Enforce which link types can be used

What it does

Rust uses markdown for doc comments. Markdown is a pretty loose standard and behaves differently based on the implementation. Rustdoc supports three different ways to specify a link with a link location:

# 1
[<text 1>](https://github.com/rust-lang/rust-clippy)

# 2
[<text 2>][https://github.com/rust-lang/rust-clippy]

# 3
[<text 3>]

[<text 3>]: https://github.com/rust-lang/rust-clippy

See: https://doc.rust-lang.org/rustdoc/write-documentation/linking-to-items-by-name.html

This lint should have a configuration, that can specify which links are allowed in doc comments. The configuration could be called: allowed-doc-link-styles or something similar, taking a list of items like:

• InlineBrackets for [][]
• InlineParenthesis for []()
• maybe Reference for the third case.

By default, I'd allow InlineParenthesis and Reference as these are normal Markdown links.

This should be a relatively simple lint, as Clippy already has a markdown parser for doc comments, however, it might require some handling to correctly detect which type of link is actually used

Lint Name

comment_link_style

Category

restriction

Advantage

• Unifies the codebase

Drawbacks

• Might cause many reports, when it's not a restriction lint

Example

/// # 1
/// [<text 1>](`Option`)
/// 
/// # 2
/// [<text 2>][`Option`]
/// 
/// # 3
/// [<text 3>]
/// 
/// [<text 3>]: `Option`
struct CoolItem;

Could be written as:

/// # 1
/// [<text 1>](`Option`)
/// 
/// # 2
/// [<text 2>](`Option`)
/// 
/// # 3
/// [<text 3>]
/// 
/// [<text 3>]: `Option`
struct CoolItem;

Start by locating Clippy's existing markdown parser for doc comments and the configuration handling for new lints. Define detection for the three link styles, apply the proposed default and configurable allowed styles, and verify that disallowed styles are reported while allowed styles are accepted using the issue's examples.