[PATCH] Print void types correctly in Rust

[Manish Goregaokar <manish@mozilla.com>]

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.

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

gdb/testsuite/ChangeLog:
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
---
 gdb/rust-lang.c                   | 22 ++++++++++++++++++----
 gdb/testsuite/gdb.rust/simple.exp |  1 +
 gdb/testsuite/gdb.rust/simple.rs  |  7 +++++++
 3 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/gdb/rust-lang.c b/gdb/rust-lang.c
index 0c56a0f..2b115ee 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/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();
 }
-- 
2.8.3