add MetaBlock interface that will represent an HCL block

hcl2template/metablock.go

@@ -0,0 +1,45 @@
+package hcl2template
+
+import (
+	"github.com/hashicorp/hcl/v2"
+	"github.com/hashicorp/packer/hcl2template/addrs"
+	"github.com/zclconf/go-cty/cty"
+)
+
+// MetaBlock defines funcs that are common to all HCL2 blocks.
+type MetaBlock interface {
+	// References returns the *addrs.Reference of everything being referenced in
+	// this block.
+	References() ([]*addrs.Reference, hcl.Diagnostics)
+
+	// Evaluate runs HCL all expression and puts them into the underlying data
+	// model.
+	//
+	// For a block that is not a Runner — like a variable block — this will make
+	// Value known.
+	Evaluate(ctx *hcl.EvalContext) hcl.Diagnostics
+
+	// Value of the block. For non-Runner types it is known from the top,
+	// otherwise it is probably unknown or not fully known before Run is called.
+	Value() cty.Value
+
+	// Expected type of Value, when unknown: cty.DynamicPseudoType is returned.
+	Type() cty.Type
+}
+
+type Runner interface {
+	// Run will be available on blocks that can execute, like the data source
+	// block or a build block.
+	//
+	// The run should result in changing the respone of Value to something
+	// known.
+	Run() hcl.Diagnostics
+}
+
+var (
+	_ MetaBlock = &Variable{}
+	_ MetaBlock = &DatasourceBlock{}
+	_ Runner    = &DatasourceBlock{}
+	_ MetaBlock = &BuildBlock{}
+	_ Runner    = &BuildBlock{}
+)