New lint: reexport_crate_in_api

What it does

If a library crate contains a type or trait in public API that was defined in a dependency, including alloc and std, then clippy should suggest to reexport the dependency using pub extern crate foo or pub use foo as bar.

Lint Name

reexport_crate_in_api

Category

style

Advantage

• The downstream crates don't need to bother with adding it to Cargo.toml
• Less version conflicts
• No need to look up which version is compatible
• No need for ugly tricks or high MSRV when the downstream dependency wants to depend optionally
• Including std and alloc helps crates that want to be no_std-aware with optional std/alloc features

Drawbacks

A few more trivial items in crates. Changing rustdoc to put these at the bottom of docs would mitigate the noise.

Example

#[derive(Serialize, Deserialize)]
pub struct Foo(u32);

pub fn bar() -> String {
    "hello".to_owned()
}

Could be written as:

pub extern crate serde;
pub extern crate alloc;

#[derive(Serialize, Deserialize)]
pub struct Foo(u32);

pub fn bar() -> String {
    "hello".to_owned()
}

The issue names no implementation files, tests, or entry points. Start by surveying existing Clippy lint implementations and their UI tests, then define detection and suggestion coverage for dependency, alloc, and std reexports before validating the proposed examples.