`effect_free` not inferred, unlike with `Base.collect`

[nsajko]

julia> using Collects

julia> Base.infer_effects(collect, Tuple{typeof((n*n for n ∈ 1:3))})
(!c,+e,!n,!t,+s,+m,!u,+o,+r)

julia> f(x) = collect_as(Vector, x)
f (generic function with 1 method)

julia> Base.infer_effects(f, Tuple{typeof((n*n for n ∈ 1:3))})
(!c,!e,!n,!t,+s,!m,+u,+o,!r)

[nsajko]

After some investigation using Cthulhu.jl, it seems like Base.collect can be inferred as effect_free in this case by avoiding using push! as long as the input iterator has a known length:

https://github.com/JuliaLang/julia/blob/602d6ee987486ec2b0953bb2bf895ca7986b7a1a/base/array.jl#L789-L866

So there seems to be a tradeoff here: Base.collect risks unnecessarily allocating huge amounts of memory, in case the input iterator:

• has known length

• the length is great

• is of heterogeneous element type, especially at the front

Will think about this more.

[nsajko]

This is where Collects.jl does push!, in this case:

Collects.jl/src/Collects.jl

Line 126 in a8aff1e

push!(coll, elem)