[PATCH 10/16] btf_encoder: Encode variant parts as union members in BTF

[Arnaldo Carvalho de Melo <acme@kernel.org>]

From: Arnaldo Carvalho de Melo <acme@redhat.com>

With the DWARF loader now populating DW_TAG_variant children (previous
commit), wire them into BTF encoding so Rust discriminated unions
(Option<T>, Result<T,E>, etc.) are no longer emitted as empty structs.

Two changes:

1. Struct-to-union promotion: when a DW_TAG_structure_type has
   variant_parts but no regular data members, encode it as BTF_KIND_UNION
   instead of BTF_KIND_STRUCT, since the variants overlap at offset 0.

2. Variant member encoding: after encoding regular data members, iterate
   the variant_parts and emit each variant as a BTF union field with the
   variant's name and resolved type reference.

Testing with the sashiko-cli Rust binary (a real-world async HTTP client
using tokio, hyper, serde, etc.):

Before:

  $ bpftool btf dump file sashiko-cli | grep -c UNION
  2073
  $ bpftool btf dump file sashiko-cli | grep 'STRUCT.*vlen=0' | grep -vc 'size=0'
  24335

After:

  $ bpftool btf dump file sashiko-cli | grep -c UNION
  25750
  $ bpftool btf dump file sashiko-cli | grep 'STRUCT.*vlen=0' | grep -vc 'size=0'
  2236

22,099 types that were previously encoded as empty structs are now
properly represented as unions with their variant members:

Before:

  $ bpftool btf dump file code_with_type.o | grep -A1 'Option<u32>'
  [11] STRUCT 'Option<u32>' size=8 vlen=0

After:

  $ bpftool btf dump file code_with_type.o | grep -A3 'Option<u32>'
  [11] UNION 'Option<u32>' size=8 vlen=2
          'None' type_id=9 bits_offset=0
          'Some' type_id=10 bits_offset=0

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 btf_encoder.c | 37 +++++++++++++++++++++++++++++++++++--
 1 file changed, 35 insertions(+), 2 deletions(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index dc1e18a986605d04..5a510afbde7afa84 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -1743,6 +1743,11 @@ static int tag__check_id_drift(struct btf_encoder *encoder, const struct tag *ta
 	return 0;
 }
 
+static bool type__has_variant_parts(const struct type *type)
+{
+	return !list_empty(&type->variant_parts);
+}
+
 static int32_t btf_encoder__add_struct_type(struct btf_encoder *encoder, struct tag *tag)
 {
 	struct type *type = tag__type(tag);
@@ -1751,8 +1756,18 @@ static int32_t btf_encoder__add_struct_type(struct btf_encoder *encoder, struct
 	int32_t type_id;
 	uint8_t kind;
 
-	kind = (tag->tag == DW_TAG_union_type) ?
-		BTF_KIND_UNION : BTF_KIND_STRUCT;
+	/*
+	 * Rust discriminated unions (enums) are represented in DWARF as
+	 * DW_TAG_structure_type with DW_TAG_variant_part children.
+	 * If the struct has only variant parts and no regular data members,
+	 * encode it as a BTF union since the variants overlap at offset 0.
+	 */
+	if (tag->tag == DW_TAG_union_type)
+		kind = BTF_KIND_UNION;
+	else if (type__has_variant_parts(type) && type->nr_members == 0)
+		kind = BTF_KIND_UNION;
+	else
+		kind = BTF_KIND_STRUCT;
 
 	type_id = btf_encoder__add_struct(encoder, kind, name, type->size);
 	if (type_id < 0)
@@ -1770,6 +1785,24 @@ static int32_t btf_encoder__add_struct_type(struct btf_encoder *encoder, struct
 			return -1;
 	}
 
+	if (type__has_variant_parts(type) && kind == BTF_KIND_UNION) {
+		struct variant_part *vpart;
+
+		type__for_each_variant_part(type, vpart) {
+			struct variant *variant;
+
+			variant_part__for_each_variant(vpart, variant) {
+				if (variant->tag.type == 0)
+					continue;
+
+				uint32_t ref_type_id = encoder->type_id_off + variant->tag.type;
+
+				if (btf_encoder__add_field(encoder, variant->name, ref_type_id, 0, 0))
+					return -1;
+			}
+		}
+	}
+
 	return type_id;
 }
 
-- 
2.54.0