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

5 files changed, 37 insertions, 4 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index ad6f5d9..40adb80 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,9 @@
+2016-06-27  Manish Goregaokar  <manish@mozilla.com>
+
+	* rust-lang.c (rust_print_type): Print unit types as "()"
+	* rust-lang.c (rust_print_type): Omit return type for functions
+	returning unit
+
 2016-06-25  Pierre-Marie de Rodat  <derodat@adacore.com>
 
 	* python/py-breakpoint.c (bppy_init): Clear bppy_pending_object
diff --git a/gdb/rust-lang.c b/gdb/rust-lang.c
index 0c56a0f..23ddd5a 100644
--- a/gdb/rust-lang.c
+++ b/gdb/rust-lang.c
@@ -446,7 +446,7 @@ static const struct generic_val_print_decorations rust_decorations =
   " * I",
   "true",
   "false",
-  "void",
+  "()",
   "[",
   "]"
 };
@@ -729,13 +729,22 @@ rust_print_type (struct type *type, const char *varstring,
   if (show <= 0
       && TYPE_NAME (type) != NULL)
     {
-      fputs_filtered (TYPE_NAME (type), stream);
+      /* Rust calls the unit type "void" in its debuginfo,
+         but we don't want to print it as that.  */
+      if (TYPE_CODE (type) == TYPE_CODE_VOID)
+        fputs_filtered ("()", stream);
+      else
+        fputs_filtered (TYPE_NAME (type), stream);
       return;
     }
 
   type = check_typedef (type);
   switch (TYPE_CODE (type))
     {
+    case TYPE_CODE_VOID:
+      fputs_filtered ("()", stream);
+      break;
+
     case TYPE_CODE_FUNC:
       /* Delegate varargs to the C printer.  */
       if (TYPE_VARARGS (type))
@@ -753,8 +762,13 @@ rust_print_type (struct type *type, const char *varstring,
 	  rust_print_type (TYPE_FIELD_TYPE (type, i), "", stream, -1, 0,
 			   flags);
 	}
-      fputs_filtered (") -> ", stream);
-      rust_print_type (TYPE_TARGET_TYPE (type), "", stream, -1, 0, flags);
+      fputs_filtered (")", stream);
+      /* If it returns unit, we can omit the return type.  */
+      if (TYPE_CODE (TYPE_TARGET_TYPE (type)) != TYPE_CODE_VOID)
+        {
+          fputs_filtered (" -> ", stream);
+          rust_print_type (TYPE_TARGET_TYPE (type), "", stream, -1, 0, flags);
+        }
       break;
 
     case TYPE_CODE_ARRAY:
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index f5bdb40..b7e30f5 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2016-06-27  Manish Goregaokar  <manish@mozilla.com>
+
+	* gdb.rust/simple.rs: Add test for returning unit in a function
+	* gdb.rust/simple.exp: Add expectation for functions returning unit
+
 2016-06-27  Pierre-Marie de Rodat  <derodat@adacore.com>
 
 	* gdb.python/py-breakpoint-create-fail.c,
diff --git a/gdb/testsuite/gdb.rust/simple.exp b/gdb/testsuite/gdb.rust/simple.exp
index 88f1c89..4622f75 100644
--- a/gdb/testsuite/gdb.rust/simple.exp
+++ b/gdb/testsuite/gdb.rust/simple.exp
@@ -149,6 +149,7 @@ gdb_test "print self::diff2(8, 9)" " = -1"
 gdb_test "print ::diff2(23, -23)" " = 46"
 
 gdb_test "ptype diff2" "fn \\(i32, i32\\) -> i32"
+gdb_test "ptype empty" "fn \\(\\)"
 
 gdb_test "print (diff2 as fn(i32, i32) -> i32)(19, -2)" " = 21"
 
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();
 }