Improved error for undefined but used left side of declarations

[adri326]

This addresses #2828

Declaring a variable at the end of a function which expects a return value or having generally a variable declaration where the compiler expects to use the return value caused in most cases an error stating that the variable is undefined but its value is used:

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

This bit of code triggers the following error on 0.33.1:

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."
          ^

The presence of the second error was also considered to be erroneous.

This patch discriminates in the is_lvalue function from pass/refer.c cases where the left-hand side of the assignment is not an lvalue and cases where it triggers an error. This way, the compiler can tell between the left side not being something that can be assigned to or it having triggered an error.

Additionally, an Info message is triggered when the left side's old value is needed in a declaration.

This is what the error message now looks like:

Error:
main.pony:6:9: the left side is undefined but its value is used
    var s = "A string."
        ^
    Info:
    main.pony:6:5: the previous value of 's' is used because you are trying to use the return value of this var declaration
        var s = "A string."
        ^

[mfelsche]

That is a cool improvement. The previous error message really didn't look like anything to me.

I don't know if it fits an error message, but i think why the compiler gives this message in the first place, which is kind of pretty unintuitive, is that the assignment to s will return its old value, as in a destructive read. It is just accumulated into a var declaration.
To me the compiler complaining here makes sense to m, if I put it this way:

Using a variable declaration as a "return" expression is not allowed as it would result in an undefined value, given an assignment always yields the previous value of the variable you are assigning to.

Just my 2 cents. I am happy with this going through as is as well.

[adri326]

I agree with you

[SeanTAllen]

After @jemc's comment is addressed this is good to be squashed and merged. If it's updated in the next couple of days, it can make it into the next release that is coming very soon.

[adri326]

(poke @SeanTAllen) I just committed it - I had no reason not to :)

[SeanTAllen]

Thanks @adri326