[FYI] Correctly compute length of DW_TAG_variant_part union

[Tom Tromey]

Currently, gdb internally transforms DW_TAG_variant_part into a union
(with some special attbributes).  When doing so, it computes the
length of this union from the length of the fields.  However, this
computation didn't include the offset of these fields, resulting in
the length being too short.

This is not a problem given the way the code currently works.
However, I have a patch series to switch gdb to value-based printing,
where this does have an impact.

Tested on x86-64 Fedora 28; and, considering that this only affects
Rust, I am checking it in.

gdb/ChangeLog
2019-11-30  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (dwarf2_add_field): Include field offset when
	computing variant part length.

Change-Id: I25d84fc237eb3c1e7f11f6eaf35ffe198efde6cc
---
 gdb/ChangeLog    |  5 +++++
 gdb/dwarf2read.c | 11 ++++++++---
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 40626a1562f..fd7d21c02e9 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -15252,13 +15252,18 @@ dwarf2_add_field (struct field_info *fip, struct die_info *die,
 
       /* Normally a DW_TAG_variant_part won't have a size, but our
 	 representation requires one, so set it to the maximum of the
-	 child sizes.  */
+	 child sizes, being sure to account for the offset at which
+	 each child is seen.  */
       if (TYPE_LENGTH (fp->type) == 0)
 	{
 	  unsigned max = 0;
 	  for (int i = 0; i < TYPE_NFIELDS (fp->type); ++i)
-	    if (TYPE_LENGTH (TYPE_FIELD_TYPE (fp->type, i)) > max)
-	      max = TYPE_LENGTH (TYPE_FIELD_TYPE (fp->type, i));
+	    {
+	      unsigned len = ((TYPE_FIELD_BITPOS (fp->type, i) + 7) / 8
+			      + TYPE_LENGTH (TYPE_FIELD_TYPE (fp->type, i)));
+	      if (len > max)
+		max = len;
+	    }
 	  TYPE_LENGTH (fp->type) = max;
 	}
     }
-- 
2.17.2