It is impossible to implement a span-like container where JIT would elide a custom bounds check

[neon-sunset]

Description

Given a custom span-like container Slice<T> and a for loop that iterates over it:

static void For(Slice<int> slice) {
    var sum = 0;
    for (nuint i = 0; i < slice.Length; i++) {
        sum += slice[i];
    }
    Console.WriteLine(sum);
}

unsafe readonly struct Slice<T>(T* ptr, nuint length)
where T: unmanaged {
    public nuint Length => length;

    public T this[nuint index] {
        get {
            ArgumentOutOfRangeException
                .ThrowIfGreaterThanOrEqual(index, length);
            return ptr[index];
        }
    }
}

The For method compiles to

G_M000_IG01:                ;; offset=0x0000
       push     rsi
       push     rbx
       sub      rsp, 40
G_M000_IG02:                ;; offset=0x0006
       mov      rax, qword ptr [rcx]
       mov      rbx, qword ptr [rcx+0x08]
       xor      ecx, ecx
       xor      esi, esi
       test     rbx, rbx
       je       SHORT G_M000_IG04
       align    [10 bytes for IG03]
G_M000_IG03:                ;; offset=0x0020
       cmp      rsi, rbx
       jae      SHORT G_M000_IG06
       add      ecx, dword ptr [rax+4*rsi]
       inc      rsi
       cmp      rsi, rbx
       jb       SHORT G_M000_IG03
G_M000_IG04:                ;; offset=0x0030
       call     [System.Console:WriteLine(int)]
       nop      
G_M000_IG05:                ;; offset=0x0037
       add      rsp, 40
       pop      rbx
       pop      rsi
       ret      
G_M000_IG06:                ;; offset=0x003E
       mov      ecx, 1
       mov      rdx, 0x7FFF9C99D780
       call     CORINFO_HELP_STRCNS
       mov      r8, rax
       mov      rdx, rbx
       mov      rcx, rsi
       call     [System.ArgumentOutOfRangeException:ThrowGreaterEqual[ulong](ulong,ulong,System.String)]
       int3     

Analysis

It appears that no matter the implementation, it is impossible to write the code in a way where the bounds check is elided.
E.g. none of these changes work:

• writing custom check and throw helper
• matching loop conditional with bounds check with !(index >= slice.Length)
• making Length a field
• using signed and plain int-based indexer and length

Configuration

.NET SDK:
 Version:           9.0.100-rtm.24512.1
 Commit:            5b9d9d4677
 Workload version:  9.0.100-manifests.87287131
 MSBuild version:   17.12.3+4ae11fa8e

Regression?

No

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