Error for variable only set but never read

[wooster0]

Main idea

pub fn main() void {
    var x = 1;
    x = 5; // Now Zig thinks `x` was "used",
           // but it was only ever written to, never read
}

This compiles without an error.
But GCC has this feature that Zig doesn't have:

int main() {
    int x = 1;
    x = 5;
}

$ gcc x.c -Wunused-but-set-variable
x.c: In function ‘main’:
x.c:2:9: warning: variable ‘x’ set but not used [-Wunused-but-set-variable]
    2 |     int x = 1;
      |         ^

Recently when messing around with C I found this warning really nice. Now I want it in Zig too. I think it's really useful and can prevent many logic errors.

So I propose having this feature in Zig too and making it an error if you only ever set a variable and never actually read it.

Extra

In fact, we might make it so smart that even this will trigger an error because the second write is useless and you aren't reading from x before the second write:

pub fn main() void {
    var x: u8 = 1;
    x = 5;
    _ = x;
}

This is more advanced but might lead to some other undesirable things (haven't thought this extra through too much) so this is optional.
So basically you'd be required to read a variable at least once before you can write it again.
This is something that GCC doesn't have.

See also: #335
See also: nektro/ziglint#15
See also: #2304