Description

#1745 introduced codegen support for unions over sub-unions. For example, we can now write JsExprOrSpread = JsExpr | JsSpread in our grammar and the codegen then knows that JsExprOrSpread can be any expression or a spread element.

The goal is to add a check to the codegen to fail if any two sub-unions of a union type are overlapping because it leads to disambiguity. For example, let's assume we have the following definition:

JsThisIsAnIssue = JsExprOrFunctionDeclaration | JsExprOrSpread
JsExprOrFunctionDeclaration = JsExpr | JsFunctionDeclaration
JsExprOrSpread = JsExpr | JsSpread

The sub-unions JsExprOrFunctionDeclaration and JsExprOrSpread are overlapping because both contain JsExpr.

The problem is that it's now unclear if an expression should be wrapped as a JsExprOrFunctionDeclaration or JsExprOrSpread.

You can trigger the code gen by running cargo xtask syntax in the project root.