Allow empty struct expressions in Rust

I learned recently that empty struct expressions, like "X{}", have been
promoted from experimental to stable in Rust.  This patch changes the
Rust expression parser to allow this case.

New test case included.
Built and regtested on x86-64 Fedora 23, using Rust 1.11 beta.

2016-07-21  Tom Tromey  <tom@tromey.com>

	* rust-lang.c (rust_tuple_struct_type_p): Return false for empty
	structs.
	* rust-exp.y (struct_expr_list): Allow empty elements.

2016-07-21  Tom Tromey  <tom@tromey.com>

	* gdb.rust/simple.rs (main): Use empty struct expression.
	* gdb.rust/simple.exp: Add tests for empty struct expression.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2016-07-21  Tom Tromey  <tom@tromey.com>
+
+	* rust-lang.c (rust_tuple_struct_type_p): Return false for empty
+	structs.
+	* rust-exp.y (struct_expr_list): Allow empty elements.
+
 2016-07-21  Tom Tromey  <tom@tromey.com>
 
 	* configure: Rebuild.

gdb/rust-exp.y

@@ -428,10 +428,14 @@ struct_expr_tail:
 		}
 ;
 
-/* S{} is documented as valid but seems to be an unstable feature, so
-   it is left out here.  */
 struct_expr_list:
-	struct_expr_tail
+	/* %empty */
+		{
+		  VEC (set_field) **result
+		    = OBSTACK_ZALLOC (&work_obstack, VEC (set_field) *);
+		  $$ = result;
+		}
+|	struct_expr_tail
 		{
 		  VEC (set_field) **result
 		    = OBSTACK_ZALLOC (&work_obstack, VEC (set_field) *);

gdb/rust-lang.c

@@ -294,7 +294,10 @@ rust_underscore_fields (struct type *type, int offset)
 int
 rust_tuple_struct_type_p (struct type *type)
 {
-  return rust_underscore_fields (type, 0);
+  /* This is just an approximation until DWARF can represent Rust more
+     precisely.  We exclude zero-length structs because they may not
+     be tuple structs, and there's no way to tell.  */
+  return TYPE_NFIELDS (type) > 0 && rust_underscore_fields (type, 0);
 }
 
 /* Return true if a variant TYPE is a tuple variant, false otherwise.  */

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2016-07-21  Tom Tromey  <tom@tromey.com>
+
+	* gdb.rust/simple.rs (main): Use empty struct expression.
+	* gdb.rust/simple.exp: Add tests for empty struct expression.
+
 2016-07-21  Yao Qi  <yao.qi@linaro.org>
 
 	* lib/gdbserver-support.exp (skip_gdbserver_tests): Return 1

gdb/testsuite/gdb.rust/simple.exp

@@ -55,7 +55,10 @@ gdb_test "print *(&c as *mut i32)" " = 0"
 
 gdb_test "print j" " = simple::Unit"
 gdb_test "ptype j" " = struct simple::Unit"
+gdb_test "print j2" " = simple::Unit"
+gdb_test "ptype j2" " = struct simple::Unit"
 gdb_test "print simple::Unit" " = simple::Unit"
+gdb_test "print simple::Unit{}" " = simple::Unit"
 
 gdb_test "print g" " = \\(u8 \\(\\*\\)\\\[6\\\]\\) $hex b\"hi bob\""
 gdb_test "ptype g" " = u8 \\(\\*\\)\\\[6\\\]"

gdb/testsuite/gdb.rust/simple.rs

@@ -81,6 +81,7 @@ fn main () {
     let i = ["whatever"; 8];
 
     let j = Unit;
+    let j2 = Unit{};
 
     let k = SpaceSaver::Nothing;
     let l = SpaceSaver::Thebox(9, Box::new(1729));