hcl: StaticExpr function for making synthetic expressions

Sometimes we want an expression that just wraps a static value, e.g. for
testing or to provide a default value for a missing attribute.

StaticExpr gives us a convenient way to do that, returning a value that
implements the Expression interface by returning just the given static
value.

hcl/static_expr.go

@@ -0,0 +1,40 @@
+package hcl
+
+import (
+	"github.com/zclconf/go-cty/cty"
+)
+
+type staticExpr struct {
+	val cty.Value
+	rng Range
+}
+
+// StaticExpr returns an Expression that always evaluates to the given value.
+//
+// This is useful to substitute default values for expressions that are
+// not explicitly given in configuration and thus would otherwise have no
+// Expression to return.
+//
+// Since expressions are expected to have a source range, the caller must
+// provide one. Ideally this should be a real source range, but it can
+// be a synthetic one (with an empty-string filename) if no suitable range
+// is available.
+func StaticExpr(val cty.Value, rng Range) Expression {
+	return staticExpr{val, rng}
+}
+
+func (e staticExpr) Value(ctx *EvalContext) (cty.Value, Diagnostics) {
+	return e.val, nil
+}
+
+func (e staticExpr) Variables() []Traversal {
+	return nil
+}
+
+func (e staticExpr) Range() Range {
+	return e.rng
+}
+
+func (e staticExpr) StartRange() Range {
+	return e.rng
+}