Recommend testing array sizes using type system rather than assertions when possible

What it does

Checks for functions accepting slices that immediately check its length for equality with a constant and fail if isn't equal.

Categories (optional)

• Kind: pedantic

What is the advantage of the recommended code over the original code

• Making illegal states unrepresentable.
• Making length requirements clear in documentation.
• Removing unnecessary bound check at runtime.

Drawbacks

Necessitates using try_into() when calling a function using a slice. Doing this change for a public API is a breaking change.

Example

fn my_func(x: &[u32]) {
    assert_eq!(x.len(), 16);
    unimplemented!();
}

Could be written as:

fn my_func(x: &[u32; 16]) {
    unimplemented!();
}

Start with the issue's slice-length equality example and review how Rust Clippy organizes comparable lint proposals. Define the cases the lint should recognize, including the documented public-API breaking-change concern, and determine the expected diagnostics and tests. Done means the behavior and limitations are specified well enough to implement and verify.