more latency improvements for #33615

[JeffBezanson]

I hit some more unpleasant delays after using Atom and CSV, e.g. in showing method lists and entering the Pkg prompt. This fixes them. It also allows rolling back one of the hacks in #33757.

[martinholters]

This can be a bit unfortunate for the common return copyto!(dest, src) construct (or otherwise using the return value instead of dest, as in foo!(copyto!(dest, src))) with typeof(dest) known but typeof(src) unknown. All matching methods of copyto! should be inferable as typeof(dest), but that won't help. If only all those places had used copyto!(dest, src); return dest...

Since #35965, this e.g. also affects the Array constructor:

julia> code_typed(Vector{Any}, Tuple{Vector}; optimize=false)
1-element Vector{Any}:
 CodeInfo(
1 ─ %1 = Core.apply_type(Base.Array, $(Expr(:static_parameter, 1)), $(Expr(:static_parameter, 2)))::Core.Const(Vector{Any}, false)
│   %2 = Base.size(x)::Tuple{Int64}
│   %3 = (%1)(Base.undef, %2)::Vector{Any}
│   %4 = Base.copyto_axcheck!(%3, x)::Any
└──      return %4
) => Any

Just increasing this to 2 restores

julia> code_typed(Vector{Any}, Tuple{Vector}; optimize=false)
1-element Vector{Any}:
 CodeInfo(
1 ─ %1 = Core.apply_type(Base.Array, $(Expr(:static_parameter, 1)), $(Expr(:static_parameter, 2)))::Core.Const(Vector{Any}, false)
│   %2 = Base.size(x)::Tuple{Int64}
│   %3 = (%1)(Base.undef, %2)::Vector{Any}
│   %4 = Base.copyto_axcheck!(%3, x)::Vector{Any}
└──      return %4
) => Vector{Any}

Not sure whether we should

• live with this
• undo this change (or at least bump max_methods to at least 2)
• audit all call sites of copyto! to not use the return value

[KristofferC]

FWIW, The copyto! method from #35965 should be removable with denizyuret/Knet.jl@bd7c162