clippy::option_option: use of `Option<Option<T>>`

What you see

warning: use of `Option<Option<T>>`
 --> main.rs:3:18
  |
3 | fn get_data() -> Option<Option<u32>> {
  |                  ^^^^^^^^^^^^^^^^^^^
  |
  = help: consider using `Option<u32>`, or a custom enum if you need to distinguish all 3 cases
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#option_option
  = note: `-W clippy::option-option` implied by `-W clippy::pedantic`

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?

Option<_> represents an optional value. Option<Option<_>> represents an optional value which itself wraps an optional. This is logically the same thing as an optional value but has an unneeded extra level of wrapping. If you have a case where Some(Some(_)), Some(None) and None are distinct cases, consider a custom enum instead, with clear names for each case.

Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.

How to fix it

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

Flagged

fn get_data() -> Option<Option<u32>> {
    None
}

Write this instead

pub enum Contents {
    Data(Vec<u8>), // Was Some(Some(Vec<u8>))
    NotYetFetched, // Was Some(None)
    None,          // Was None
}

fn get_data() -> Contents {
    Contents::None
}

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

[lints.clippy]
option_option = "warn"

#![warn(clippy::option_option)]

cargo clippy -- -W clippy::option-option

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

• avoid-breaking-exported-api: Suppress lints whenever the suggested change would cause breakage for other crates.