clippy::result_large_err: the `Err`-variant returned from this function is very large

What you see

warning: the `Err`-variant returned from this function is very large
 --> main.rs:9:19
  |
4 |     UnparsedBytes([u8; 512]),
  |     ------------------------ the largest variant contains at least 512 bytes
...
9 | pub fn parse() -> Result<(), ParseError> {
  |                   ^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: try reducing the size of `main::ParseError`, for example by boxing large elements or replacing it with `Box<main::ParseError>`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#result_large_err
  = note: `#[warn(clippy::result_large_err)]` on by default

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

A Result is at least as large as the Err-variant. While we expect that variant to be seldom used, the compiler needs to reserve and move that much memory every single time. Furthermore, errors are often simply passed up the call-stack, making use of the ?-operator and its type-conversion mechanics. If the Err-variant further up the call-stack stores the Err-variant in question (as library code often does), it itself needs to be at least as large, propagating the problem.

Code that could be written to run faster. Warns by default.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

pub enum ParseError {
    UnparsedBytes([u8; 512]),
    UnexpectedEof,
}

// The `Result` has at least 512 bytes, even in the `Ok`-case
pub fn parse() -> Result<(), ParseError> {
    Ok(())
}

Write this instead

pub enum ParseError {
    UnparsedBytes(Box<[u8; 512]>),
    UnexpectedEof,
}

// The `Result` is slightly larger than a pointer
pub fn parse() -> Result<(), ParseError> {
    Ok(())
}

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

#[allow(clippy::result_large_err)]
fn my_function() { /* ... */ }

[lints.clippy]
result_large_err = "allow"

cargo clippy -- -A clippy::result-large-err

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Settings in clippy.toml

• large-error-ignored: A list of paths to types that should be ignored as overly large Err-variants in a Result returned from a function
• large-error-threshold: The maximum size of the Err-variant in a Result returned from a function