WIP: handle zero_tangent in presence of cyclic structures (v1)

[oxinabox]

Follow on to #626
this was my first take at it.
I do not like it because it depends on Core.Compiler.return_type which is unreliable for getting tight types and in real cases I have tried falls back to returning Any.

and this code adds a lot of complexity and still doesn't catch everything.

[github-actions]

[JuliaFormatter] _{reported by reviewdog 🐶}

Suggested change

[github-actions]

[JuliaFormatter] _{reported by reviewdog 🐶}

Suggested change

	$(
	map(fieldnames(primal), zfield_exprs) do fname, fval_expr
	:(setproperty!(tangent, $(QuoteNode(fname)), $fval_expr))
	end...
	)
	return tangent
	$(map(fieldnames(primal), zfield_exprs) do fname, fval_expr
	:(setproperty!(tangent, $(QuoteNode(fname)), $fval_expr))
	end...)
	return tangent

[github-actions]

[JuliaFormatter] _{reported by reviewdog 🐶}

Suggested change

	:($Tangent{$primal}($(Expr(:parameters, Expr.(:kw, fieldnames(primal), zfield_exprs)...))))
	:($Tangent{$primal}(
	$(Expr(:parameters, Expr.(:kw, fieldnames(primal), zfield_exprs)...))
	))

[github-actions]

[JuliaFormatter] _{reported by reviewdog 🐶}

Suggested change

	end
	end

[github-actions]

[JuliaFormatter] _{reported by reviewdog 🐶}

Suggested change

	Ref{Union{ZeroTangent, tangent_type}} # allow a Zero which will be used for uninitialized values
	Ref{Union{ZeroTangent,tangent_type}} # allow a Zero which will be used for uninitialized values

[github-actions]

[JuliaFormatter] _{reported by reviewdog 🐶}

Suggested change

	return new{P, typeof(backing)}(backing)
	return new{P,typeof(backing)}(backing)

[github-actions]

[JuliaFormatter] _{reported by reviewdog 🐶}

Suggested change

	function MutableTangent{P}(fvals) where P
	function MutableTangent{P}(fvals) where {P}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fe63c33) 94.15% compared to head (5fbbe5b) 94.34%.
Report is 1 commits behind head on ox/mutabletangent.

Additional details and impacted files

@@                  Coverage Diff                  @@
##           ox/mutabletangent     #654      +/-   ##
=====================================================
+ Coverage              94.15%   94.34%   +0.18%     
=====================================================
  Files                     15       15              
  Lines                    976      991      +15     
=====================================================
+ Hits                     919      935      +16     
+ Misses                    57       56       -1     

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[oxinabox]

Closing in favor of #655 (comment)