Improve performance of depth-limited type printing in stack traces

[charleskawczynski]

Can we improve the performance of depth-limited type printing in stack traces? Here is an example where printing truncated types takes quite a while with a highly nested struct:

Base.@kwdef struct Nested{A,B}
    num::Int = 1
end
nest_val(na, nb, ::Val{1}) = Nested{na, nb}()
nest_val(na, nb, ::Val{n}) where {n} = nest_val(Nested{na, nb}, Nested{na, nb}, Val(n-1))
nest_val(na, nb, n::Int) = nest_val(na, nb, Val(n))
nest_val(n) = nest_val(1, 1, n)
foo1(t::Nested) = error("oops")
foo2(t::Nested) = foo1(t)
foo3(t::Nested) = foo2(t)
foo4(t::Nested) = foo3(t)
foo5(t::Nested) = foo4(t)
foo6(t::Nested) = foo5(t)
foo7(t::Nested) = foo6(t)
foo8(t::Nested) = foo7(t)
foo9(t::Nested) = foo8(t)
foo10(t::Nested) = foo9(t)
NV = nest_val(20); # be careful with changing 20 to a larger number
foo10(NV) # errors, takes ~45 seconds on Julia 1.10

I haven't looked at the implementation in a while, but I imagine that this could be related to #55807.

[nsajko]

Try the latest nightly, xref #55575.

[charleskawczynski]

Try the latest nightly, xref #55575.

I just tried it. It's actually slower on nightly. ~ 148 seconds

[KristofferC]

I think this is the same as #37990.

The code tries to check if this type could potentially have a type alias somewhere and in that case print the type alias. This check is very slow and it is done over and over and over.

julia/base/show.jl

Lines 729 to 754 in 1cfda3f

	function show_typealias(io::IO, name::GlobalRef, x::Type, env::SimpleVector, wheres::Vector)
	if !(get(io, :compact, false)::Bool)
	# Print module prefix unless alias is visible from module passed to
	# IOContext. If :module is not set, default to Main.
	# nothing can be used to force printing prefix.
	from = get(io, :module, Main)
	if (from === nothing || !isvisible(name.name, name.mod, from))
	show(io, name.mod)
	print(io, ".")
	end
	end
	print(io, name.name)
	isempty(env) && return
	io = IOContext(io)
	for p in wheres
	io = IOContext(io, :unionall_env => p)
	end
	orig = getfield(name.mod, name.name)
	vars = TypeVar[]
	while orig isa UnionAll
	push!(vars, orig.var)
	orig = orig.body
	end
	show_typeparams(io, env, Core.svec(vars...), wheres)
	nothing
	end