zclsyntax: Expression.Variables implementation using AST walk

This function is effectively the implementation of Variables for all
expressions, but unfortunately we still need to declare a wrapper around
it as a method on every single expression type.

zcl/zclsyntax/expression.go

@@ -25,8 +25,6 @@ var assertExprImplExpr zcl.Expression = Expression(nil)
 type LiteralValueExpr struct {
 	Val      cty.Value
 	SrcRange zcl.Range
-
-	hasNoVariables
 }
 
 func (e *LiteralValueExpr) walkChildNodes(w internalWalkFunc) {
@@ -41,11 +39,37 @@ func (e *LiteralValueExpr) Range() zcl.Range {
 	return e.SrcRange
 }
 
-// Embed this in an expression struct to get a default implementation of
-// Variables that returns no variables.
-type hasNoVariables struct {
+func (e *LiteralValueExpr) StartRange() zcl.Range {
+	return e.SrcRange
 }
 
-func (e hasNoVariables) Variables() []zcl.Traversal {
-	return nil
+func (e *LiteralValueExpr) Variables() []zcl.Traversal {
+	return Variables(e)
+}
+
+// ScopeTraversalExpr is an Expression that retrieves a value from the scope
+// using a traversal.
+type ScopeTraversalExpr struct {
+	Traversal zcl.Traversal
+	SrcRange  zcl.Range
+}
+
+func (e *ScopeTraversalExpr) walkChildNodes(w internalWalkFunc) {
+	// Scope traversals have no child nodes
+}
+
+func (e *ScopeTraversalExpr) Value(ctx *zcl.EvalContext) (cty.Value, zcl.Diagnostics) {
+	panic("ScopeTraversalExpr.Value not yet implemented")
+}
+
+func (e *ScopeTraversalExpr) Range() zcl.Range {
+	return e.SrcRange
+}
+
+func (e *ScopeTraversalExpr) StartRange() zcl.Range {
+	return e.SrcRange
+}
+
+func (e *ScopeTraversalExpr) Variables() []zcl.Traversal {
+	return Variables(e)
 }

zcl/zclsyntax/variables.go

@@ -0,0 +1,20 @@
+package zclsyntax
+
+import (
+	"github.com/apparentlymart/go-zcl/zcl"
+)
+
+// Variables returns all of the variables referenced within a given experssion.
+//
+// This is the implementation of the "Variables" method on every native
+// expression.
+func Variables(expr Expression) []zcl.Traversal {
+	var vars []zcl.Traversal
+	VisitAll(expr, func(n Node) zcl.Diagnostics {
+		if ste, ok := n.(*ScopeTraversalExpr); ok {
+			vars = append(vars, ste.Traversal)
+		}
+		return nil
+	})
+	return vars
+}