Re: [PATCH] Print void types correctly in Rust

[Manish Goregaokar <manish@mozilla.com>]

> This is fine but there is another case in rust_val_print:
>
>     case TYPE_CODE_INT:
>       /* Recognize the unit type.  */
>       if (TYPE_UNSIGNED (type) && TYPE_LENGTH (type) == 0
>           && TYPE_NAME (type) != NULL && strcmp (TYPE_NAME (type), "()") == 0)
>         {
>           fputs_filtered ("()", stream);
>           break;
>         }
>       goto generic_print;
>
>
> ... I wonder if that is something that changed after 1.8, or if it's the
> case that the unit type can be represented in multiple ways.  (Or maybe
> this only handles the unit type constructed by rust_language_arch_info?)
>
> Anyway I wonder if a case like this is needed in rust_print_type, or if
> the one in rust_val_print can be removed or changed.


I don't think this has changed. The issue is with functions, where the
return type of
functions that return unit is called void because we pretend to be C.
Values have a
different type code.

> Manish> +      if (TYPE_CODE (TYPE_TARGET_TYPE (type)) != TYPE_CODE_VOID)
> Manish> +      {
> Manish> +        fputs_filtered (" -> ", stream);
> Manish> +        rust_print_type (TYPE_TARGET_TYPE (type), "", stream, -1, 0, flags);
>
> This should be formatted like:
>
> +      if (TYPE_CODE (TYPE_TARGET_TYPE (type)) != TYPE_CODE_VOID)
> +        {
> +          fputs_filtered (" -> ", stream);
> +          rust_print_type (TYPE_TARGET_TYPE (type), "", stream, -1, 0, flags);
>
> Manish> +// Empty function, should not have "void"
> Manish> +// or "()" in its return type
> Manish> +fn empty() {
>
> I'm curious what happens if it does say "-> ()"?

It's okay to do so, just not idiomatic Rust. You rarely see `-> ()`
for void functions.


=========

new patch below (changelog and formatting fixed):

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:
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..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/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