Cache the hash code when requested.

[gafter]

In many client scenarios, thee are types for which the generated code would be more efficient if it (1) computed the hash code at construction time and cached it, and (2) compares the hash codes when comparing for equality, to shortcut the potentially more expensive equality comparisons on the elements.

Specifically, code like this

Using AutoHashEquals

@auto_hash_equals_cached struct Q
    x::Int
end

would be translated to something like

struct Q
    x::Int
    _cached_hash::UInt
    Q(x) = new(x, hash(x))
end

@show_structure Base.hash(x::Q) = x._cached_hash
@show_structure Base.hash(x::Q, h::UInt) = hash(x._cached_hash, h)
@show_structure Base.:(==)(a::Q, b::Q) = a._cached_hash == b._cached_hash && isequal(a.x, b.x)
@show_structure Base.show(io::IO, x::Q) = print(io, "Q(", x.x, ")")

(I'm willing to do the work of implementing this.)

[gafter]

The function show shouldn't be defined if the user has already done so.

You cannot specialize a function on a type that hasn't been declared yet.

[gafter]

The implementation of show should correctly handle cyclic data structures in which Q in on the cycle.

[gafter]

The current latest version (not registered yet) supports all this.