Lint `sparse_imports`

[Veetaha]

Lint explanation

Detect use statements that are separated with empty lines, and suggest removing them. The reasoning is that imports that don't have empty lines between them are nicely sorted by rustfmt in an alphabetical order.

This does apply to use statements with different visibility.

This lint is rather subjective and should be opt-in.

Example code

Bad

use std::path::Path;

use serde::Deserialize;

Good

use serde::Deserialize;
use std::path::Path;

Notes

marker must have the ability to provide information about whitespace in code to lints to implement this. I know it is not obvious what AST nodes whitespace (just like comments) should be nested under, but this is not very important. Rust-analyzer, for example has a lossless AST, and it attaches the leading whitespace to AST nodes (more details in rust-marker/marker#189 (comment)).

[xFrednet]

It should be possible to extract the line location from the spans of the use items. That's how Clippy deals with similar lints. Though right now, spans are a bit broken 😅 rust-marker/marker#175