Print void types correctly in Rust

Rust prefers to not specify the return type of a function when it is unit
(`()`). The type is also referred to as "void" in debuginfo but not in actual
usage, so we should never be printing "void" when the language is Rust.

2016-06-27  Manish Goregaokar  <manish@mozilla.com>

gdb/ChangeLog:
    * rust-lang.c (rust_print_type): Print unit types as "()"
    * rust-lang.c (rust_print_type): Omit return type for functions
    returning unit

gdb/testsuite/ChangeLog:
    * gdb.rust/simple.rs: Add test for returning unit in a function
    * gdb.rust/simple.exp: Add expectation for functions returning unit

1 files changed, 7 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.rust/simple.rs b/gdb/testsuite/gdb.rust/simple.rs
index 32da580..3d28e27 100644
--- a/gdb/testsuite/gdb.rust/simple.rs
+++ b/gdb/testsuite/gdb.rust/simple.rs
@@ -48,6 +48,12 @@ fn diff2(x: i32, y: i32) -> i32 {
     x - y
 }
 
+// Empty function, should not have "void"
+// or "()" in its return type
+fn empty() {
+
+}
+
 pub struct Unit;
 
 // This triggers the non-zero optimization that yields a different
@@ -111,4 +117,5 @@ fn main () {
 
     println!("{}, {}", x.0, x.1);        // set breakpoint here
     println!("{}", diff2(92, 45));
+    empty();
 }