Empty structs and struct lits (#920)

* Handle empty struct lits

Closes #835

* Don't crash on empty struct defs.

Not a great fix, but better than crashing.

src/expr.rs

@@ -1390,7 +1390,6 @@ fn rewrite_struct_lit<'a>(context: &RewriteContext,
                           offset: Indent)
                           -> Option<String> {
     debug!("rewrite_struct_lit: width {}, offset {:?}", width, offset);
-    assert!(!fields.is_empty() || base.is_some());
 
     enum StructLitField<'a> {
         Regular(&'a ast::Field),
@@ -1502,6 +1501,10 @@ fn rewrite_struct_lit<'a>(context: &RewriteContext,
     };
     let fields_str = try_opt!(write_list(&item_vec, &fmt));
 
+    if fields_str.is_empty() {
+        return Some(format!("{} {{}}", path_str));
+    }
+
     let format_on_newline = || {
         let inner_indent = context.block_indent
                                   .block_indent(context.config)

src/items.rs

@@ -786,7 +786,7 @@ fn format_struct_struct(context: &RewriteContext,
     };
     result.push_str(&generics_str);
 
-    // FIXME: properly format empty structs and their comments.
+    // FIXME(#919): properly format empty structs and their comments.
     if fields.is_empty() {
         result.push_str(&context.snippet(mk_sp(body_lo, span.hi)));
         return Some(result);
@@ -838,13 +838,17 @@ fn format_tuple_struct(context: &RewriteContext,
                        span: Span,
                        offset: Indent)
                        -> Option<String> {
-    assert!(!fields.is_empty(), "Tuple struct with no fields?");
     let mut result = String::with_capacity(1024);
 
     let header_str = format_header(item_name, ident, vis);
     result.push_str(&header_str);
 
-    let body_lo = fields[0].span.lo;
+    // FIXME(#919): don't lose comments on empty tuple structs.
+    let body_lo = if fields.is_empty() {
+        span.hi
+    } else {
+        fields[0].span.lo
+    };
 
     let where_clause_str = match generics {
         Some(ref generics) => {

tests/source/struct_lits.rs

@@ -123,3 +123,12 @@ fn issue698() {
         ffffffffffffffffffffffffffields: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa,
     }
 }
+
+fn issue835() {
+    MyStruct {};
+    MyStruct { /* a comment */ };
+    MyStruct {
+        // Another comment
+    };
+    MyStruct {}
+}

tests/source/structs.rs

@@ -151,3 +151,6 @@ struct Issue677 {
     pub trace: fn(  obj: 
           *const libc::c_void, tracer   : *mut   JSTracer   ),
 }
+
+struct Foo {}
+struct Foo();

tests/target/struct_lits.rs

@@ -159,3 +159,12 @@ fn issue698() {
             aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa,
     }
 }
+
+fn issue835() {
+    MyStruct {};
+    MyStruct { /* a comment */ };
+    MyStruct {
+        // Another comment
+    };
+    MyStruct {}
+}

tests/target/structs.rs

@@ -158,3 +158,6 @@ struct Issue677 {
     pub ptr: *const libc::c_void,
     pub trace: fn(obj: *const libc::c_void, tracer: *mut JSTracer),
 }
+
+struct Foo {}
+struct Foo();