hclpack: Support literal JSON as an additional expression syntax

hclpack can potentially be used as an intermediary to more easily bring
non-HCL input into the HCL API, and so allowing a literal value in JSON
format as an expression type means that such a transcoder doesn't need to
worry about formatting values it encounters using HCL syntax and can
instead just encode directly to JSON, but at the expense of then not being
able to use the full expression/template syntax.

hclpack/expression.go

@@ -3,9 +3,11 @@ package hclpack
 import (
 	"fmt"
 
+	"github.com/zclconf/go-cty/cty"
+	ctyjson "github.com/zclconf/go-cty/cty/json"
+
 	"github.com/hashicorp/hcl2/hcl"
 	"github.com/hashicorp/hcl2/hcl/hclsyntax"
-	"github.com/zclconf/go-cty/cty"
 )
 
 // Expression is an implementation of hcl.Expression in terms of some raw
@@ -74,6 +76,30 @@ func (e *Expression) Parse() (hcl.Expression, hcl.Diagnostics) {
 		return hclsyntax.ParseExpression(e.Source, e.Range_.Filename, e.Range_.Start)
 	case ExprTemplate:
 		return hclsyntax.ParseTemplate(e.Source, e.Range_.Filename, e.Range_.Start)
+	case ExprLiteralJSON:
+		ty, err := ctyjson.ImpliedType(e.Source)
+		if err != nil {
+			return nil, hcl.Diagnostics{
+				{
+					Severity: hcl.DiagError,
+					Summary:  "Invalid JSON value",
+					Detail:   fmt.Sprintf("The JSON representation of this expression is invalid: %s.", err),
+					Subject:  &e.Range_,
+				},
+			}
+		}
+		val, err := ctyjson.Unmarshal(e.Source, ty)
+		if err != nil {
+			return nil, hcl.Diagnostics{
+				{
+					Severity: hcl.DiagError,
+					Summary:  "Invalid JSON value",
+					Detail:   fmt.Sprintf("The JSON representation of this expression is invalid: %s.", err),
+					Subject:  &e.Range_,
+				},
+			}
+		}
+		return hcl.StaticExpr(val, e.Range_), nil
 	default:
 		// This should never happen for a valid Expression.
 		return nil, hcl.Diagnostics{
@@ -103,7 +129,12 @@ const (
 	// expression syntax, with hclsyntax.ParseExpression.
 	ExprNative ExprSourceType = 'N'
 
-	// ExprTemplate indicates that an expression must be parsed as nave
+	// ExprTemplate indicates that an expression must be parsed as native
 	// template syntax, with hclsyntax.ParseTemplate.
 	ExprTemplate ExprSourceType = 'T'
+
+	// ExprLiteralJSON indicates that an expression must be parsed as JSON and
+	// treated literally, using cty/json. This can be used when populating
+	// literal attribute values from a non-HCL source.
+	ExprLiteralJSON ExprSourceType = 'L'
 )

hclpack/json_marshal.go

@@ -56,6 +56,8 @@ func (a *Attribute) forJSON(pos map[string]map[hcl.Pos]posOfs) attrJSON {
 		ret.Syntax = 0
 	case ExprTemplate:
 		ret.Syntax = 1
+	case ExprLiteralJSON:
+		ret.Syntax = 2
 	}
 	ret.Ranges = make(rangesPacked, 4)
 	ret.Ranges[0] = packRange(a.Range, pos)