Fix crash in quirk_rust_enum

I noticed that quirk_rust_enum can crash when presented with a union
whose fields are all scalar types.

This patch adds a new test case and fixes the bug.

Regression tested on Fedora 26 x86-64.

2018-04-17  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (quirk_rust_enum): Handle unions correctly.

2018-04-17  Tom Tromey  <tom@tromey.com>

	* gdb.rust/simple.rs (Union): New type.
	(main): New local "u".
	* gdb.rust/simple.exp (test_one_slice): Add new test case.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2018-04-17  Tom Tromey  <tom@tromey.com>
+
+	* dwarf2read.c (quirk_rust_enum): Handle unions correctly.
+
 2018-04-17  Andreas Arnez  <arnez@linux.vnet.ibm.com>
 
 	* symtab.c (print_symbol_info): Skip printing filename and line

gdb/dwarf2read.c

@@ -9989,9 +9989,15 @@ quirk_rust_enum (struct type *type, struct objfile *objfile)
 	{
 	  disr_type = TYPE_FIELD_TYPE (type, i);
 
-	  if (TYPE_NFIELDS (disr_type) == 0)
+	  if (TYPE_CODE (disr_type) != TYPE_CODE_STRUCT)
+	    {
+	      /* All fields of a true enum will be structs.  */
+	      return;
+	    }
+	  else if (TYPE_NFIELDS (disr_type) == 0)
 	    {
 	      /* Could be data-less variant, so keep going.  */
+	      disr_type = nullptr;
 	    }
 	  else if (strcmp (TYPE_FIELD_NAME (disr_type, 0),
 			   "RUST$ENUM$DISR") != 0)

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2018-04-17  Tom Tromey  <tom@tromey.com>
+
+	* gdb.rust/simple.rs (Union): New type.
+	(main): New local "u".
+	* gdb.rust/simple.exp (test_one_slice): Add new test case.
+
 2018-04-16  Andreas Arnez  <arnez@linux.vnet.ibm.com>
 
 	* gdb.base/dbx.exp (test_whereis): Adjust regexp to added line

gdb/testsuite/gdb.rust/simple.exp

@@ -277,6 +277,7 @@ gdb_test "print parametrized.next.val" \
 gdb_test "print parametrized" \
     " = simple::ParametrizedStruct<i32> \\{next: simple::ParametrizedEnum<\[a-z:\]*Box<simple::ParametrizedStruct<i32>>>::Val\\{val: $hex\\}, value: 0\\}"
 
+gdb_test "print u" " = simple::Union {f1: -1, f2: 255}"
 
 load_lib gdb-python.exp
 if {[skip_python_tests]} {

gdb/testsuite/gdb.rust/simple.rs

@@ -80,6 +80,11 @@ struct ParametrizedStruct<T> {
     value: T
 }
 
+union Union {
+    f1: i8,
+    f2: u8,
+}
+
 fn main () {
     let a = ();
     let b : [i32; 0] = [];
@@ -153,6 +158,8 @@ fn main () {
         value: 0,
     };
 
+    let u = Union { f2: 255 };
+
     println!("{}, {}", x.0, x.1);        // set breakpoint here
     println!("{}", diff2(92, 45));
     empty();