Context

JsonString is a type in the schema for arbitrary JSON values. In SCALE it is encoded as a string (hence the name), while in JSON it is inlined as any JSON value.

There are a few issues:

• Ambiguity of Option<JsonString> in JSON: it is impossible to distinguish between None and Some("null"), since both serialize as null in actual JSON.
• Schema's purpose is solely to describe SCALE encoding and it doesn't tell anything about JsonString representation in JSON. In JSON it is not a string, which caused confusion for SDK developers.

This PR fixes #4900 by disallowing Option<JsonString> and essentially forcing SDKs to handle JSON values as a special case and account for its differences in SCALE vs JSON.

Solution

by these changes:

• Rename JsonString to JsonValue and make stronger emphasis on using the native Rust JSON type, i.e. serde_json::Value. Remove ambiguous conversions and delegate it to the serde_json::Value itself.
• Add struct JsonValueWrap { value: JsonValue } for use with Option
• In schema, treat JsonValue as a special type, i.e. not as an alias to String, and forbid use of Option<JsonValue>.

Migration Guide

There is a breaking change for SDK developers. Apart from JsonString being renamed to JsonValue, it is also no longer an alias to String type, but instead is a special type JsonValue. SDK should treat it as a special case in a way that is suitable for their platform.

Binary and JSON serialisation compatibility is not broken. However, JSON form of ExecuteTrigger instruction and ExecuteTriggerEvent changed: its args field was an Option<JsonString> with ambiguous null value. Now it is Option<JsonValueWrap>, meaning that you need to use it this way:

// Before
{ "args": null }
{ "args": 42 }

// After
{ "args": null }
{ "args": { "value": null } }
{ "args": { "value": 42 } }

Review notes

• Start from primitives/json.rs, then schema, then the rest
• I am not sure about JsonValue method names, especially get(&self) -> &Value.

Checklist

• I've read CONTRIBUTING.md.
• (optional) I've written unit tests for the code changes.
• All review comments have been resolved.