Why it is not possible to use for loops inside the setup block?

[bvdmitri]

julia> using BenchmarkTools

julia> b = @benchmarkable sum(a) setup = begin
           a = zeros(10)
           for i in 1:10
               a[i] = i
           end
       end
Benchmark(evals=1, seconds=5.0, samples=10000)

julia> run(b)
ERROR: UndefVarError: `i` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
Stacktrace:
  [1] var"##sample#231"(::Tuple{}, __params::BenchmarkTools.Parameters)
    @ Main ~/.julia/packages/BenchmarkTools/1i1mY/src/execution.jl:607
  [2] _run(b::BenchmarkTools.Benchmark, p::BenchmarkTools.Parameters; verbose::Bool, pad::String, warmup::Bool, kwargs::@Kwargs{})
    @ BenchmarkTools ~/.julia/packages/BenchmarkTools/1i1mY/src/execution.jl:113
  [3] _run(b::BenchmarkTools.Benchmark, p::BenchmarkTools.Parameters)
    @ BenchmarkTools ~/.julia/packages/BenchmarkTools/1i1mY/src/execution.jl:109
  [4] #invokelatest#2
    @ ./essentials.jl:1055 [inlined]
  [5] invokelatest
    @ ./essentials.jl:1052 [inlined]
  [6] #run_result#45
    @ ~/.julia/packages/BenchmarkTools/1i1mY/src/execution.jl:48 [inlined]
  [7] run_result
    @ ~/.julia/packages/BenchmarkTools/1i1mY/src/execution.jl:47 [inlined]
  [8] run(b::BenchmarkTools.Benchmark, p::BenchmarkTools.Parameters; progressid::Nothing, nleaves::Float64, ndone::Float64, kwargs::@Kwargs{})
    @ BenchmarkTools ~/.julia/packages/BenchmarkTools/1i1mY/src/execution.jl:143
  [9] run
    @ ~/.julia/packages/BenchmarkTools/1i1mY/src/execution.jl:135 [inlined]
 [10] run(b::BenchmarkTools.Benchmark)
    @ BenchmarkTools ~/.julia/packages/BenchmarkTools/1i1mY/src/execution.jl:135
 [11] top-level scope
    @ REPL[4]:1

foreach works just fine

julia> b = @benchmarkable sum(a) setup=begin
           a = zeros(10)
           foreach(1:10) do i
               a[i] = i
           end
       end
Benchmark(evals=1, seconds=5.0, samples=10000)

julia> run(b)
BenchmarkTools.Trial: 10000 samples with 1 evaluation per sample.
 Range (min … max):   0.001 ns … 292.000 ns  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):      0.001 ns               ┊ GC (median):    0.00%
 Time  (mean ± σ):   10.709 ns ±  18.605 ns  ┊ GC (mean ± σ):  0.00% ± 0.00%

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  0.001 ns        Histogram: frequency by time           42 ns <

 Memory estimate: 0 bytes, allocs estimate: 0.

julia>