Array bounds check not elided when indexing with ZST enum

[moxian]

I tried this code:

#[repr(C)]
pub enum E {
    A, 
}

pub fn index(x: &[u32; 3], ind: E) -> u32{
    x[ind as usize]
}

I expected to see this happen: no bounds check

Instead, this happened (with -C opt-level=3)

example::index:
        push    rax
        mov     rax, rdi
        mov     edi, esi
        cmp     esi, 2
        ja      .LBB0_2
        mov     eax, dword ptr [rax + 4*rdi]
        pop     rcx
        ret
.LBB0_2:
        lea     rdx, [rip + .L__unnamed_1]
        mov     esi, 3
        call    qword ptr [rip + core::panicking::panic_bounds_check@GOTPCREL]
        ud2

However adding more variants to the enum makes the bound check go away:

#[repr(C)]
pub enum E {
    A,
    B,
}

pub fn index(x: &[u32; 3], ind: E) -> u32{
    x[ind as usize]
}

generates:

example::index:
        mov     eax, esi
        mov     eax, dword ptr [rdi + 4*rax]
        ret

Here's a godbolt link to play around further: https://rust.godbolt.org/z/dK9e1z
The MIR of the two cases is identical, but the llvm IR in ZST case is lacking @llvm.assume, which I assume (heh!) leads to the bounds check being preserved. (However i don't actually understand LLVM, I'm just reading the diff...)

Meta

rustc 1.50.0

This sounds very closely related to #13926, but I'm not actually sure if it is. Please reroute as appropriate.

[elomatreb]

Removing any #[repr(...)] attributes from the enum also leads to the optimization occurring as expected (https://rust.godbolt.org/z/fPn613).

[nikic]

We don't emit the enum range assumption if the range only contains one element:

rust/compiler/rustc_codegen_ssa/src/mir/rvalue.rs

Line 328 in 1d6b0f6

&& scalar.valid_range.end() > scalar.valid_range.start()

[Rustin170506]

@rustbot claim