diff --git a/benchmark/file_join.yml b/benchmark/file_join.yml
new file mode 100644
index 00000000000000..845257cf1e4a31
--- /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 cfafc77703b9fa..7372902666b97b 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/doc/contributing/documentation_guide.md b/doc/contributing/documentation_guide.md
index 29a1c72b02f0de..4b1e2ac9adaf2e 100644
--- a/doc/contributing/documentation_guide.md
+++ b/doc/contributing/documentation_guide.md
@@ -265,16 +265,16 @@ and _never_ when referring to the class itself.
 
 When writing an explicit link, follow these guidelines.
 
-#### +rdoc-ref+ Scheme
+#### `rdoc-ref` Scheme
 
-Use the +rdoc-ref+ scheme for:
+Use the `rdoc-ref` scheme for:
 
 - A link in core documentation to other core documentation.
 - A link in core documentation to documentation in a standard library package.
 - A link in a standard library package to other documentation in that same
   standard library package.
 
-See section "+rdoc-ref+ Scheme" in [links].
+See section "`rdoc-ref` Scheme" in [links].
 
 #### URL-Based Link
 
@@ -297,7 +297,7 @@ The name of a variable (as specified in its call-seq) should be marked up as
 [monofont].
 
 Also, use monofont text for the name of a transient variable
-(i.e., one defined and used only in the discussion, such as +n+).
+(i.e., one defined and used only in the discussion, such as `n`).
 
 ### HTML Tags
 
@@ -491,10 +491,10 @@ Return types:
 
 - If the method can return multiple different types,
   separate the types with "or" and, if necessary, commas.
-- If the method can return multiple types, use +object+.
-- If the method returns the receiver, use +self+.
+- If the method can return multiple types, use `object`.
+- If the method returns the receiver, use `self`.
 - If the method returns an object of the same class,
-  prefix `new_` if and only if the object is not +self+;
+  prefix `new_` if and only if the object is not `self`;
   example: `new_array`.
 
 Aliases:
diff --git a/doc/language/exceptions.md b/doc/language/exceptions.md
index 2c47455911376f..5f8f0ece69d2e6 100644
--- a/doc/language/exceptions.md
+++ b/doc/language/exceptions.md
@@ -504,18 +504,18 @@ These methods return backtrace information:
 By default, Ruby sets the backtrace of the exception to the location where it
 was raised.
 
-The developer might adjust this by either providing +backtrace+ argument
+The developer might adjust this by either providing `backtrace` argument
 to Kernel#raise, or using Exception#set_backtrace.
 
 Note that:
 
-- by default, both +backtrace+ and +backtrace_locations+ represent the same backtrace;
+- by default, both `backtrace` and `backtrace_locations` represent the same backtrace;
 - if the developer sets the backtrace by one of the above methods to an array of
   Thread::Backtrace::Location, they still represent the same backtrace;
 - if the developer sets the backtrace to a string or an array of strings:
-  - by Kernel#raise: +backtrace_locations+ become +nil+;
-  - by Exception#set_backtrace: +backtrace_locations+ preserve the original
+  - by Kernel#raise: `backtrace_locations` become `nil`;
+  - by Exception#set_backtrace: `backtrace_locations` preserve the original
     value;
-- if the developer sets the backtrace to +nil+ by Exception#set_backtrace,
-  +backtrace_locations+ preserve the original value; but if the exception is then
-  reraised, both +backtrace+ and +backtrace_locations+ become the location of reraise.
+- if the developer sets the backtrace to `nil` by Exception#set_backtrace,
+  `backtrace_locations` preserve the original value; but if the exception is then
+  reraised, both `backtrace` and `backtrace_locations` become the location of reraise.
diff --git a/doc/language/options.md b/doc/language/options.md
index cca87f42dedf0f..3421d73f552b40 100644
--- a/doc/language/options.md
+++ b/doc/language/options.md
@@ -640,7 +640,7 @@ Option `--encoding` is an alias for
 
 Option `--external-encoding`
 sets the default external encoding for the invoked Ruby program;
-for values of +encoding+,
+for values of `encoding`,
 see {Encoding: Names and Aliases}[rdoc-ref:encodings.rdoc@Names+and+Aliases].
 
 ```console
@@ -662,7 +662,7 @@ For a shorter help message, use option `-h`.
 
 Option `--internal-encoding`
 sets the default internal encoding for the invoked Ruby program;
-for values of +encoding+,
+for values of `encoding`,
 see {Encoding: Names and Aliases}[rdoc-ref:encodings.rdoc@Names+and+Aliases].
 
 ```console
diff --git a/ext/-test-/stack/depend b/ext/-test-/stack/depend
index 31571c882e6eb7..77e93bb201db09 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 de6e775accf395..478ae3b82b7500 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 04b26eb6c26567..d9dfc0c42b4740 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 bd2de759065ba2..944da25ee94f38 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 3573dc45e2fa9f..77f6239a3da52b 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 89294ea009e7b3..867b041a44af0f 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"
@@ -3723,7 +3724,7 @@ 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 +4652,7 @@ 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 +4693,8 @@ 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 +5257,17 @@ 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 +5332,77 @@ 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);
+
+    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];
+        long len = RSTRING_LEN(result);
+
+        const char *tmp_s;
+        long tmp_len;
+        RSTRING_GETMEM(tmp, tmp_s, tmp_len);
+
+        if (isdirsep(tmp_s[0])) {
+            // right side has a leading separator, remove left side separators.
+            long trailing_seps = 0;
+            while (isdirsep(name[len - trailing_seps - 1])) {
+                trailing_seps++;
+            }
+            rb_str_set_len(result, len - trailing_seps);
+        }
+        else if (!isdirsep(name[len - 1])) {
+            // neither side have a separator, append one;
+            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_cat(result, tmp_s, tmp_len);
+    }
+
+    rb_str_null_check(result);
+    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 +5415,9 @@ 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)
@@ -5482,7 +5557,7 @@ rb_thread_flock(void *data)
  *  call-seq:
  *    flock(locking_constant) -> 0 or false
  *
- *  Locks or unlocks file +self+ according to the given `locking_constant`,
+ *  Locks or unlocks file `self` according to the given `locking_constant`,
  *  a bitwise OR of the values in the table below.
  *
  *  Not available on all platforms.
@@ -5492,10 +5567,10 @@ rb_thread_flock(void *data)
  *
  *  | Constant        | Lock         | Effect
  *  |-----------------|--------------|-----------------------------------------------------------------------------------------------------------------|
- *  | +File::LOCK_EX+ | Exclusive    | Only one process may hold an exclusive lock for +self+ at a time.                                               |
- *  | +File::LOCK_NB+ | Non-blocking | No blocking; may be combined with +File::LOCK_SH+ or +File::LOCK_EX+ using the bitwise OR operator <tt>\|</tt>. |
- *  | +File::LOCK_SH+ | Shared       | Multiple processes may each hold a shared lock for +self+ at the same time.                                     |
- *  | +File::LOCK_UN+ | Unlock       | Remove an existing lock held by this process.                                                                   |
+ *  | `File::LOCK_EX` | Exclusive    | Only one process may hold an exclusive lock for `self` at a time.                                               |
+ *  | `File::LOCK_NB` | Non-blocking | No blocking; may be combined with `File::LOCK_SH` or `File::LOCK_EX` using the bitwise OR operator <tt>\|</tt>. |
+ *  | `File::LOCK_SH` | Shared       | Multiple processes may each hold a shared lock for `self` at the same time.                                     |
+ *  | `File::LOCK_UN` | Unlock       | Remove an existing lock held by this process.                                                                   |
  *
  *  Example:
  *
@@ -5622,11 +5697,11 @@ test_check(int n, int argc, VALUE *argv)
  *      | <tt>'z'</tt> | Whether the entity exists and is of length zero.                          |
  *
  *  - This test operates only on the entity at `path0`,
- *    and returns an integer size or +nil+:
+ *    and returns an integer size or `nil`:
  *
  *      | Character    | Test                                                                                         |
  *      |:------------:|:---------------------------------------------------------------------------------------------|
- *      | <tt>'s'</tt> | Returns positive integer size if the entity exists and has non-zero length, +nil+ otherwise. |
+ *      | <tt>'s'</tt> | Returns positive integer size if the entity exists and has non-zero length, `nil` otherwise. |
  *
  *  - Each of these tests operates only on the entity at `path0`,
  *    and returns a Time object;
@@ -7584,7 +7659,7 @@ 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/gc.c b/gc.c
index 1fe3dbf0aebb4b..ab0539cd3358a3 100644
--- a/gc.c
+++ b/gc.c
@@ -1709,63 +1709,68 @@ rb_gc_copy_finalizer(VALUE dest, VALUE obj)
 
 /*
  *  call-seq:
- *     ObjectSpace.define_finalizer(obj, aProc=proc())
+ *     ObjectSpace.define_finalizer(obj) {|id| ... } -> array
+ *     ObjectSpace.define_finalizer(obj, finalizer) -> array
  *
- *  Adds <i>aProc</i> as a finalizer, to be called after <i>obj</i>
- *  was destroyed. The object ID of the <i>obj</i> will be passed
- *  as an argument to <i>aProc</i>. If <i>aProc</i> is a lambda or
- *  method, make sure it can be called with a single argument.
+ *  Adds a new finalizer for +obj+ that is called when +obj+ is destroyed
+ *  by the garbage collector or when Ruby shuts down (which ever comes first).
  *
- *  The return value is an array <code>[0, aProc]</code>.
+ *  With a block given, uses the block as the callback. Without a block given,
+ *  uses a callable object +finalizer+ as the callback. The callback is called
+ *  when +obj+ is destroyed with a single argument +id+ which is the object
+ *  ID of +obj+ (see Object#object_id).
  *
- *  The two recommended patterns are to either create the finaliser proc
- *  in a non-instance method where it can safely capture the needed state,
- *  or to use a custom callable object that stores the needed state
- *  explicitly as instance variables.
+ *  The return value is an array <code>[0, callback]</code>, where +callback+
+ *  is a Proc created from the block if one was given or +finalizer+ otherwise.
  *
- *      class Foo
- *        def initialize(data_needed_for_finalization)
- *          ObjectSpace.define_finalizer(self, self.class.create_finalizer(data_needed_for_finalization))
- *        end
+ *  Note that defining a finalizer in an instance method of the object may prevent
+ *  the object from being garbage collected since if the block or +finalizer+ refers
+ *  to +obj+ then +obj+ will never be reclaimed by the garbage collector. For example,
+ *  the following script demonstrates the issue:
  *
- *        def self.create_finalizer(data_needed_for_finalization)
- *          proc {
- *            puts "finalizing #{data_needed_for_finalization}"
- *          }
+ *      class Foo
+ *        def define_final
+ *          ObjectSpace.define_finalizer(self) do |id|
+ *            puts "Running finalizer for #{id}!"
+ *          end
  *        end
  *      end
  *
- *      class Bar
- *       class Remover
- *          def initialize(data_needed_for_finalization)
- *            @data_needed_for_finalization = data_needed_for_finalization
- *          end
+ *      obj = Foo.new
+ *      obj.define_final
  *
- *          def call(id)
- *            puts "finalizing #{@data_needed_for_finalization}"
- *          end
+ *  There are two patterns to solve this issue:
+ *
+ *  - Create the finalizer in a non-instance method so it can safely capture
+ *    the needed state:
+ *
+ *      class Foo
+ *        def define_final
+ *          ObjectSpace.define_finalizer(self, self.class.create_finalizer)
  *        end
  *
- *        def initialize(data_needed_for_finalization)
- *          ObjectSpace.define_finalizer(self, Remover.new(data_needed_for_finalization))
+ *        def self.create_finalizer
+ *          proc do |id|
+ *            puts "Running finalizer for #{id}!"
+ *          end
  *        end
  *      end
  *
- *  Note that if your finalizer references the object to be
- *  finalized it will never be run on GC, although it will still be
- *  run at exit. You will get a warning if you capture the object
- *  to be finalized as the receiver of the finalizer.
+ *  - Use a callable object:
+ *
+ *      class Foo
+ *        class Finalizer
+ *          def call(id)
+ *            puts "Running finalizer for #{id}!"
+ *          end
+ *        end
  *
- *      class CapturesSelf
- *        def initialize(name)
- *          ObjectSpace.define_finalizer(self, proc {
- *            # this finalizer will only be run on exit
- *            puts "finalizing #{name}"
- *          })
+ *        def define_final
+ *          ObjectSpace.define_finalizer(self, Finalizer.new)
  *        end
  *      end
  *
- *  Also note that finalization can be unpredictable and is never guaranteed
+ *  Note that finalization can be unpredictable and is never guaranteed
  *  to be run except on exit.
  */
 
diff --git a/internal/string.h b/internal/string.h
index d6fea62061ddfa..cd1e8d79296ef4 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);
@@ -63,6 +84,7 @@ bool rb_str_reembeddable_p(VALUE);
 VALUE rb_str_upto_endless_each(VALUE, int (*each)(VALUE, VALUE), VALUE);
 VALUE rb_str_with_debug_created_info(VALUE, VALUE, int);
 VALUE rb_str_frozen_bare_string(VALUE);
+const char *rb_str_null_check(VALUE);
 
 /* error.c */
 void rb_warn_unchilled_literal(VALUE str);
diff --git a/lib/prism/lex_compat.rb b/lib/prism/lex_compat.rb
index 46f6130357e024..b7c54178ac3193 100644
--- a/lib/prism/lex_compat.rb
+++ b/lib/prism/lex_compat.rb
@@ -225,14 +225,6 @@ def state
       end
     end
 
-    # Ripper doesn't include the rest of the token in the event, so we need to
-    # trim it down to just the content on the first line when comparing.
-    class EndContentToken < Token
-      def ==(other) # :nodoc:
-        [self[0], self[1], self[2][0..self[2].index("\n")], self[3]] == other
-      end
-    end
-
     # Tokens where state should be ignored
     # used for :on_comment, :on_heredoc_end, :on_embexpr_end
     class IgnoreStateToken < Token
@@ -680,7 +672,10 @@ def result
         token =
           case event
           when :on___end__
-            EndContentToken.new([[lineno, column], event, value, lex_state])
+            # Ripper doesn't include the rest of the token in the event, so we need to
+            # trim it down to just the content on the first line.
+            value = value[0..value.index("\n")]
+            Token.new([[lineno, column], event, value, lex_state])
           when :on_comment
             IgnoreStateToken.new([[lineno, column], event, value, lex_state])
           when :on_heredoc_end
diff --git a/string.c b/string.c
index 2d74c46a360aa8..1e0b9929ef150c 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;
@@ -2917,6 +2902,34 @@ str_null_check(VALUE str, int *w)
     return s;
 }
 
+const char *
+rb_str_null_check(VALUE str)
+{
+    RUBY_ASSERT(RB_TYPE_P(str, T_STRING));
+
+    char *s;
+    long len;
+    RSTRING_GETMEM(str, s, len);
+
+    if (RB_LIKELY(rb_str_enc_fastpath(str))) {
+        if (!s || memchr(s, 0, len)) {
+            rb_raise(rb_eArgError, "string contains null byte");
+        }
+    }
+    else {
+        int w;
+        const char *s = str_null_check(str, &w);
+        if (!s) {
+            if (w) {
+                rb_raise(rb_eArgError, "string contains null char");
+            }
+            rb_raise(rb_eArgError, "string contains null byte");
+        }
+    }
+
+    return s;
+}
+
 char *
 rb_str_to_cstr(VALUE str)
 {
@@ -3765,7 +3778,7 @@ 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
diff --git a/test/prism/fixtures/__END__.txt b/test/prism/fixtures/__END__.txt
new file mode 100644
index 00000000000000..c0f4f28004cdc5
--- /dev/null
+++ b/test/prism/fixtures/__END__.txt
@@ -0,0 +1,3 @@
+foo
+__END__
+Available in DATA constant