Implement Eq derive (#163)

[tyranron]

Part of #163

Synopsis

As our PartialEq derive infers trait bounds correctly for generic cases, we need also an Eq derive for doing the same.

Solution

derive(Eq) in std uses core::cmp::AssertParamIsEq to assert that fields' types are indeed Eq in its Eq::assert_receiver_is_total_eq() method. However, they both are #[doc(hidden)] and not a stable part of std, so we cannot reuse them blindly.

However, instead, we could just put the appropriate trait bounds in the where clause of the generated Eq impl:

impl Eq for Foo where FieldType: Eq {}

The only situation where this fails is when the field type uses the self-type recursively:

struct Foo(Option<Box<Foo>>);

In this case, we cannot generate Option<Box<Foo>>: Eq trait bound, because will hit the compilation error:

error[E0275]: overflow evaluating the requirement

For such fields, we need to do the similar static assertion as std does, that's why introduce our own derive_more::__private::AssertParamIsEq helper type. In contrast with std, we do generate these assertions (if needed) in an inherent method, rather than introducing a trait for it.

Checklist

• Documentation is updated
• Tests are added/updated
• CHANGELOG entry is added
• README is updated