json: Add ParseExpression function

json/parser.go

@@ -30,6 +30,21 @@ func parseFileContent(buf []byte, filename string) (node, hcl.Diagnostics) {
 	return node, diags
 }
 
+func parseExpression(buf []byte, filename string, start hcl.Pos) (node, hcl.Diagnostics) {
+	tokens := scan(buf, pos{Filename: filename, Pos: start})
+	p := newPeeker(tokens)
+	node, diags := parseValue(p)
+	if len(diags) == 0 && p.Peek().Type != tokenEOF {
+		diags = diags.Append(&hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Extraneous data after value",
+			Detail:   "Extra characters appear after the JSON value.",
+			Subject:  p.Peek().Range.Ptr(),
+		})
+	}
+	return node, diags
+}
+
 func parseValue(p *peeker) (node, hcl.Diagnostics) {
 	tok := p.Peek()
 

json/public.go

@@ -62,6 +62,13 @@ func Parse(src []byte, filename string) (*hcl.File, hcl.Diagnostics) {
 	return file, diags
 }
 
+// ParseExpression parses the given buffer as a standalone JSON expression,
+// returning it as an instance of Expression.
+func ParseExpression(src []byte, filename string, start hcl.Pos) (hcl.Expression, hcl.Diagnostics) {
+	node, diags := parseExpression(src, filename, start)
+	return &expression{src: node}, diags
+}
+
 // ParseFile is a convenience wrapper around Parse that first attempts to load
 // data from the given filename, passing the result to Parse if successful.
 //