Redundant bounds check when indexing array with enum in Rust 1.90

Code

I tried this code:

fn test(instructions: &[Instruction], registers: [u32; 4]) -> u32 {
    if let Some(&instruction) = instructions.get(0) {
        match instruction {
            Instruction::A(a) => registers[a as usize],
            Instruction::B(b) => registers[b as usize],
        }
    } else {
        0
    }
}

#[derive(Copy, Clone)]
enum Instruction {
    A(Register),
    B(Register),
}

#[derive(Copy, Clone)]
enum Register {
    A,
    B,
    C,
    D,
}

I expected to see this happen: No bounds check is emitted

Instead, this happened: There's a bounds check accessing registers

Version it worked on

It most recently worked on: Rust 1.89

Version with regression

rustc --version --verbose:

rustc 1.90.0 (1159e78c4 2025-09-14)
binary: rustc
commit-hash: 1159e78c4747b02ef996e55082b704c09b970588
commit-date: 2025-09-14
host: x86_64-unknown-linux-gnu
release: 1.90.0
LLVM version: 20.1.8

Also checked in nightly:

rustc 1.92.0-nightly (53a741fc4 2025-10-16)
binary: rustc
commit-hash: 53a741fc4b8cf2d8e7b1b2336ed8edf889db84f4
commit-date: 2025-10-16
host: x86_64-unknown-linux-gnu
release: 1.92.0-nightly
LLVM version: 21.1.3

Bisected to b56aaec52bc0fa35591a872fb4aac81f606e265c / #144389 using cargo-bisect-rustc