Error/lint on non-use of return values once purity is completely tracked

[calebzulawski]

I just spent some time debugging a simple case where I meant to write ^func args and instead had func args

[afranchuk]

Yeah this would be very easy if the function is known to be pure. Unfortunately if the function is impure the same mistake can be made :( If there were more type information (static analysis) available, there could be a lint that warns when e.g. a function returns a map but isn't being merged.

[calebzulawski]

Good point, checking the type would probably be better. This seems like a relatively lightweight check, some lints like this could probably be turned on by default (without specifically enabling linting)