Optimize File.join common use case

`File.join` is a hotspot for common libraries such as Zeitwerk
and Bootsnap. It has a fairly flexible signature, but 99% of
the time it's called with just two (or a small number of) UTF-8 strings.

If we optimistically optimize for that use case we can cut down a large
number of type and encoding checks, significantly speeding up the method.

The one remaining expensive check we could try to optimize is `str_null_check`.
Given it's common to use the same base string for joining, we could memoize it.
Also we could precompute it for literal strings.

```
compare-ruby: ruby 4.1.0dev (2026-01-17T14:40:03Z master 00a3b71eaf) +PRISM [arm64-darwin25]
built-ruby: ruby 4.1.0dev (2026-01-18T12:10:38Z spedup-file-join 069bab58d4) +PRISM [arm64-darwin25]
warming up....

|              |compare-ruby|built-ruby|
|:-------------|-----------:|---------:|
|two_strings   |      2.475M|    9.444M|
|              |           -|     3.82x|
|many_strings  |    551.975k|    2.346M|
|              |           -|     4.25x|
|array         |    514.946k|  522.034k|
|              |           -|     1.01x|
|mixed         |    621.236k|  633.189k|
|              |           -|     1.02x|
```

10 files changed, 197 insertions, 39 deletions

diff --git a/benchmark/file_join.yml b/benchmark/file_join.yml
new file mode 100644
index 0000000000..845257cf1e
--- /dev/null
+++ b/benchmark/file_join.yml
@@ -0,0 +1,7 @@
+prelude: |
+  # frozen_string_literal: true
+benchmark:
+  two_strings: File.join(__FILE__, "path")
+  many_strings: File.join(__FILE__, "path", "a", "b", "c", "d")
+  array: File.join([__FILE__, "path", "a", "b", "c", "d"])
+  mixed: File.join(__FILE__, "path", "a", "b", ["c", "d"])
diff --git a/depend b/depend
index cfafc77703..7372902666 100644
--- a/depend
+++ b/depend
@@ -799,6 +799,7 @@ box.$(OBJEXT): {$(VPATH)}constant.h
 box.$(OBJEXT): {$(VPATH)}darray.h
 box.$(OBJEXT): {$(VPATH)}debug_counter.h
 box.$(OBJEXT): {$(VPATH)}defines.h
+box.$(OBJEXT): {$(VPATH)}encindex.h
 box.$(OBJEXT): {$(VPATH)}encoding.h
 box.$(OBJEXT): {$(VPATH)}eval_intern.h
 box.$(OBJEXT): {$(VPATH)}id.h
@@ -1250,6 +1251,7 @@ class.$(OBJEXT): {$(VPATH)}config.h
 class.$(OBJEXT): {$(VPATH)}constant.h
 class.$(OBJEXT): {$(VPATH)}debug_counter.h
 class.$(OBJEXT): {$(VPATH)}defines.h
+class.$(OBJEXT): {$(VPATH)}encindex.h
 class.$(OBJEXT): {$(VPATH)}encoding.h
 class.$(OBJEXT): {$(VPATH)}id.h
 class.$(OBJEXT): {$(VPATH)}id_table.h
@@ -1449,6 +1451,7 @@ compar.$(OBJEXT): {$(VPATH)}backward/2/stdarg.h
 compar.$(OBJEXT): {$(VPATH)}compar.c
 compar.$(OBJEXT): {$(VPATH)}config.h
 compar.$(OBJEXT): {$(VPATH)}defines.h
+compar.$(OBJEXT): {$(VPATH)}encindex.h
 compar.$(OBJEXT): {$(VPATH)}encoding.h
 compar.$(OBJEXT): {$(VPATH)}id.h
 compar.$(OBJEXT): {$(VPATH)}intern.h
@@ -1921,6 +1924,7 @@ complex.$(OBJEXT): {$(VPATH)}config.h
 complex.$(OBJEXT): {$(VPATH)}constant.h
 complex.$(OBJEXT): {$(VPATH)}debug_counter.h
 complex.$(OBJEXT): {$(VPATH)}defines.h
+complex.$(OBJEXT): {$(VPATH)}encindex.h
 complex.$(OBJEXT): {$(VPATH)}encoding.h
 complex.$(OBJEXT): {$(VPATH)}id.h
 complex.$(OBJEXT): {$(VPATH)}id_table.h
@@ -2126,6 +2130,7 @@ concurrent_set.$(OBJEXT): {$(VPATH)}concurrent_set.c
 concurrent_set.$(OBJEXT): {$(VPATH)}config.h
 concurrent_set.$(OBJEXT): {$(VPATH)}debug_counter.h
 concurrent_set.$(OBJEXT): {$(VPATH)}defines.h
+concurrent_set.$(OBJEXT): {$(VPATH)}encindex.h
 concurrent_set.$(OBJEXT): {$(VPATH)}encoding.h
 concurrent_set.$(OBJEXT): {$(VPATH)}id.h
 concurrent_set.$(OBJEXT): {$(VPATH)}id_table.h
@@ -2364,6 +2369,7 @@ cont.$(OBJEXT): {$(VPATH)}constant.h
 cont.$(OBJEXT): {$(VPATH)}cont.c
 cont.$(OBJEXT): {$(VPATH)}debug_counter.h
 cont.$(OBJEXT): {$(VPATH)}defines.h
+cont.$(OBJEXT): {$(VPATH)}encindex.h
 cont.$(OBJEXT): {$(VPATH)}encoding.h
 cont.$(OBJEXT): {$(VPATH)}eval_intern.h
 cont.$(OBJEXT): {$(VPATH)}fiber/scheduler.h
@@ -4906,6 +4912,7 @@ enumerator.$(OBJEXT): {$(VPATH)}config.h
 enumerator.$(OBJEXT): {$(VPATH)}constant.h
 enumerator.$(OBJEXT): {$(VPATH)}debug_counter.h
 enumerator.$(OBJEXT): {$(VPATH)}defines.h
+enumerator.$(OBJEXT): {$(VPATH)}encindex.h
 enumerator.$(OBJEXT): {$(VPATH)}encoding.h
 enumerator.$(OBJEXT): {$(VPATH)}enumerator.c
 enumerator.$(OBJEXT): {$(VPATH)}id.h
@@ -5126,6 +5133,7 @@ error.$(OBJEXT): {$(VPATH)}config.h
 error.$(OBJEXT): {$(VPATH)}constant.h
 error.$(OBJEXT): {$(VPATH)}debug_counter.h
 error.$(OBJEXT): {$(VPATH)}defines.h
+error.$(OBJEXT): {$(VPATH)}encindex.h
 error.$(OBJEXT): {$(VPATH)}encoding.h
 error.$(OBJEXT): {$(VPATH)}error.c
 error.$(OBJEXT): {$(VPATH)}id.h
@@ -5373,6 +5381,7 @@ eval.$(OBJEXT): {$(VPATH)}config.h
 eval.$(OBJEXT): {$(VPATH)}constant.h
 eval.$(OBJEXT): {$(VPATH)}debug_counter.h
 eval.$(OBJEXT): {$(VPATH)}defines.h
+eval.$(OBJEXT): {$(VPATH)}encindex.h
 eval.$(OBJEXT): {$(VPATH)}encoding.h
 eval.$(OBJEXT): {$(VPATH)}eval.c
 eval.$(OBJEXT): {$(VPATH)}eval_error.c
@@ -5584,6 +5593,7 @@ file.$(OBJEXT): $(top_srcdir)/internal/array.h
 file.$(OBJEXT): $(top_srcdir)/internal/class.h
 file.$(OBJEXT): $(top_srcdir)/internal/compilers.h
 file.$(OBJEXT): $(top_srcdir)/internal/dir.h
+file.$(OBJEXT): $(top_srcdir)/internal/encoding.h
 file.$(OBJEXT): $(top_srcdir)/internal/error.h
 file.$(OBJEXT): $(top_srcdir)/internal/file.h
 file.$(OBJEXT): $(top_srcdir)/internal/gc.h
@@ -5865,6 +5875,7 @@ gc.$(OBJEXT): {$(VPATH)}darray.h
 gc.$(OBJEXT): {$(VPATH)}debug.h
 gc.$(OBJEXT): {$(VPATH)}debug_counter.h
 gc.$(OBJEXT): {$(VPATH)}defines.h
+gc.$(OBJEXT): {$(VPATH)}encindex.h
 gc.$(OBJEXT): {$(VPATH)}encoding.h
 gc.$(OBJEXT): {$(VPATH)}eval_intern.h
 gc.$(OBJEXT): {$(VPATH)}gc.c
@@ -6373,6 +6384,7 @@ hash.$(OBJEXT): {$(VPATH)}config.h
 hash.$(OBJEXT): {$(VPATH)}constant.h
 hash.$(OBJEXT): {$(VPATH)}debug_counter.h
 hash.$(OBJEXT): {$(VPATH)}defines.h
+hash.$(OBJEXT): {$(VPATH)}encindex.h
 hash.$(OBJEXT): {$(VPATH)}encoding.h
 hash.$(OBJEXT): {$(VPATH)}hash.c
 hash.$(OBJEXT): {$(VPATH)}hash.rbinc
@@ -7203,6 +7215,7 @@ io_buffer.$(OBJEXT): {$(VPATH)}backward/2/stdalign.h
 io_buffer.$(OBJEXT): {$(VPATH)}backward/2/stdarg.h
 io_buffer.$(OBJEXT): {$(VPATH)}config.h
 io_buffer.$(OBJEXT): {$(VPATH)}defines.h
+io_buffer.$(OBJEXT): {$(VPATH)}encindex.h
 io_buffer.$(OBJEXT): {$(VPATH)}encoding.h
 io_buffer.$(OBJEXT): {$(VPATH)}fiber/scheduler.h
 io_buffer.$(OBJEXT): {$(VPATH)}id.h
@@ -7454,6 +7467,7 @@ iseq.$(OBJEXT): {$(VPATH)}config.h
 iseq.$(OBJEXT): {$(VPATH)}constant.h
 iseq.$(OBJEXT): {$(VPATH)}debug_counter.h
 iseq.$(OBJEXT): {$(VPATH)}defines.h
+iseq.$(OBJEXT): {$(VPATH)}encindex.h
 iseq.$(OBJEXT): {$(VPATH)}encoding.h
 iseq.$(OBJEXT): {$(VPATH)}eval_intern.h
 iseq.$(OBJEXT): {$(VPATH)}id.h
@@ -7702,6 +7716,7 @@ jit.$(OBJEXT): {$(VPATH)}config.h
 jit.$(OBJEXT): {$(VPATH)}constant.h
 jit.$(OBJEXT): {$(VPATH)}debug_counter.h
 jit.$(OBJEXT): {$(VPATH)}defines.h
+jit.$(OBJEXT): {$(VPATH)}encindex.h
 jit.$(OBJEXT): {$(VPATH)}encoding.h
 jit.$(OBJEXT): {$(VPATH)}id.h
 jit.$(OBJEXT): {$(VPATH)}id_table.h
@@ -7956,6 +7971,7 @@ load.$(OBJEXT): {$(VPATH)}constant.h
 load.$(OBJEXT): {$(VPATH)}darray.h
 load.$(OBJEXT): {$(VPATH)}defines.h
 load.$(OBJEXT): {$(VPATH)}dln.h
+load.$(OBJEXT): {$(VPATH)}encindex.h
 load.$(OBJEXT): {$(VPATH)}encoding.h
 load.$(OBJEXT): {$(VPATH)}eval_intern.h
 load.$(OBJEXT): {$(VPATH)}id.h
@@ -9979,6 +9995,7 @@ numeric.$(OBJEXT): {$(VPATH)}builtin.h
 numeric.$(OBJEXT): {$(VPATH)}config.h
 numeric.$(OBJEXT): {$(VPATH)}constant.h
 numeric.$(OBJEXT): {$(VPATH)}defines.h
+numeric.$(OBJEXT): {$(VPATH)}encindex.h
 numeric.$(OBJEXT): {$(VPATH)}encoding.h
 numeric.$(OBJEXT): {$(VPATH)}id.h
 numeric.$(OBJEXT): {$(VPATH)}id_table.h
@@ -10200,6 +10217,7 @@ object.$(OBJEXT): {$(VPATH)}config.h
 object.$(OBJEXT): {$(VPATH)}constant.h
 object.$(OBJEXT): {$(VPATH)}debug_counter.h
 object.$(OBJEXT): {$(VPATH)}defines.h
+object.$(OBJEXT): {$(VPATH)}encindex.h
 object.$(OBJEXT): {$(VPATH)}encoding.h
 object.$(OBJEXT): {$(VPATH)}id.h
 object.$(OBJEXT): {$(VPATH)}id_table.h
@@ -10418,6 +10436,7 @@ pack.$(OBJEXT): {$(VPATH)}builtin.h
 pack.$(OBJEXT): {$(VPATH)}config.h
 pack.$(OBJEXT): {$(VPATH)}constant.h
 pack.$(OBJEXT): {$(VPATH)}defines.h
+pack.$(OBJEXT): {$(VPATH)}encindex.h
 pack.$(OBJEXT): {$(VPATH)}encoding.h
 pack.$(OBJEXT): {$(VPATH)}id.h
 pack.$(OBJEXT): {$(VPATH)}id_table.h
@@ -10644,6 +10663,7 @@ parse.$(OBJEXT): {$(VPATH)}config.h
 parse.$(OBJEXT): {$(VPATH)}constant.h
 parse.$(OBJEXT): {$(VPATH)}defines.h
 parse.$(OBJEXT): {$(VPATH)}defs/keywords
+parse.$(OBJEXT): {$(VPATH)}encindex.h
 parse.$(OBJEXT): {$(VPATH)}encoding.h
 parse.$(OBJEXT): {$(VPATH)}id.h
 parse.$(OBJEXT): {$(VPATH)}id_table.h
@@ -12125,6 +12145,7 @@ proc.$(OBJEXT): {$(VPATH)}config.h
 proc.$(OBJEXT): {$(VPATH)}constant.h
 proc.$(OBJEXT): {$(VPATH)}debug_counter.h
 proc.$(OBJEXT): {$(VPATH)}defines.h
+proc.$(OBJEXT): {$(VPATH)}encindex.h
 proc.$(OBJEXT): {$(VPATH)}encoding.h
 proc.$(OBJEXT): {$(VPATH)}eval_intern.h
 proc.$(OBJEXT): {$(VPATH)}id.h
@@ -12356,6 +12377,7 @@ process.$(OBJEXT): {$(VPATH)}constant.h
 process.$(OBJEXT): {$(VPATH)}debug_counter.h
 process.$(OBJEXT): {$(VPATH)}defines.h
 process.$(OBJEXT): {$(VPATH)}dln.h
+process.$(OBJEXT): {$(VPATH)}encindex.h
 process.$(OBJEXT): {$(VPATH)}encoding.h
 process.$(OBJEXT): {$(VPATH)}fiber/scheduler.h
 process.$(OBJEXT): {$(VPATH)}hrtime.h
@@ -12585,6 +12607,7 @@ ractor.$(OBJEXT): {$(VPATH)}config.h
 ractor.$(OBJEXT): {$(VPATH)}constant.h
 ractor.$(OBJEXT): {$(VPATH)}debug_counter.h
 ractor.$(OBJEXT): {$(VPATH)}defines.h
+ractor.$(OBJEXT): {$(VPATH)}encindex.h
 ractor.$(OBJEXT): {$(VPATH)}encoding.h
 ractor.$(OBJEXT): {$(VPATH)}eval_intern.h
 ractor.$(OBJEXT): {$(VPATH)}id.h
@@ -13018,6 +13041,7 @@ range.$(OBJEXT): {$(VPATH)}backward/2/stdalign.h
 range.$(OBJEXT): {$(VPATH)}backward/2/stdarg.h
 range.$(OBJEXT): {$(VPATH)}config.h
 range.$(OBJEXT): {$(VPATH)}defines.h
+range.$(OBJEXT): {$(VPATH)}encindex.h
 range.$(OBJEXT): {$(VPATH)}encoding.h
 range.$(OBJEXT): {$(VPATH)}id.h
 range.$(OBJEXT): {$(VPATH)}id_table.h
@@ -14688,6 +14712,7 @@ ruby.$(OBJEXT): {$(VPATH)}constant.h
 ruby.$(OBJEXT): {$(VPATH)}debug_counter.h
 ruby.$(OBJEXT): {$(VPATH)}defines.h
 ruby.$(OBJEXT): {$(VPATH)}dln.h
+ruby.$(OBJEXT): {$(VPATH)}encindex.h
 ruby.$(OBJEXT): {$(VPATH)}encoding.h
 ruby.$(OBJEXT): {$(VPATH)}eval_intern.h
 ruby.$(OBJEXT): {$(VPATH)}id.h
@@ -14896,6 +14921,7 @@ ruby_parser.$(OBJEXT): {$(VPATH)}backward/2/stdalign.h
 ruby_parser.$(OBJEXT): {$(VPATH)}backward/2/stdarg.h
 ruby_parser.$(OBJEXT): {$(VPATH)}config.h
 ruby_parser.$(OBJEXT): {$(VPATH)}defines.h
+ruby_parser.$(OBJEXT): {$(VPATH)}encindex.h
 ruby_parser.$(OBJEXT): {$(VPATH)}encoding.h
 ruby_parser.$(OBJEXT): {$(VPATH)}intern.h
 ruby_parser.$(OBJEXT): {$(VPATH)}internal.h
@@ -15306,6 +15332,7 @@ set.$(OBJEXT): {$(VPATH)}backward/2/stdarg.h
 set.$(OBJEXT): {$(VPATH)}config.h
 set.$(OBJEXT): {$(VPATH)}constant.h
 set.$(OBJEXT): {$(VPATH)}defines.h
+set.$(OBJEXT): {$(VPATH)}encindex.h
 set.$(OBJEXT): {$(VPATH)}encoding.h
 set.$(OBJEXT): {$(VPATH)}id.h
 set.$(OBJEXT): {$(VPATH)}id_table.h
@@ -15678,6 +15705,7 @@ shape.$(OBJEXT): {$(VPATH)}config.h
 shape.$(OBJEXT): {$(VPATH)}constant.h
 shape.$(OBJEXT): {$(VPATH)}debug_counter.h
 shape.$(OBJEXT): {$(VPATH)}defines.h
+shape.$(OBJEXT): {$(VPATH)}encindex.h
 shape.$(OBJEXT): {$(VPATH)}encoding.h
 shape.$(OBJEXT): {$(VPATH)}id.h
 shape.$(OBJEXT): {$(VPATH)}id_table.h
@@ -15892,6 +15920,7 @@ signal.$(OBJEXT): {$(VPATH)}config.h
 signal.$(OBJEXT): {$(VPATH)}constant.h
 signal.$(OBJEXT): {$(VPATH)}debug_counter.h
 signal.$(OBJEXT): {$(VPATH)}defines.h
+signal.$(OBJEXT): {$(VPATH)}encindex.h
 signal.$(OBJEXT): {$(VPATH)}encoding.h
 signal.$(OBJEXT): {$(VPATH)}eval_intern.h
 signal.$(OBJEXT): {$(VPATH)}id.h
@@ -16101,6 +16130,7 @@ sprintf.$(OBJEXT): {$(VPATH)}backward/2/stdarg.h
 sprintf.$(OBJEXT): {$(VPATH)}config.h
 sprintf.$(OBJEXT): {$(VPATH)}constant.h
 sprintf.$(OBJEXT): {$(VPATH)}defines.h
+sprintf.$(OBJEXT): {$(VPATH)}encindex.h
 sprintf.$(OBJEXT): {$(VPATH)}encoding.h
 sprintf.$(OBJEXT): {$(VPATH)}id.h
 sprintf.$(OBJEXT): {$(VPATH)}id_table.h
@@ -16457,6 +16487,7 @@ strftime.$(OBJEXT): {$(VPATH)}backward/2/stdalign.h
 strftime.$(OBJEXT): {$(VPATH)}backward/2/stdarg.h
 strftime.$(OBJEXT): {$(VPATH)}config.h
 strftime.$(OBJEXT): {$(VPATH)}defines.h
+strftime.$(OBJEXT): {$(VPATH)}encindex.h
 strftime.$(OBJEXT): {$(VPATH)}encoding.h
 strftime.$(OBJEXT): {$(VPATH)}intern.h
 strftime.$(OBJEXT): {$(VPATH)}internal.h
@@ -16925,6 +16956,7 @@ struct.$(OBJEXT): {$(VPATH)}config.h
 struct.$(OBJEXT): {$(VPATH)}constant.h
 struct.$(OBJEXT): {$(VPATH)}debug_counter.h
 struct.$(OBJEXT): {$(VPATH)}defines.h
+struct.$(OBJEXT): {$(VPATH)}encindex.h
 struct.$(OBJEXT): {$(VPATH)}encoding.h
 struct.$(OBJEXT): {$(VPATH)}id.h
 struct.$(OBJEXT): {$(VPATH)}id_table.h
@@ -17141,6 +17173,7 @@ symbol.$(OBJEXT): {$(VPATH)}constant.h
 symbol.$(OBJEXT): {$(VPATH)}darray.h
 symbol.$(OBJEXT): {$(VPATH)}debug_counter.h
 symbol.$(OBJEXT): {$(VPATH)}defines.h
+symbol.$(OBJEXT): {$(VPATH)}encindex.h
 symbol.$(OBJEXT): {$(VPATH)}encoding.h
 symbol.$(OBJEXT): {$(VPATH)}id.c
 symbol.$(OBJEXT): {$(VPATH)}id.h
@@ -17398,6 +17431,7 @@ thread.$(OBJEXT): {$(VPATH)}constant.h
 thread.$(OBJEXT): {$(VPATH)}debug.h
 thread.$(OBJEXT): {$(VPATH)}debug_counter.h
 thread.$(OBJEXT): {$(VPATH)}defines.h
+thread.$(OBJEXT): {$(VPATH)}encindex.h
 thread.$(OBJEXT): {$(VPATH)}encoding.h
 thread.$(OBJEXT): {$(VPATH)}eval_intern.h
 thread.$(OBJEXT): {$(VPATH)}fiber/scheduler.h
@@ -17628,6 +17662,7 @@ time.$(OBJEXT): {$(VPATH)}builtin.h
 time.$(OBJEXT): {$(VPATH)}config.h
 time.$(OBJEXT): {$(VPATH)}constant.h
 time.$(OBJEXT): {$(VPATH)}defines.h
+time.$(OBJEXT): {$(VPATH)}encindex.h
 time.$(OBJEXT): {$(VPATH)}encoding.h
 time.$(OBJEXT): {$(VPATH)}id.h
 time.$(OBJEXT): {$(VPATH)}id_table.h
@@ -17830,6 +17865,7 @@ transcode.$(OBJEXT): {$(VPATH)}config.h
 transcode.$(OBJEXT): {$(VPATH)}constant.h
 transcode.$(OBJEXT): {$(VPATH)}debug_counter.h
 transcode.$(OBJEXT): {$(VPATH)}defines.h
+transcode.$(OBJEXT): {$(VPATH)}encindex.h
 transcode.$(OBJEXT): {$(VPATH)}encoding.h
 transcode.$(OBJEXT): {$(VPATH)}id.h
 transcode.$(OBJEXT): {$(VPATH)}id_table.h
@@ -18211,6 +18247,7 @@ variable.$(OBJEXT): {$(VPATH)}config.h
 variable.$(OBJEXT): {$(VPATH)}constant.h
 variable.$(OBJEXT): {$(VPATH)}debug_counter.h
 variable.$(OBJEXT): {$(VPATH)}defines.h
+variable.$(OBJEXT): {$(VPATH)}encindex.h
 variable.$(OBJEXT): {$(VPATH)}encoding.h
 variable.$(OBJEXT): {$(VPATH)}id.h
 variable.$(OBJEXT): {$(VPATH)}id_table.h
@@ -18687,6 +18724,7 @@ vm.$(OBJEXT): {$(VPATH)}constant.h
 vm.$(OBJEXT): {$(VPATH)}debug_counter.h
 vm.$(OBJEXT): {$(VPATH)}defines.h
 vm.$(OBJEXT): {$(VPATH)}defs/opt_operand.def
+vm.$(OBJEXT): {$(VPATH)}encindex.h
 vm.$(OBJEXT): {$(VPATH)}encoding.h
 vm.$(OBJEXT): {$(VPATH)}eval_intern.h
 vm.$(OBJEXT): {$(VPATH)}id.h
@@ -18951,6 +18989,7 @@ vm_backtrace.$(OBJEXT): {$(VPATH)}constant.h
 vm_backtrace.$(OBJEXT): {$(VPATH)}debug.h
 vm_backtrace.$(OBJEXT): {$(VPATH)}debug_counter.h
 vm_backtrace.$(OBJEXT): {$(VPATH)}defines.h
+vm_backtrace.$(OBJEXT): {$(VPATH)}encindex.h
 vm_backtrace.$(OBJEXT): {$(VPATH)}encoding.h
 vm_backtrace.$(OBJEXT): {$(VPATH)}eval_intern.h
 vm_backtrace.$(OBJEXT): {$(VPATH)}id.h
@@ -20087,6 +20126,7 @@ yjit.$(OBJEXT): {$(VPATH)}constant.h
 yjit.$(OBJEXT): {$(VPATH)}debug.h
 yjit.$(OBJEXT): {$(VPATH)}debug_counter.h
 yjit.$(OBJEXT): {$(VPATH)}defines.h
+yjit.$(OBJEXT): {$(VPATH)}encindex.h
 yjit.$(OBJEXT): {$(VPATH)}encoding.h
 yjit.$(OBJEXT): {$(VPATH)}id.h
 yjit.$(OBJEXT): {$(VPATH)}id_table.h
@@ -20342,6 +20382,7 @@ zjit.$(OBJEXT): {$(VPATH)}constant.h
 zjit.$(OBJEXT): {$(VPATH)}debug.h
 zjit.$(OBJEXT): {$(VPATH)}debug_counter.h
 zjit.$(OBJEXT): {$(VPATH)}defines.h
+zjit.$(OBJEXT): {$(VPATH)}encindex.h
 zjit.$(OBJEXT): {$(VPATH)}encoding.h
 zjit.$(OBJEXT): {$(VPATH)}id.h
 zjit.$(OBJEXT): {$(VPATH)}id_table.h
diff --git a/ext/-test-/stack/depend b/ext/-test-/stack/depend
index 31571c882e..77e93bb201 100644
--- a/ext/-test-/stack/depend
+++ b/ext/-test-/stack/depend
@@ -172,6 +172,7 @@ stack.o: $(hdrdir)/ruby/oniguruma.h
 stack.o: $(hdrdir)/ruby/ruby.h
 stack.o: $(hdrdir)/ruby/st.h
 stack.o: $(hdrdir)/ruby/subst.h
+stack.o: $(top_srcdir)/encindex.h
 stack.o: $(top_srcdir)/internal/compilers.h
 stack.o: $(top_srcdir)/internal/string.h
 stack.o: stack.c
diff --git a/ext/-test-/string/depend b/ext/-test-/string/depend
index de6e775acc..478ae3b82b 100644
--- a/ext/-test-/string/depend
+++ b/ext/-test-/string/depend
@@ -172,6 +172,7 @@ capacity.o: $(hdrdir)/ruby/oniguruma.h
 capacity.o: $(hdrdir)/ruby/ruby.h
 capacity.o: $(hdrdir)/ruby/st.h
 capacity.o: $(hdrdir)/ruby/subst.h
+capacity.o: $(top_srcdir)/encindex.h
 capacity.o: $(top_srcdir)/internal/compilers.h
 capacity.o: $(top_srcdir)/internal/string.h
 capacity.o: capacity.c
@@ -679,6 +680,7 @@ cstr.o: $(hdrdir)/ruby/oniguruma.h
 cstr.o: $(hdrdir)/ruby/ruby.h
 cstr.o: $(hdrdir)/ruby/st.h
 cstr.o: $(hdrdir)/ruby/subst.h
+cstr.o: $(top_srcdir)/encindex.h
 cstr.o: $(top_srcdir)/internal.h
 cstr.o: $(top_srcdir)/internal/compilers.h
 cstr.o: $(top_srcdir)/internal/string.h
@@ -1535,6 +1537,7 @@ fstring.o: $(hdrdir)/ruby/oniguruma.h
 fstring.o: $(hdrdir)/ruby/ruby.h
 fstring.o: $(hdrdir)/ruby/st.h
 fstring.o: $(hdrdir)/ruby/subst.h
+fstring.o: $(top_srcdir)/encindex.h
 fstring.o: $(top_srcdir)/internal/compilers.h
 fstring.o: $(top_srcdir)/internal/string.h
 fstring.o: fstring.c
diff --git a/ext/objspace/depend b/ext/objspace/depend
index 04b26eb6c2..d9dfc0c42b 100644
--- a/ext/objspace/depend
+++ b/ext/objspace/depend
@@ -602,6 +602,7 @@ objspace_dump.o: $(top_srcdir)/ccan/list/list.h
 objspace_dump.o: $(top_srcdir)/ccan/str/str.h
 objspace_dump.o: $(top_srcdir)/constant.h
 objspace_dump.o: $(top_srcdir)/debug_counter.h
+objspace_dump.o: $(top_srcdir)/encindex.h
 objspace_dump.o: $(top_srcdir)/id_table.h
 objspace_dump.o: $(top_srcdir)/internal.h
 objspace_dump.o: $(top_srcdir)/internal/array.h
diff --git a/ext/ripper/depend b/ext/ripper/depend
index bd2de75906..944da25ee9 100644
--- a/ext/ripper/depend
+++ b/ext/ripper/depend
@@ -578,6 +578,7 @@ ripper.o: $(top_srcdir)/ccan/container_of/container_of.h
 ripper.o: $(top_srcdir)/ccan/list/list.h
 ripper.o: $(top_srcdir)/ccan/str/str.h
 ripper.o: $(top_srcdir)/constant.h
+ripper.o: $(top_srcdir)/encindex.h
 ripper.o: $(top_srcdir)/id_table.h
 ripper.o: $(top_srcdir)/internal.h
 ripper.o: $(top_srcdir)/internal/array.h
diff --git a/ext/socket/depend b/ext/socket/depend
index 3573dc45e2..77f6239a3d 100644
--- a/ext/socket/depend
+++ b/ext/socket/depend
@@ -193,6 +193,7 @@ ancdata.o: $(top_srcdir)/ccan/check_type/check_type.h
 ancdata.o: $(top_srcdir)/ccan/container_of/container_of.h
 ancdata.o: $(top_srcdir)/ccan/list/list.h
 ancdata.o: $(top_srcdir)/ccan/str/str.h
+ancdata.o: $(top_srcdir)/encindex.h
 ancdata.o: $(top_srcdir)/id_table.h
 ancdata.o: $(top_srcdir)/internal.h
 ancdata.o: $(top_srcdir)/internal/array.h
@@ -408,6 +409,7 @@ basicsocket.o: $(top_srcdir)/ccan/check_type/check_type.h
 basicsocket.o: $(top_srcdir)/ccan/container_of/container_of.h
 basicsocket.o: $(top_srcdir)/ccan/list/list.h
 basicsocket.o: $(top_srcdir)/ccan/str/str.h
+basicsocket.o: $(top_srcdir)/encindex.h
 basicsocket.o: $(top_srcdir)/id_table.h
 basicsocket.o: $(top_srcdir)/internal.h
 basicsocket.o: $(top_srcdir)/internal/array.h
@@ -623,6 +625,7 @@ constants.o: $(top_srcdir)/ccan/check_type/check_type.h
 constants.o: $(top_srcdir)/ccan/container_of/container_of.h
 constants.o: $(top_srcdir)/ccan/list/list.h
 constants.o: $(top_srcdir)/ccan/str/str.h
+constants.o: $(top_srcdir)/encindex.h
 constants.o: $(top_srcdir)/id_table.h
 constants.o: $(top_srcdir)/internal.h
 constants.o: $(top_srcdir)/internal/array.h
@@ -839,6 +842,7 @@ ifaddr.o: $(top_srcdir)/ccan/check_type/check_type.h
 ifaddr.o: $(top_srcdir)/ccan/container_of/container_of.h
 ifaddr.o: $(top_srcdir)/ccan/list/list.h
 ifaddr.o: $(top_srcdir)/ccan/str/str.h
+ifaddr.o: $(top_srcdir)/encindex.h
 ifaddr.o: $(top_srcdir)/id_table.h
 ifaddr.o: $(top_srcdir)/internal.h
 ifaddr.o: $(top_srcdir)/internal/array.h
@@ -1054,6 +1058,7 @@ init.o: $(top_srcdir)/ccan/check_type/check_type.h
 init.o: $(top_srcdir)/ccan/container_of/container_of.h
 init.o: $(top_srcdir)/ccan/list/list.h
 init.o: $(top_srcdir)/ccan/str/str.h
+init.o: $(top_srcdir)/encindex.h
 init.o: $(top_srcdir)/id_table.h
 init.o: $(top_srcdir)/internal.h
 init.o: $(top_srcdir)/internal/array.h
@@ -1269,6 +1274,7 @@ ipsocket.o: $(top_srcdir)/ccan/check_type/check_type.h
 ipsocket.o: $(top_srcdir)/ccan/container_of/container_of.h
 ipsocket.o: $(top_srcdir)/ccan/list/list.h
 ipsocket.o: $(top_srcdir)/ccan/str/str.h
+ipsocket.o: $(top_srcdir)/encindex.h
 ipsocket.o: $(top_srcdir)/id_table.h
 ipsocket.o: $(top_srcdir)/internal.h
 ipsocket.o: $(top_srcdir)/internal/array.h
@@ -1484,6 +1490,7 @@ option.o: $(top_srcdir)/ccan/check_type/check_type.h
 option.o: $(top_srcdir)/ccan/container_of/container_of.h
 option.o: $(top_srcdir)/ccan/list/list.h
 option.o: $(top_srcdir)/ccan/str/str.h
+option.o: $(top_srcdir)/encindex.h
 option.o: $(top_srcdir)/id_table.h
 option.o: $(top_srcdir)/internal.h
 option.o: $(top_srcdir)/internal/array.h
@@ -1699,6 +1706,7 @@ raddrinfo.o: $(top_srcdir)/ccan/check_type/check_type.h
 raddrinfo.o: $(top_srcdir)/ccan/container_of/container_of.h
 raddrinfo.o: $(top_srcdir)/ccan/list/list.h
 raddrinfo.o: $(top_srcdir)/ccan/str/str.h
+raddrinfo.o: $(top_srcdir)/encindex.h
 raddrinfo.o: $(top_srcdir)/id_table.h
 raddrinfo.o: $(top_srcdir)/internal.h
 raddrinfo.o: $(top_srcdir)/internal/array.h
@@ -1914,6 +1922,7 @@ socket.o: $(top_srcdir)/ccan/check_type/check_type.h
 socket.o: $(top_srcdir)/ccan/container_of/container_of.h
 socket.o: $(top_srcdir)/ccan/list/list.h
 socket.o: $(top_srcdir)/ccan/str/str.h
+socket.o: $(top_srcdir)/encindex.h
 socket.o: $(top_srcdir)/id_table.h
 socket.o: $(top_srcdir)/internal.h
 socket.o: $(top_srcdir)/internal/array.h
@@ -2129,6 +2138,7 @@ sockssocket.o: $(top_srcdir)/ccan/check_type/check_type.h
 sockssocket.o: $(top_srcdir)/ccan/container_of/container_of.h
 sockssocket.o: $(top_srcdir)/ccan/list/list.h
 sockssocket.o: $(top_srcdir)/ccan/str/str.h
+sockssocket.o: $(top_srcdir)/encindex.h
 sockssocket.o: $(top_srcdir)/id_table.h
 sockssocket.o: $(top_srcdir)/internal.h
 sockssocket.o: $(top_srcdir)/internal/array.h
@@ -2344,6 +2354,7 @@ tcpserver.o: $(top_srcdir)/ccan/check_type/check_type.h
 tcpserver.o: $(top_srcdir)/ccan/container_of/container_of.h
 tcpserver.o: $(top_srcdir)/ccan/list/list.h
 tcpserver.o: $(top_srcdir)/ccan/str/str.h
+tcpserver.o: $(top_srcdir)/encindex.h
 tcpserver.o: $(top_srcdir)/id_table.h
 tcpserver.o: $(top_srcdir)/internal.h
 tcpserver.o: $(top_srcdir)/internal/array.h
@@ -2559,6 +2570,7 @@ tcpsocket.o: $(top_srcdir)/ccan/check_type/check_type.h
 tcpsocket.o: $(top_srcdir)/ccan/container_of/container_of.h
 tcpsocket.o: $(top_srcdir)/ccan/list/list.h
 tcpsocket.o: $(top_srcdir)/ccan/str/str.h
+tcpsocket.o: $(top_srcdir)/encindex.h
 tcpsocket.o: $(top_srcdir)/id_table.h
 tcpsocket.o: $(top_srcdir)/internal.h
 tcpsocket.o: $(top_srcdir)/internal/array.h
@@ -2774,6 +2786,7 @@ udpsocket.o: $(top_srcdir)/ccan/check_type/check_type.h
 udpsocket.o: $(top_srcdir)/ccan/container_of/container_of.h
 udpsocket.o: $(top_srcdir)/ccan/list/list.h
 udpsocket.o: $(top_srcdir)/ccan/str/str.h
+udpsocket.o: $(top_srcdir)/encindex.h
 udpsocket.o: $(top_srcdir)/id_table.h
 udpsocket.o: $(top_srcdir)/internal.h
 udpsocket.o: $(top_srcdir)/internal/array.h
@@ -2989,6 +3002,7 @@ unixserver.o: $(top_srcdir)/ccan/check_type/check_type.h
 unixserver.o: $(top_srcdir)/ccan/container_of/container_of.h
 unixserver.o: $(top_srcdir)/ccan/list/list.h
 unixserver.o: $(top_srcdir)/ccan/str/str.h
+unixserver.o: $(top_srcdir)/encindex.h
 unixserver.o: $(top_srcdir)/id_table.h
 unixserver.o: $(top_srcdir)/internal.h
 unixserver.o: $(top_srcdir)/internal/array.h
@@ -3204,6 +3218,7 @@ unixsocket.o: $(top_srcdir)/ccan/check_type/check_type.h
 unixsocket.o: $(top_srcdir)/ccan/container_of/container_of.h
 unixsocket.o: $(top_srcdir)/ccan/list/list.h
 unixsocket.o: $(top_srcdir)/ccan/str/str.h
+unixsocket.o: $(top_srcdir)/encindex.h
 unixsocket.o: $(top_srcdir)/id_table.h
 unixsocket.o: $(top_srcdir)/internal.h
 unixsocket.o: $(top_srcdir)/internal/array.h
diff --git a/file.c b/file.c
index 89294ea009..8658b30677 100644
--- a/file.c
+++ b/file.c
@@ -169,6 +169,7 @@ typedef struct timespec stat_timestamp;
 #include "internal.h"
 #include "internal/compilers.h"
 #include "internal/dir.h"
+#include "internal/encoding.h"
 #include "internal/error.h"
 #include "internal/file.h"
 #include "internal/io.h"
@@ -3713,6 +3714,22 @@ chompdirsep(const char *path, const char *end, rb_encoding *enc)
     return (char *)path;
 }
 
+static char *
+single_byte_chompdirsep(const char *path, const char *end)
+{
+    while (path < end) {
+        if (isdirsep(*path)) {
+            const char *last = path++;
+            while (path < end && isdirsep(*path)) path++;
+            if (path >= end) return (char *)last;
+        }
+        else {
+            path++;
+        }
+    }
+    return (char *)path;
+}
+
 char *
 rb_enc_path_end(const char *path, const char *end, rb_encoding *enc)
 {
@@ -3723,7 +3740,7 @@ rb_enc_path_end(const char *path, const char *end, rb_encoding *enc)
 static rb_encoding *
 fs_enc_check(VALUE path1, VALUE path2)
 {
-    rb_encoding *enc = rb_enc_check(path1, path2);
+    rb_encoding *enc = rb_enc_check_str(path1, path2);
     int encidx = rb_enc_to_index(enc);
     if (encidx == ENCINDEX_US_ASCII) {
         encidx = rb_enc_get_index(path1);
@@ -4651,7 +4668,7 @@ rb_check_realpath_emulate(VALUE basedir, VALUE path, rb_encoding *origenc, enum
     return resolved;
 }
 
-static VALUE rb_file_join(VALUE ary);
+static VALUE rb_file_join(long argc, VALUE *args);
 
 #ifndef HAVE_REALPATH
 static VALUE
@@ -4692,7 +4709,8 @@ rb_check_realpath_internal(VALUE basedir, VALUE path, rb_encoding *origenc, enum
 
     unresolved_path = rb_str_dup_frozen(path);
     if (*RSTRING_PTR(unresolved_path) != '/' && !NIL_P(basedir)) {
-        unresolved_path = rb_file_join(rb_assoc_new(basedir, unresolved_path));
+        VALUE paths[2] = {basedir, unresolved_path};
+        unresolved_path = rb_file_join(2, paths);
     }
     if (origenc) unresolved_path = TO_OSPATH(unresolved_path);
 
@@ -5255,15 +5273,17 @@ rb_file_s_split(VALUE klass, VALUE path)
     return rb_assoc_new(rb_file_dirname(path), rb_file_s_basename(1,&path,Qundef));
 }
 
+static VALUE rb_file_join_ary(VALUE ary);
+
 static VALUE
 file_inspect_join(VALUE ary, VALUE arg, int recur)
 {
     if (recur || ary == arg) rb_raise(rb_eArgError, "recursive array");
-    return rb_file_join(arg);
+    return rb_file_join_ary(arg);
 }
 
 static VALUE
-rb_file_join(VALUE ary)
+rb_file_join_ary(VALUE ary)
 {
     long len, i;
     VALUE result, tmp;
@@ -5328,6 +5348,69 @@ rb_file_join(VALUE ary)
     return result;
 }
 
+static inline VALUE
+rb_file_join_fastpath(long argc, VALUE *args)
+{
+    long size = argc;
+
+    long i;
+    for (i = 0; i < argc; i++) {
+        VALUE tmp = args[i];
+        if (RB_LIKELY(RB_TYPE_P(tmp, T_STRING) && rb_str_enc_fastpath(tmp))) {
+            size += RSTRING_LEN(tmp);
+        }
+        else {
+            return 0;
+        }
+    }
+
+    VALUE result = rb_str_buf_new(size);
+
+    StringValueCStr(args[0]);
+    int encidx = ENCODING_GET_INLINED(args[0]);
+    ENCODING_SET_INLINED(result, encidx);
+    rb_str_buf_append(result, args[0]);
+
+    const char *name = RSTRING_PTR(result);
+    for (i = 1; i < argc; i++) {
+        VALUE tmp = args[i];
+        StringValueCStr(tmp);
+        long len = RSTRING_LEN(result);
+
+        const char *tail = single_byte_chompdirsep(name, name + len);
+        if (RSTRING_PTR(tmp) && isdirsep(RSTRING_PTR(tmp)[0])) {
+            rb_str_set_len(result, tail - name);
+        }
+        else if (!*tail) {
+            rb_str_cat(result, "/", 1);
+        }
+
+        if (RB_UNLIKELY(ENCODING_GET_INLINED(tmp) != encidx)) {
+            rb_encoding *new_enc = fs_enc_check(result, tmp);
+            rb_enc_associate(result, new_enc);
+            encidx = rb_enc_to_index(new_enc);
+        }
+
+        rb_str_buf_append(result, tmp);
+    }
+
+    return result;
+}
+
+static inline VALUE
+rb_file_join(long argc, VALUE *args)
+{
+    if (RB_UNLIKELY(argc == 0)) {
+        return rb_str_new(0, 0);
+    }
+
+    VALUE result = rb_file_join_fastpath(argc, args);
+    if (RB_LIKELY(result)) {
+        return result;
+    }
+
+    return rb_file_join_ary(rb_ary_new_from_values(argc, args));
+}
 /*
  *  call-seq:
  *     File.join(string, ...)  ->  string
@@ -5340,9 +5423,9 @@ rb_file_join(VALUE ary)
  */
 
 static VALUE
-rb_file_s_join(VALUE klass, VALUE args)
+rb_file_s_join(int argc, VALUE *argv, VALUE klass)
 {
-    return rb_file_join(args);
+    return rb_file_join(argc, argv);
 }
 
 #if defined(HAVE_TRUNCATE)
@@ -7584,7 +7667,7 @@ Init_File(void)
     /* separates directory parts in path */
     rb_define_const(rb_cFile, "SEPARATOR", separator);
     rb_define_singleton_method(rb_cFile, "split",  rb_file_s_split, 1);
-    rb_define_singleton_method(rb_cFile, "join",   rb_file_s_join, -2);
+    rb_define_singleton_method(rb_cFile, "join",   rb_file_s_join, -1);
 
 #ifdef DOSISH
     /* platform specific alternative separator */
diff --git a/internal/string.h b/internal/string.h
index d6fea62061..ea81db7ed3 100644
--- a/internal/string.h
+++ b/internal/string.h
@@ -14,6 +14,7 @@
 #include "ruby/internal/stdbool.h"     /* for bool */
 #include "ruby/encoding.h"      /* for rb_encoding */
 #include "ruby/ruby.h"          /* for VALUE */
+#include "encindex.h"
 
 #define STR_SHARED                  FL_USER0 /* = ELTS_SHARED */
 #define STR_NOEMBED                 FL_USER1
@@ -29,6 +30,26 @@ enum ruby_rstring_private_flags {
 # undef rb_fstring_cstr
 #endif
 
+static inline bool
+rb_str_encindex_fastpath(int encindex)
+{
+    // The overwhelming majority of strings are in one of these 3 encodings.
+    switch (encindex) {
+      case ENCINDEX_ASCII_8BIT:
+      case ENCINDEX_UTF_8:
+      case ENCINDEX_US_ASCII:
+        return true;
+      default:
+        return false;
+    }
+}
+
+static inline bool
+rb_str_enc_fastpath(VALUE str)
+{
+    return rb_str_encindex_fastpath(ENCODING_GET_INLINED(str));
+}
+
 /* string.c */
 VALUE rb_str_dup_m(VALUE str);
 VALUE rb_fstring(VALUE);
diff --git a/string.c b/string.c
index 2d74c46a36..cfadabd379 100644
--- a/string.c
+++ b/string.c
@@ -146,27 +146,7 @@ VALUE rb_cSymbol;
     RSTRING(str)->len = (n); \
 } while (0)
 
-static inline bool
-str_encindex_fastpath(int encindex)
-{
-    // The overwhelming majority of strings are in one of these 3 encodings.
-    switch (encindex) {
-      case ENCINDEX_ASCII_8BIT:
-      case ENCINDEX_UTF_8:
-      case ENCINDEX_US_ASCII:
-        return true;
-      default:
-        return false;
-    }
-}
-
-static inline bool
-str_enc_fastpath(VALUE str)
-{
-    return str_encindex_fastpath(ENCODING_GET_INLINED(str));
-}
-
-#define TERM_LEN(str) (str_enc_fastpath(str) ? 1 : rb_enc_mbminlen(rb_enc_from_index(ENCODING_GET(str))))
+#define TERM_LEN(str) (rb_str_enc_fastpath(str) ? 1 : rb_enc_mbminlen(rb_enc_from_index(ENCODING_GET(str))))
 #define TERM_FILL(ptr, termlen) do {\
     char *const term_fill_ptr = (ptr);\
     const int term_fill_len = (termlen);\
@@ -960,7 +940,7 @@ static inline bool
 rb_enc_str_asciicompat(VALUE str)
 {
     int encindex = ENCODING_GET_INLINED(str);
-    return str_encindex_fastpath(encindex) || rb_enc_asciicompat(rb_enc_get_from_index(encindex));
+    return rb_str_encindex_fastpath(encindex) || rb_enc_asciicompat(rb_enc_get_from_index(encindex));
 }
 
 int
@@ -2796,7 +2776,7 @@ rb_must_asciicompat(VALUE str)
         rb_raise(rb_eTypeError, "not encoding capable object");
     }
 
-    if (RB_LIKELY(str_encindex_fastpath(encindex))) {
+    if (RB_LIKELY(rb_str_encindex_fastpath(encindex))) {
         return;
     }
 
@@ -2897,16 +2877,21 @@ str_null_check(VALUE str, int *w)
 {
     char *s = RSTRING_PTR(str);
     long len = RSTRING_LEN(str);
-    rb_encoding *enc = rb_enc_get(str);
-    const int minlen = rb_enc_mbminlen(enc);
+    int minlen = 1;
+
+    if (RB_UNLIKELY(!rb_str_enc_fastpath(str))) {
+        rb_encoding *enc = rb_enc_get(str);
+        minlen = rb_enc_mbminlen(enc);
 
-    if (minlen > 1) {
-        *w = 1;
-        if (str_null_char(s, len, minlen, enc)) {
-            return NULL;
+        if (minlen > 1) {
+            *w = 1;
+            if (str_null_char(s, len, minlen, enc)) {
+                return NULL;
+            }
+            return str_fill_term(str, s, len, minlen);
         }
-        return str_fill_term(str, s, len, minlen);
     }
+
     *w = 0;
     if (!s || memchr(s, 0, len)) {
         return NULL;
@@ -3765,7 +3750,7 @@ rb_str_buf_append(VALUE str, VALUE str2)
 {
     int str2_cr = rb_enc_str_coderange(str2);
 
-    if (str_enc_fastpath(str)) {
+    if (rb_str_enc_fastpath(str)) {
         switch (str2_cr) {
           case ENC_CODERANGE_7BIT:
             // If RHS is 7bit we can do simple concatenation