Confusing error message when method only includes variable declarations

[patternspandemic]

ponyc version:

0.24.0 [release]
compiled with: llvm 3.9.1 -- gcc (GCC) 7.3.0
Defaults: pic=false ssl=openssl_0.9.0

source:

actor Main
  new create(env:Env) =>
    confusing_error_msg()
    
  fun confusing_error_msg(): String =>
    var s = "A string."
    
    // Forget to return a string
    //s

compiler output:

0.24.0 [release]
compiled with: llvm 3.9.1 -- cc (Ubuntu 5.4.0-6ubuntu1~16.04.10) 5.4.0 20160609
Defaults: pic=false ssl=openssl_0.9.0
Error:
main.pony:26:9: the left side is undefined but its value is used
    var s = "A string."
        ^
Error:
main.pony:26:11: left side must be something that can be assigned to
    var s = "A string."
          ^

Furthermore, changing the return type of confusing_error_msg to None results in a successful build, without any error output mentioning unused variables. I remember receiving errors regarding unused declarations previously, and am unsure if they are still a compilation error.

[aturley]

I think this is more generally a problem with the error message. At the very least maybe we could have a hint that says something about "destructive read" to give people a place to look? I don't necessarily think we need a specific error message for this situation (where the code indicates that the result of an initial destructive read should be returned by the function). I think the second error is also confusing (or even inaccurate) and should probably not appear.

[adri326]

I'd like to tackle this issue.

[SeanTAllen]

@adri326 have at it!