disallow slicing many-pointer with sentinel without end index

[Techatrix]

The following code currently compiles without errors:

const std = @import("std");
const assert = std.debug.assert;

test {
    var foo: [*]const u8 = &.{ 1, 2 };
    _ = foo[0.. :0];
    comptime assert(@TypeOf(foo[0.. :0]) == [*]const u8);
}

The sentinel expression is being analyzed but has does nothing.

I can think of two possible ways to resolve this:

• disallow ptr[start.. :sentinel] on a multi item pointer (or C pointer)
• resolve to [*:0]const u8

The problem with the second option is that it is not possible to implement a safety check for it.

Related:

• Allow slicing multi-pointers as ptr[start..] #15561
• Optimize lowering of s[start..][0..len] #15519

[Rexicon226]

disallow slicing with a sentinel on a multi item pointer (or C pointer)

foo[0..10 :0] this is considered slicing with a sentinel on a multi-item pointer. There's absolutely no reason it shouldn't be allowed.
However slicing a multi-item pointer where end_is_len is true, with a sentinel, I agree doesn't make sense and shouldn't be allowed.

[Techatrix]

foo[0..10 :0] this is considered slicing with a sentinel on a multi-item pointer. There's absolutely no reason it shouldn't be allowed.

I meant to focus on slicing without an end index. I will clarify the issue description.

[andrewrk]

Thanks for the writeup. Let's go with the conservative strategy - disallow it. This choice reflects the goal of minimizing the amount of uncheckable illegal behavior that the language allows.