JIT: Use importer stack type to elide ldelema covariance check

[EgorBo]

When ldelema is imported inside a shared generic inlinee (e.g. ref _items[i] in MyList<T>), the array tree only knows __Canon[], so the existing "T[] with exact T" check fails and we emit CORINFO_HELP_LDELEMA_REF. The importer's stack type for that array was resolved in the exact context (Instruction[]), so use it as a fallback.

sealed class Instruction { }

class MyList<T>
{
    private T[] _items;
    public ref T this[int i] => ref _items[i];
}

static ref Instruction Test(MyList<Instruction> list, int i) => ref list[i];

 G_M40277_IG01:
        sub      rsp, 40
 G_M40277_IG02:
-       mov      rcx, gword ptr [rcx+0x08]
-       movsxd   rdx, edx
-       mov      r8, 0x7FFB49FFA3E0      ; Instruction
-       call     [CORINFO_HELP_LDELEMA_REF]
-       nop
+       mov      rax, gword ptr [rcx+0x08]
+       cmp      edx, dword ptr [rax+0x08]
+       jae      SHORT G_M40277_IG04
+       mov      ecx, edx
+       lea      rax, bword ptr [rax+8*rcx+0x10]
 G_M40277_IG03:
        add      rsp, 40
        ret
+G_M40277_IG04:
+       call     CORINFO_HELP_RNGCHKFAIL
+       int3

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[EgorBo]

@MihuBot

[EgorBo]

@MihuBot -nuget

[EgorBo]

Doesn't look to be worth it