Missing compile error for reassigning parameter

[jayschwa]

Possibly related to #788.

I have a piece of code that processes smaller sequential slices of a larger slice. Rather than getting a slice of length zero at the end, I get a segmentation fault.

const std = @import("std");

const CHUNK_LEN = 2;

fn slice_to_zero(input: []const u8) void {
    while (input.len > 0) {
        const take = std.math.min(input.len, CHUNK_LEN);
        _ = input[0..take]; // Imagine this slice chunk is used for something
        input = input[take..];
    }
}

test "slicing down to zero length" {
    slice_to_zero("");
    slice_to_zero("foo");
}

[LemonBoy]

input = input[take..];

This is illegal as parameters are const by default => #4015

_ = input[0..take];

This is a long-standing issue that affects many builtins (and also the slice operator), when the destionation is _ the codegen gets a nullptr for the result_loc and so it segfaults. I'm pretty sure there are a few tickets open about that.

[jayschwa]

It looks like my code works if I create a new slice variable from the const slice parameter. Lack of compiler error was allowing me to footgun. Thanks.

[andrewrk]

I wasnt aware of this missing compile error, so I'm going to keep this open to track if that's ok

Linking #3800 as well