"**_NOTE_**:" are changes to be aware of.

The language

mruby now supports private and protected visibility (b0db0bd)
Maximum length of inlined symbols reduced from 5 to 4 characters to provide space for visibility flags (6442a01)
Many methods are made private according to CRuby visibility (4a0e806)
Generate OP_SSEND for self.method type calls (111fe4b)
initialize method will be always private (eb8b412)
Add new hooks method_removed, method_undefined (9c74f6e)
Add new hooks singleton_method_removed, singleton_method_undefined (0863c08)
Updated OP_DEF output from codedump (3a3e877)
Better handling of binary strings, e.g. String#b (b0127f0)
Hash to_s format has changed (baeeb5e)
Some encoding related method such as #force_encoding (e47b4ca), #b (b0127f0)
Constant folding for String#+ (6687bdd)
Remove Float bit-operation (db8368f)
use SWAR technique for strlen performance (cbb31e6)
use merge sort for Array#sort (5bd63d6)

Changes in C API

pool.c renamed to mempool.c (and mrb_pool to mrb_mempool) (49525fa)
mrb_pool_value renamed to mrb_irep_pool to reduce confusion (62ef5db)
rename BOXNIX_SET_VALUE to BOXNO_SET_VALUE (#6397)
MRB_FROZEN_P() is replaced by mrb_frozen_p() (c11d18e)
rename color to gc_color (0e79f6b, 1e36d76)
add obj->frozen instead of flags MRB_SET_FROZEN_FLAG/MRB_UNSET_FROZEN_FLAG (8276143)

Build & Configuration

New Build Target: test:run:serial, test:run:serial:lib, test:run:serial:bin (#6423)
New Platform: PlayStation Portable (#6022)
New Platform: emscripten (#6487)
New Config: no-float (with MRB_NO_FLOAT) (32200f1)

Changes in mrbgems

mruby-print: removed; if you do not use mruby-io, mruby use #print etc. in the core (8c8bbd9)
mruby-enum-lazy: Add Enumerable::Lazy#grep_v to mruby-enum-lazy (#6171)
mruby-io: Add level argument to File.dirname (#6463)
mruby-io: File.absolute_path? (#6482)
mruby-io: File.absolute_path (96113a2)
mruby-toplevel-ext: top-level public/private/protected moved to the core (2a876d2)
mruby-metaprog: method list methods now works according to the visibility (9229da1)
mruby-metaprog: public_instance_methods, private_instance_methods, protected_instance_methods (9e3e7b2)
mruby-encoding: MRB_UTF8_STRING turned on automatically with this gem (74bdae9)

Fixed GitHub Issues

#6173 Fails to build with tcc(Tiny C Compiler)
#6156 '/LIBPATH' issue
#6183 ".e".to_f returns NAN
#6182 mrb_read_float() converts "0.3" with a small error compared to strtod()
#6210 Unary minus seems broken
#6255 Wrong number of characters in broken UTF-8 string
#4038 Heap buffer overflow in OP_ENTER
#6262 Unable to define == for objects when using Array#delete
#6267 When MRB_UTF8_STRING is enabled, giving byte characters for String#index and String#split gives wrong results
#6277 MSVC: can't use malloc() in string.c with WIN32_LEAN_AND_MEAN
#6240 Differentiate between lib and lib64 in the build settings.
#6304 Calling method_missing with only Kwargs passes arguments incorrectly
#6317 mrb_gc_register() may cause GC and collect the object being protected
#6307 Planned change patch for mrb_vm_exec()
#6298 foo :bar {} is legal in mruby?
#6326 Detect “Use-after-free” with address sanitizer
#5358 static warning from getpwnam
#6339 mrb_ary_delete() may refer to an invalid address (use-after-free)
#6346 Block kwargs are passed as last positional arg when using yield
#6365 powl() not available when compiling for Dreamcast
#6369 100x Performance Regression from 3.1
#6270 NODE_ZSUPER from deeply nested blocks will result in a truncated digits for block index in OP_ARGARY
#6297 Assigning to a block variable changes the actual block (thus affecting block_given? and yield)
#6389 instance_exec named block args don't work properly
#6388 Recent commit broke my windows build
#6411 Wrong function unwinding when using return in a block
#6439 OP_JMPUW does not call the ensure block when it jumps to the beginning of the begin block
#6441 break inside while loop will execute the ensure block outside of the while loop
#6453 Bigint: incorrect behavior of ^ operator
#6452 Bigint: weird mod behavior
#6451 Bigint: incorrect division behavior
#6456 bigint: bug with division of a small number by a bigint
#6466 mruby-bin-mruby using Kernel#p and Kernel#print in bintest fails test
#6467 Heap-Use-After-Free due to Recursive group_by Calls
#6471 Discrepancy in codegen for binary operations between master branch and 3.3.0
#6477 heap-buffer-overflow in mrb_vm_exec
#6485 Hash#rehash does not check if the hash is frozen
#6483 Hash#default_proc= accepts arbitrary objects that are not callable
#6491 Destroy existing string literals when composing string literals

Merged Pull Requests (User Visible Ones)

#6171 Add Enumerable::Lazy#grep_v to mruby-enum-lazy
#6174 Fix MRUBY_PACKAGE_DIR in mruby-config.bat
#6175 Corrected strange conditional in mrb_vm_run()
#6176 Stop assuming alias proc in CI_PROC_SET()
#6177 gha: add macOS 14 to the build
#6184 Remove the L_STOP label
#6185 Added mrb_callinfo::u.keep_context for clarity
#6186 Omit NULL check of e->cxt in OP_RETURN_BLK
#6191 Speed up symbol equality comparison
#6192 Fix OP_STOP with exception
#6193 Fix wrong assertion in OP_SENDB.
#6194 Simplify the calculation of the number of closures in MRB_TT_FIBER
#6197 Fix int_xor to call flo_xor.
#6201 tasks/doc.rake: standardize the rake doc error messages
#6202 Remove the .yardoc folder with rake doc:clean:api
#6204 Clean up the .editorconfig file
#6209 Minor .gitignore clean up
#6211 Minor .gitignore clean up; order entries
#6216 Shared empty iv_tbl of module
#6217 Strict declaration for mrb_istruct_size()
#6219 Avoid assigning a fixed value in the loop
#6220 Reorganize mrb_cache_entry and mrb_method_t types
#6221 Arranging each_backtrace()
#6222 Need to synchronize dbg->regs after VM call in mrdb
#6224 mrb_env_unshare() to break the link to fiber
#6225 Revert "Adjust environment when `mrb_exec_irep` happened."
#6227 fix: Array#shuffle(!) result distribution
#6228 Revert "`env` referred from top-level callinfo should not be unshared; fix #4019"
#6230 Fix status of fiber after switched by exception raised
#6231 Add a way to let other gems handle closing of fds in mruby-io
#6232 Fold the code for freeing env
#6233 Free stack memory at end of fiber
#6235 fix Array#delete always firing the block when deleting nil
#6236 unify the code for filter methods (and speed up #reject!)
#6237 Stricter env objects to attach to ci
#6238 Minimize zero initialization of the stack
#6243 Fixed base64 decoding in mruby-pack
#6244 Revise scope of role of mrb_vm_run()
#6246 Fix typo in test/t/hash.rb
#6249 Fix spelling in src/vm.c
#6250 Fix spelling
#6251 Clean up root move CODEOWNERS to .github directory
#6253 Allow recycling fibers by GC if not referenced directly
#6256 Update documentation for mrb_top_run()
#6257 fix some mrbconf.md typos
#6260 Remove exc_caught from mrb_vm_exec()
#6261 fix: to_a integer ranges with begin > end failing
#6263 fix: Array#delete mistakenly calling block even if not passed
#6264 Must pass keyword arguments for Kernel#to_enum
#6265 Fixes Dir.children and Dir.each_child
#6266 Passes the nonexistent key as a block argument in Array#delete
#6273 Improvements to mrb_protect_atexit()
#6275 Fixed Binding#eval that failed to assign to the same variable
#6276 Always run atexit on the top-level call frame
#6279 Include headers for malloc() explicitly; fix #6277
#6280 Remove MRB_ENV_CLOSED flag
#6281 Fixes local variables in mruby-binding.
#6283 Simplify uvenv()
#6288 Detach env of ci explicitly on atexit
#6289 Simplify OP_RETURN_BLK and OP_BREAK
#6290 Allow to change the output directory name of the libmruby file
#6293 Changed the instruction table in opcode.md
#6294 Optimise mrb_iv_get
#6302 Minor cleanup in mrb_str_init
#6303 mrb_str_aset_m() should return replace instead of str
#6305 Protect keyword arguments in prepare_missing()
#6308 Assume that MRB_CATCH() has mrb->exc set
#6310 Doubling the call stack when extending it
#6311 Added fast-path for positional arguments less than 15 in OP_SEND
#6312 Omit error checking at OP_RETURN, OP_RETURN_BLK and OP_BREAK
#6313 Fix wrong column number in opcode.md
#6314 Optimize even?/odd? for big integers
#6318 Shrink variables in mrb_vm_exec()
#6320 Shrinking the code in OP_BREAK and OP_RETURN_BLK
#6321 Avoid warnings in lib/**/*.rb
#6322 Fix mrb_ro_data_p on Intel Mac
#6324 Remove localjump_error()
#6327 fix ncurses linking issues
#6328 Fix use-after-free in obj_free() for env objects
#6329 Fix use-after-free in mrb_obj_alloc()
#6330 Add a precondition to call mrb_env_unshare().
#6331 Restore the GC arena with tests
#6332 Must not depend on the “host” build to generate mruby-compiler/core/y.tab.c
#6333 Reduce the number of branch instructions in the heap_p()
#6335 Add return_ci in CHECKPOINT_MAIN() of OP_RETURN
#6338 Need to place static proc objects into 8-byte alignments
#6340 Fix use-after-free for Array#<=>
#6341 Need to restore the GC arena after some function calls
#6344 prefer using mrb_yield to call block arguments
#6347 codegen.c,parse.y: remove flattening of yield arguments; fix #6346
#6348 Cancel the warning disablement
#6349 Perform GC before deleting directories
#6350 Fixed character encoding conversion function mismatch
#6351 Remove unnecessary mrb_gc_arena_restore()
#6353 Fix use-after-free in mrb_ary_delete()
#6356 Making splat argument objects invisible from Ruby side
#6373 Add build config for Milk-V Duo (RISC-V Linux) board
#6382 Make array objects invisible in mrb_gc_register()
#6385 Small improvements for mrb_gc_register()
#6386 Avoid calling mrb_gv_set() from mrb_gc_unregister()
#6387 Small improvements for mrb_gc_unregister()
#6390 Fix use-after-free by mrb_gc_unregistor()
#6391 Fixed argument forwarding in instance_exec
#6392 Fix argument forwarding in mrb_exec_irep()
#6393 Follow-up to #6391
#6395 Storing method-id inside Symbol#to_proc
#6396 Milk-V Build Config: update GPIO gem URL
#6397 boxing_no.h: rename BOXNIX_SET_VALUE -> BOXNO_SET_VALUE
#6399 Add macOS 15 to the build
#6405 io_read: use i instead of d in call to mrb_raisef
#6407 Allow to exclude specific files in rake install
#6408 Improve compliance with C++ standards
#6410 Put #include <stdlib.h> in parse.y
#6412 Distinguish the call frame of the generator with OP_RETURN_BLK
#6413 Add links to documentation in README.md
#6415 Fix numbered parameters when used as a singleton
#6416 Optimize the "new" method's iseq
#6419 Follow Ruby's behavior for numbered parameters in -> {}
#6420 Update labeler.yml: add label for the tools directory
#6422 Add annotations for function names defined in the preprocessor
#6423 Adding a serialized test task
#6427 Need to update ci variable after re-entry to VM
#6428 Change the limits of OP_ADDI and OP_SUBI from 0-127 to 0-255.
#6429 Fix numbered parameters when used as hash keys
#6432 Moving code in macro arguments out of macros
#6434 Added document "Layout of the mruby filesystem"
#6436 Make rake doc:update-index prettier friendly
#6437 Add more details to the pre-commit config
#6438 Remove unused MRuby::Build#list_install_excludes method
#6440 Fix redo keyword
#6442 Fixed wrong range condition in OP_JMPUW
#6443 Fix NODE_NEGATE for bigints
#6444 Add test cases for bigints
#6446 Omit the _WIN64 definition check
#6447 Fixed File.expand_path
#6448 Suppress presym in mruby/ext/io.h file
#6449 Using presym in the mruby-io/src/file_test.c file
#6450 Change MRB_WITH_IO_PREAD_PWRITE configuration name
#6454 mruby-bigint: handle rhs bigint in int_mod and int_divmod functions
#6455 mruby-bigint: fix rounding behavior in mpz_mdiv and mpz_mdivmod functions
#6457 bigint: fix bug with division of a small number by a bigint
#6459 FileTest is a module
#6461 To create a release package file in draft
#6462 Properly cast the return value of memchr()
#6463 Add level argument to File.dirname
#6465 Add initial PlayStation portable crossbuild support
#6468 Fixed missing changes to IB_FIND_BY_KEY() parameter names
#6469 Add more const qualifier for RProc
#6472 Moved tests for Integer#quo
#6473 Hide mpz_and() symbol
#6473 Hide mpz_and() symbol
#6474 Avoid array object creation with “unknown keyword” error
#6475 Don't include deleted mruby-print
#6478 Fixed buffer overrun in function chars2bytes()
#6479 Reimplementation of File.expand_path method
#6482 Add File.absolute_path? method
#6487 Add Emscripten toolchain & build_config