improved handling of index of a sentinel-terminated array which is comptime-known to be len

[andrewrk]

Extracted from #3770.

When the index used for loading or storing to a sentinel-terminated slice or array is comptime-known to be equal to the length, we know at compile-time the value is the sentinel. So, loading should give a comptime value of the sentinel, with no runtime code emitted, and storing should be a no-op with a safety check to make sure the stored value is equal to the sentinel value.

const std = @import("std");
const expect = std.testing.expect;

test "comptime known load of sentinel-terminated slice" {
    // here we have a normal array
    var buf: [50]u8 = undefined;

    buf[0] = 'a';
    buf[1] = 'b';
    buf[2] = 'c';
    buf[3] = 0;

    // now we obtain a null terminated slice:
    const slice = buf[0..3 :0];

    comptime expect(slice[3] == 0);
}

The safety check thing:

test "trip runtime safety check" {
    // here we have a normal array
    var buf: [50]u8 = undefined;

    buf[0] = 'a';
    buf[1] = 'b';
    buf[2] = 'c';
    buf[3] = 0;

    // now we obtain a null terminated slice:
    const slice = buf[0..3 :0];
    var runtime_elem: u8 = 42;

    slice[3] = runtime_elem; // panic: mangled the sentinel value
}

Potentially could be further improved:

test "trip runtime safety check 2" {
    // here we have a normal array
    var buf: [50]u8 = undefined;

    buf[0] = 'a';
    buf[1] = 'b';
    buf[2] = 'c';
    buf[3] = 0;

    // now we obtain a null terminated slice:
    const slice = buf[0..3 :0];
    var runtime_elem: u8 = 42;
    var runtime_index: usize = 3;

    slice[runtime_index] = runtime_elem; // panic: mangled the sentinel value
}