pydantic/monty · info

nibble fits usize

Error message

nibble fits usize

What it means

Internal `expect` in the table-driven `crc32`: `crc & 0x0f` yields a value in 0..=15, which always fits `usize`, so indexing `NIBBLE_TABLE` cannot fail. The message ('nibble fits usize') documents why the conversion is infallible. Users cannot trigger it via `binascii.crc32` with any input data or seed.

Source

Thrown at crates/monty/src/modules/binascii.rs:554

        0x26d9_30ac,
        0x76dc_4190,
        0x6b6b_51f4,
        0x4db2_6158,
        0x5005_713c,
        0xedb8_8320,
        0xf00f_9344,
        0xd6d6_a3e8,
        0xcb61_b38c,
        0x9b64_c2b0,
        0x86d3_d2d4,
        0xa00a_e278,
        0xbdbd_f21c,
    ];

    let mut crc = !seed;
    for byte in data {
        crc ^= u32::from(*byte);
        crc = (crc >> 4) ^ NIBBLE_TABLE[usize::try_from(crc & 0x0f).expect("nibble fits usize")];
        crc = (crc >> 4) ^ NIBBLE_TABLE[usize::try_from(crc & 0x0f).expect("nibble fits usize")];
    }
    !crc
}

/// Computes the CRC-16 of `data`, continuing from `seed`.
///
/// CRC-16/XMODEM: polynomial `0x1021`, unreflected, no final xor — the
/// checksum BinHex 4.0 carried, which `crc_hqx` outlived.
fn crc_hqx(data: &[u8], seed: u16) -> u16 {
    let mut crc = seed;
    for byte in data {
        crc ^= u16::from(*byte) << 8;
        for _ in 0..8 {
            crc = if crc & 0x8000 == 0 {
                crc << 1
            } else {
                (crc << 1) ^ 0x1021

View on GitHub (pinned to adc986b362)

Solutions

1. No runtime action; keep the `& 0x0f` mask matched to the 16-entry table.

When it happens

Trigger: Unreachable; would require the mask `0x0f` and the table size (16) to fall out of sync in an edit.

Common situations: Contributor-only, e.g. swapping to a different nibble-table width.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• reduced below 2**32
• gather commit frame id is not a GatherFuture
• gather item is not a Coroutine, ExternalFuture, or GatherFut
• Cannot get identity of Dereferenced object
• invalid ascii byte

AI-assisted analysis of pydantic/monty@adc986b362 (2026-09-13). Data as JSON: /api/errors/930f09b932a8bd38. Report an issue: GitHub.