Disallow interfaces definitions except at the top-level

[Chriscbr]

Use Case

For the same reason we disallow structs except at the top-level of a file (see #5135). Interfaces only define contracts (they don't contain any source code) so there aren't strong use cases to support defining interfaces within if branches or as a result of code execution etc.

bring math;

if math.random() < 0.5 {
  interface Foo {
    foo(): str;
  }
} else {
  interface Foo {
    foo(): num;
  }
}

Proposed Solution

No response

Implementation Notes

No response

Component

Compiler

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[Chriscbr]

@yoav-steinberg @eladb any objections?

[staycoolcall911]

@Chriscbr, please see #5507 - we may have to support interface definitions inside classes through jsii.

[Chriscbr]

Got it, after reading Mark's comment I see how they're related - good callout.

(2 cents) Even if we support interface definitions existing inside classes in our type system (for backwards compatibility with existing JSII libraries, like the issue above shows), I think it might still be reasonable to prevent the creation of new interfaces in this way within Wing code.

[monadabot]

Congrats! 🚀 This was released in Wing 0.83.0.