lint idea: `useless_default_generic_parameters`

What it does

Given a type with a generic argument that has a default:

type Result<T = ()> = core::result::Result<T, MyError>;

Usage of this type when specifying the default should not use generics:

fn foo() -> Result<()> { Ok(()) }
//                ^^^^ unnecessary generic, `()` already is the default
//                hint: use `Result` instead.

Advantage

• Removes duplication of the default value
• Reduces visual clutter
• Reminds people that the type has a default value

Drawbacks

• when a library adds a default value, one gets this warning when updating
• macros might trigger this involuntarily

Example

type Result<T = ()> = core::result::Result<T, MyError>;

fn foo() -> Result<()> {
    Ok(())
}

Could be written as:

type Result<T = ()> = core::result::Result<T, MyError>;

fn foo() -> Result {
    Ok(())
}