compile-time use of undefined value detection

[andrewrk]

extern fn bar();
export fn entry() {
    var a: i32 = undefined;
    if (a > 10) {
        bar();
    }
}

Zig should be able to detect that we will with 100% certainty branch on an undefined value.

Related:

export fn entry() {
    unreachable;
}

Should be an error, because it's 100% certain at compile-time that we will reach unreachable code.

Not to be confused with #211 which is adding extra code and data in safety-enabled builds to detect use of undefined value at runtime.

[pluto439]

After a = undefined, will I be able to create another variable with the same name, but a different type?

[andrewrk]

No, you would need to make a go out of scope. This use case is not supported because we want code readers to have the guarantee that within a scope, if you see a variable name, that same variable name is the same type and refers to the same memory location in the entire scope.

[pluto439]

It's not the same as deleting a variable then. I want it to stop existing, completely. I will only ever delete variables that are temporary anyway.

Probably a wrong place to talk about it, should've I wrote this in the closed issue?

[PavelVozenilek]

@pluto439:

After a = undefined, will I be able to create another variable with the same name, but a different type?

You are asking for feature similar to "hyperstatic scope" used in Forths ( http://wiki.c2.com/?HyperStaticGlobalEnvironment ). Here it is used for functions (Forth doesn't have anything else) and allows to reuse their names, reducing the need to invent unique names all the time. It is one of Forth's strong points.

[pluto439]

@PavelVozenilek

That's too smart for me. I'm just trying to steal the del keyword from python. Which in c-like languages will mean "free stack, or mark this place on stack as free to reuse".

[PavelVozenilek]

@pluto439

Which in c-like languages will mean "free stack, or mark this place on stack as free to reuse".

LLVM does this automatically, it is called "stack slot coloring".

[pluto439]

@PavelVozenilek I know, but didn't knew it's called "stack slot coloring". Just wanted to say that it's possible for compiled languages too, and not for interpreted ones only.

It's more of a hint for a programmer that this variable will not be used again, and also a safety net. More details here #594, further discussion should be there I guess.

[andrewrk]

Related: #1947