Use Base.IdSet

GitHub tells me this has been available since at least 0.7.

src/Zygote.jl

@@ -18,7 +18,6 @@ export rrule_via_ad
 
 const Numeric{T<:Number} = Union{T, AbstractArray{<:T}}
 
-include("tools/idset.jl")
 include("tools/buffer.jl")
 include("tools/builtins.jl")
 

src/compiler/interface.jl

@@ -1,7 +1,7 @@
 using InteractiveUtils
 using InteractiveUtils: typesof
 using Core: Typeof
-import Base: copy!
+import Base: copy!, IdSet
 import Base.Broadcast: broadcasted, materialize!
 
 mutable struct Context <: AContext
@@ -144,7 +144,7 @@ struct Params
 end
 
 Params() = Params(Buffer([], false), IdSet())
-Params(xs) = Params(Buffer(xs, false), IdSet(xs))
+Params(xs) = Params(Buffer(xs, false), IdSet{Any}(xs))
 Params(ps::Params) = ps
 Params(xs::Tuple) = Params(collect(xs))
 

src/profiler/Profile.jl

@@ -1,7 +1,7 @@
 module Profile
 
 using Requires
-using ..Zygote: Pullback, IdSet, meta, stacklines
+using ..Zygote: Pullback, meta, stacklines
 
 function loc(f)
   # TODO perhaps find most general method
@@ -36,7 +36,7 @@ function mem(x, seen)
   sum(x -> mem(x, seen), fields(x))
 end
 
-mem(x) = mem(x, IdSet())
+mem(x) = mem(x, Base.IdSet())
 
 struct Node
   func::Symbol