hcltest: new implementations for ExprList and AbsTraversalForExpr

To assist in testing code that depends on hcl.ExprList and
hcl.AbsTraversalForExpr we now implement the necessary interfaces on our
existing MockExprLiteral and MockExprVariable, as well as adding new
functions MockExprList and MockExprTraversal that more directly serve
those interfaces with full functionality.

hcltest/mock.go

@@ -3,6 +3,8 @@ package hcltest
 import (
 	"fmt"
 
+	"github.com/hashicorp/hcl2/hcl/hclsyntax"
+
 	"github.com/hashicorp/hcl2/hcl"
 	"github.com/zclconf/go-cty/cty"
 )
@@ -149,6 +151,21 @@ func (e mockExprLiteral) StartRange() hcl.Range {
 	return e.Range()
 }
 
+// Implementation for hcl.ExprList
+func (e mockExprLiteral) ExprList() []hcl.Expression {
+	v := e.V
+	ty := v.Type()
+	if v.IsKnown() && !v.IsNull() && (ty.IsListType() || ty.IsTupleType()) {
+		ret := make([]hcl.Expression, 0, v.LengthInt())
+		for it := v.ElementIterator(); it.Next(); {
+			_, v := it.Element()
+			ret = append(ret, MockExprLiteral(v))
+		}
+		return ret
+	}
+	return nil
+}
+
 // MockExprVariable returns a hcl.Expression that evaluates to the value of
 // the variable with the given name.
 func MockExprVariable(name string) hcl.Expression {
@@ -197,6 +214,111 @@ func (e mockExprVariable) StartRange() hcl.Range {
 	return e.Range()
 }
 
+// Implementation for hcl.AbsTraversalForExpr and hcl.RelTraversalForExpr.
+func (e mockExprVariable) AsTraversal() hcl.Traversal {
+	return hcl.Traversal{
+		hcl.TraverseRoot{
+			Name:     string(e),
+			SrcRange: e.Range(),
+		},
+	}
+}
+
+// MockExprTraversal returns a hcl.Expression that evaluates the given
+// absolute traversal.
+func MockExprTraversal(traversal hcl.Traversal) hcl.Expression {
+	return mockExprTraversal{
+		Traversal: traversal,
+	}
+}
+
+// MockExprTraversalSrc is like MockExprTraversal except it takes a
+// traversal string as defined by the native syntax and parses it first.
+//
+// This method is primarily for testing with hard-coded traversal strings, so
+// it will panic if the given string is not syntactically correct.
+func MockExprTraversalSrc(src string) hcl.Expression {
+	traversal, diags := hclsyntax.ParseTraversalAbs([]byte(src), "MockExprTraversal", hcl.Pos{})
+	if diags.HasErrors() {
+		panic("invalid traversal string")
+	}
+	return MockExprTraversal(traversal)
+}
+
+type mockExprTraversal struct {
+	Traversal hcl.Traversal
+}
+
+func (e mockExprTraversal) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
+	return e.Traversal.TraverseAbs(ctx)
+}
+
+func (e mockExprTraversal) Variables() []hcl.Traversal {
+	return []hcl.Traversal{e.Traversal}
+}
+
+func (e mockExprTraversal) Range() hcl.Range {
+	return e.Traversal.SourceRange()
+}
+
+func (e mockExprTraversal) StartRange() hcl.Range {
+	return e.Range()
+}
+
+// Implementation for hcl.AbsTraversalForExpr and hcl.RelTraversalForExpr.
+func (e mockExprTraversal) AsTraversal() hcl.Traversal {
+	return e.Traversal
+}
+
+func MockExprList(exprs []hcl.Expression) hcl.Expression {
+	return mockExprList{
+		Exprs: exprs,
+	}
+}
+
+type mockExprList struct {
+	Exprs []hcl.Expression
+}
+
+func (e mockExprList) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics) {
+	if len(e.Exprs) == 0 {
+		return cty.ListValEmpty(cty.DynamicPseudoType), nil
+	}
+	vals := make([]cty.Value, 0, len(e.Exprs))
+	var diags hcl.Diagnostics
+
+	for _, expr := range e.Exprs {
+		val, valDiags := expr.Value(ctx)
+		diags = append(diags, valDiags...)
+		vals = append(vals, val)
+	}
+
+	return cty.ListVal(vals), diags
+}
+
+func (e mockExprList) Variables() []hcl.Traversal {
+	var traversals []hcl.Traversal
+	for _, expr := range e.Exprs {
+		traversals = append(traversals, expr.Variables()...)
+	}
+	return traversals
+}
+
+func (e mockExprList) Range() hcl.Range {
+	return hcl.Range{
+		Filename: "MockExprList",
+	}
+}
+
+func (e mockExprList) StartRange() hcl.Range {
+	return e.Range()
+}
+
+// Implementation for hcl.ExprList
+func (e mockExprList) ExprList() []hcl.Expression {
+	return e.Exprs
+}
+
 // MockAttrs constructs and returns a hcl.Attributes map with attributes
 // derived from the given expression map.
 //