btf: add Array.Index

The kernel refuses to load an array type without an index type, so probably
btf.rst is outdated.

btf/core_test.go

@@ -27,8 +27,8 @@ func TestCOREAreTypesCompatible(t *testing.T) {
 		{&Int{Name: "a", Size: 2}, &Int{Name: "b", Size: 4}, true},
 		{&Pointer{Target: &Void{}}, &Pointer{Target: &Void{}}, true},
 		{&Pointer{Target: &Void{}}, &Void{}, false},
-		{&Array{Type: &Void{}}, &Array{Type: &Void{}}, true},
-		{&Array{Type: &Int{}}, &Array{Type: &Void{}}, false},
+		{&Array{Index: &Void{}, Type: &Void{}}, &Array{Index: &Void{}, Type: &Void{}}, true},
+		{&Array{Index: &Void{}, Type: &Int{}}, &Array{Index: &Void{}, Type: &Void{}}, false},
 		{&FuncProto{Return: &Int{}}, &FuncProto{Return: &Void{}}, false},
 		{
 			&FuncProto{Return: &Void{}, Params: []FuncParam{{Name: "a", Type: &Void{}}}},

btf/types.go

@@ -151,17 +151,22 @@ func (p *Pointer) copy() Type {
 
 // Array is an array with a fixed number of elements.
 type Array struct {
+	Index  Type
 	Type   Type
 	Nelems uint32
 }
 
 func (arr *Array) Format(fs fmt.State, verb rune) {
-	formatType(fs, verb, arr, "type=", arr.Type, "n=", arr.Nelems)
+	formatType(fs, verb, arr, "index=", arr.Index, "type=", arr.Type, "n=", arr.Nelems)
 }
 
 func (arr *Array) TypeName() string { return "" }
 
-func (arr *Array) walk(tdq *typeDeque) { tdq.push(&arr.Type) }
+func (arr *Array) walk(tdq *typeDeque) {
+	tdq.push(&arr.Index)
+	tdq.push(&arr.Type)
+}
+
 func (arr *Array) copy() Type {
 	cpy := *arr
 	return &cpy
@@ -904,10 +909,8 @@ func inflateRawTypes(rawTypes []rawType, rawStrings *stringTable) ([]Type, error
 
 		case kindArray:
 			btfArr := raw.data.(*btfArray)
-
-			// IndexType is unused according to btf.rst.
-			// Don't make it available right now.
-			arr := &Array{nil, btfArr.Nelems}
+			arr := &Array{nil, nil, btfArr.Nelems}
+			fixup(btfArr.IndexType, &arr.Index)
 			fixup(btfArr.Type, &arr.Type)
 			typ = arr