset: Indent) -> Option<String>;
 }
 
+#[derive(Clone)]
 pub struct RewriteContext<'a> {
     pub parse_session: &'a ParseSess,
     pub codemap: &'a CodeMap,