support slurping for svec (#43020)

It's an internal type, but this is occasionally useful (ref
JuliaDebug/Cthulhu.jl#245).

Author: simeonschaub

base/tuple.jl

@@ -133,6 +133,7 @@ function rest end
 rest(t::Tuple) = t
 rest(t::Tuple, i::Int) = ntuple(x -> getfield(t, x+i-1), length(t)-i+1)
 rest(a::Array, i::Int=1) = a[i:end]
+rest(a::Core.SimpleVector, i::Int=1) = a[i:end]
 rest(itr, state...) = Iterators.rest(itr, state...)
 
 # Use dispatch to avoid a branch in first

test/core.jl

@@ -7607,3 +7607,12 @@ end
     @test !occursin(r"(interpreter|do_call)", trim_after_eval(string(stacktrace(btc, true))))
     @test  occursin(r"(interpreter|do_call)", trim_after_eval(string(stacktrace(bti, true))))
 end
+
+@testset "rest(svec, ...)" begin
+    x = Core.svec(1, 2, 3)
+    a..., = x
+    @test a == Core.svec(1, 2, 3)
+    a, b... = x
+    @test a == 1
+    @test b == Core.svec(2, 3)
+end