New lint: `invariant_collect_option`

What it does

Checks for use of collect::<Option<_>>() where the result is always Some or always None. If the iterator always contains None, then the collect will always yield None and is pointless. If the iterator items are always Some, then the collect will always yield Some and can most likely be simplified.

Detection of these cases could iteratively improve over time, but we can start with the example below.

Categories (optional)

• Kind: complexity

Drawbacks

None.

Example

(0..5).map(|n| Some(n + 1)).collect::<Option<Vec<u32>>>();

Could be written as:

(0..5).map(|n| n + 1).collect::<Vec<u32>>();

The lint could work with filter or take (and more?) operations between map and collect.

It is probably too difficult to provide a specific suggestion to fix this lint. The user will have to change not only the iterator, but the usage of the collect output.

The programmer may have a misconception that None will be returned if the iterator is empty. This may be addressed in the lint output.

Start with the issue's example and review existing Rust Clippy lint implementations and test conventions; no specific source file or test is named. Define detection for iterator chains whose Option items are always Some or None, and verify the lint reports the example without requiring an automatic suggestion.