faster mapfoldl for tuples (#30471)

(cherry picked from commit 6dc205a)

base/tuple.jl

@@ -209,6 +209,18 @@ function map(f, t1::Any16, t2::Any16, ts::Any16...)
     (A...,)
 end
 
+# mapafoldl, based on afold in operators.jl
+mapafoldl(F,op,a) = a
+mapafoldl(F,op,a,b) = op(a,F(b))
+mapafoldl(F,op,a,b,c...) = mapafoldl(F, op, op(a,F(b)), c...)
+function mapafoldl(F,op,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,qs...)
+    y = op(op(op(op(op(op(op(op(op(op(op(op(op(op(op(a,F(b)),F(c)),F(d)),F(e)),F(f)),F(g)),F(h)),F(i)),F(j)),F(k)),F(l)),F(m)),F(n)),F(o)),F(p))
+    for x in qs; y = op(y,F(x)); end
+    y
+end
+mapfoldl_impl(f, op, nt::NamedTuple{(:init,)}, t::Tuple) = mapafoldl(f, op, nt.init, t...)
+mapfoldl_impl(f, op, nt::NamedTuple{()}, t::Tuple) = mapafoldl(f, op, f(t[1]), tail(t)...)
+mapfoldl_impl(f, op, nt::NamedTuple{()}, t::Tuple{}) = mapreduce_empty_iter(f, op, t, IteratorEltype(t))
 
 # type-stable padding
 fill_to_length(t::NTuple{N,Any}, val, ::Val{N}) where {N} = t

test/tuple.jl

@@ -237,6 +237,15 @@ end
     end
 end
 
+@testset "mapfoldl" begin
+    @test (((1=>2)=>3)=>4) == foldl(=>, (1,2,3,4)) ==
+          mapfoldl(identity, =>, (1,2,3,4)) == mapfoldl(abs, =>, (-1,-2,-3,-4))
+    @test mapfoldl(abs, =>, (-1,-2,-3,-4), init=-10) == ((((-10=>1)=>2)=>3)=>4)
+    @test mapfoldl(abs, =>, (), init=-10) == -10
+    @test mapfoldl(abs, Pair{Any,Any}, (-30:-1...,)) == mapfoldl(abs, Pair{Any,Any}, [-30:-1...,])
+    @test_throws ArgumentError mapfoldl(abs, =>, ())
+end
+
 @testset "comparison and hash" begin
     @test isequal((), ())
     @test isequal((1,2,3), (1,2,3))