User visible changes in `mruby4.0` from `mruby3.4`

"NOTE:" are changes to be aware of.

The language

Pattern Matching

mruby now supports pattern matching (case/in) syntax:

Basic pattern matching with case/in syntax (dadfac6)
Array pattern matching (ec67fd9)
Hash pattern matching (2147263)
Find pattern matching ([*pre, target, *post]) (6c4d98b)
Pin operator (^variable) (1de6340)
Guard clauses (if/unless conditions) (07ac110)
One-line pattern matching (expr in pattern) (e76ce24)
Brace-less hash pattern support (e8096bf)

Other Language Changes

&nil in formal parameters to explicitly opt out of block arguments (b07518e)
Trailing comma in method definition parameters: def foo(a, b,) (f78334b)
Array/Hash/String subclasses can now override [] and []= methods (#6675)
OP_SETIDX optimization for Array and Hash (ddd8fe1)
case/in without else now raises NoMatchingPatternError (d8de35b)
Allow compound statement in parenthesized argument context (919cbd8)

Changes in C API

NOTE: mrb_alloca() renamed to mrb_temp_alloc() (7fe5c2e)
NOTE: mruby/ext/io.h renamed to mruby/io.h (2813f79)
mrb_gc_add_region() for contiguous heap region support (072855a)
mrb_class_outer() to get the outer class/module (3a1b771)
MRB_ENSURE() macro for exception-safe cleanup (3ac682b)
mrb_time_get_tm() for accessing struct tm (daaaafe)
MRB_OPEN_FAILURE() macro for checking mrb_open result (40b0cb9)
mrb_print_error() now handles NULL gracefully (8e50a45)
mrb_open() returns mrb_state with exc set on init failure (05ffe0c)
mrb_utf8_to_buf() for UTF-8 encoding consolidation (7e28e68)
kh_is_end() macro for safe khash iteration (893cc75)
mrb_bigint_p() always defined regardless of bigint gem presence (6c4a8c0)
RInteger and RFloat added to RVALUE union (13dbca0)

ROM Method Tables

All built-in classes and most extension gems now use read-only method tables stored in .rodata instead of heap-allocated hash tables. Method definitions no longer consume heap memory, significantly reducing memory footprint for embedded use.

Core classes converted: BasicObject, Object, Module, Class, Kernel, String, Array, Hash, Numeric, Integer, Float, NilClass, TrueClass, FalseClass, Range, Symbol, Exception, Proc.

Extension gems converted: mruby-string-ext, mruby-array-ext, mruby-set, mruby-struct, mruby-class-ext, mruby-numeric-ext, mruby-random, mruby-kernel-ext, mruby-complex, mruby-rational, mruby-io, mruby-socket, mruby-method, mruby-metaprog, mruby-time, mruby-hash-ext, mruby-proc-ext, mruby-symbol-ext, mruby-range-ext, mruby-object-ext.

GC and Memory

NOTE: MRB_NO_PRESYM removed; presym is now always enabled (81689045)
Replace gcnext gray linked list with fixed-size gray stack, reducing per-object overhead (31fea170)
mrb_gc_add_region() for providing contiguous memory buffers as GC heap pages (072855a)
Chunk-based pool for symbol string allocation (e05bd8f)
Reduce IV_INITIAL_SIZE from 4 to 2 (6bd1f51)
Lossless float encoding using rotation in word boxing (b6148c8)
Lossless rotation encoding for 32-bit float32 word boxing (14a5cfb)
Consolidated irep allocation for .mrb loading (74fb045)
Object shapes (hidden classes) for MRB_TT_OBJECT IV storage, sharing key layouts across objects with the same instance variable assignment order (8d10056)

Build & Configuration

NOTE: MRB_WORDBOX_NO_FLOAT_TRUNCATE renamed to MRB_WORDBOX_NO_INLINE_FLOAT (old name still works) (59e1fe2)
NOTE: MRB_INT64 on 32-bit now requires MRB_NO_BOXING (other boxing modes cannot guarantee alignment for heap-allocated 64-bit integers) (eaaa66b)
Amalgamation support via rake amalgam task (d995ca2)
New Platform: Cosmopolitan Libc (#6681)
Emscripten: use native WASM exception handling (ca364e3)
HAL (Hardware Abstraction Layer) for platform abstraction in mruby-io, mruby-socket, mruby-dir, mruby-task (74ca22f)
MRUBY_MIRB_READLINE environment variable to control readline library selection (0aafb83)
MSYS2 drive letter support in build script (77f6ffe)
Inter-gem headers separated from external API headers (#6671)

Changes in mrbgems

New Gems

mruby-task: Cooperative multitasking with preemptive scheduling (ae0d7a0)
mruby-benchmark: Benchmarking gem (2f40f3d)
mruby-strftime: Time#strftime implementation (b31e22f)

mruby-bin-mirb Improvements

Custom multi-line editor replacing readline (527018c)
Syntax highlighting for keywords, strings, result values, hash key symbols (624272b, 1713d4a)
Automatic light/dark theme detection via OSC 11 (db4c8d9)
Tab completion support (2f15282)
Colored output for prompts and errors (b36e0b4)
Auto-indentation and auto-dedent (d52f318, e901b6d)
Command history with Up/Down navigation (5f85c1b)
Line numbers in multi-line prompts (5a3f0e2)
UTF-8 multibyte character support (4a97da3)

mruby-bigint Improvements

Toom-3 multiplication for large numbers (99620804)
Karatsuba multiplication for medium-sized numbers (85e81072)
Balance multiplication for asymmetric operands (0220ec2b)
Divide-and-conquer optimization for to_s (990ff90f)
Consolidated mpn layer for low-level limb operations (9ef3362f)
Always use 32-bit limbs by default (c747c77f)

Other Gem Changes

NOTE: Hash#deconstruct_keys removed for CRuby compatibility (34b9412)
mruby-enum-lazy: Fix Lazy#flat_map to handle non-enumerable block return values (#6765)
mruby-array-ext: Add Array#find and Array#rfind methods
mruby-io: Add IO#putc and Kernel#putc (baff6e6)
mruby-random: Replace xoshiro with PCG for better memory efficiency (f1bab01)
mruby-compiler: Variable-sized AST nodes for reduced memory usage
mruby-compiler: no_return_value context flag for script optimization (613b03a)
initialize_copy and respond_to_missing? defined as private (#6708)
Struct keyword argument initialization (#6574)

Compiler Improvements

Variable-sized AST nodes for reduced memory consumption (821b989)
Pattern matching bytecode optimizations (21d4135)
Optimized masgn to generate literals directly into target registers (fb5d966)
Optimized splat of literal arrays in args/literals (1cb8d73)
Early termination after too many parse errors (510ebd7)
Chunk array literals at 64 elements to reduce register pressure (f98d641)
Chunk %w() and %i() literals to reduce register pressure (62cf0dc)

VM Optimizations

New super-instructions that fuse common opcode sequences to reduce bytecode size and improve performance:

OP_SEND0/OP_SSEND0: Zero-argument method call, avoiding argument count setup (9123ef4)
OP_TDEF/OP_SDEF: Fused method definition combining TCLASS/SCLASS+METHOD+DEF into single instruction, saving 4 bytes per method (8d4f47e)
OP_GETIDX0: Fast path for array[0] and Array#first access (680f7ec)
OP_ADDILV/OP_SUBILV: Local variable increment/decrement fusion for i += n patterns (43f64b9)
OP_RETSELF: Single-byte instruction for return self pattern (a71db8c)
OP_RETNIL: Single-byte instruction for return nil pattern (64e30bf)
OP_RETTRUE/OP_RETFALSE: Single-byte instructions for return true/return false patterns (0b15727)
OP_MATCHERR: Pattern matching error with conditional execution (944168a)
OP_BLKCALL: Direct block call for yield, bypassing method dispatch (13-17% faster) (3aa2872)

Other optimizations:

1.5x stack growth instead of linear growth for reduced reallocations (f7988c93)
Skip keyword argument hash duplication (5970e350)

Fixed GitHub Issues

#5531 Hash recursion detection
#6506 Constant lookup in singleton class
#6507 tally multi-values
#6508 Enumerable#sum index
#6509 scope_new nregs initialization
#6515 y.tab.c in repository
#6516 Private backquote
#6554 Socket private #initialize
#6570 instance_eval crash
#6613 const_added hook during bootstrapping
#6635, #6636 Colon3 constant lookup
#6637 arm64 mingw64 builtin setjmp/longjmp
#6642 Task segfault when sleep called from C
#6645 Set memory leak from double initialization
#6646 IO#gets negative length
#6647 IO#ungetc buffer overflow
#6648 sprintf buffer overread
#6649 Array#sort! use-after-realloc
#6650 Array#fill validation
#6652 Array comparison use-after-realloc
#6657 Exception handling for ||= on class variables
#6659 Super with keyword arguments
#6660 Regression on struct/array/hash == override with super
#6662 Array set operations use-after-free
#6664 Set#flatten memory leak
#6666 Regexp literal with encoding
#6668 Method#== for aliased methods and comparison bug
#6671 Separate inter-gem headers from external API headers
#6674 Document pattern matching limitations
#6675 Allow Hash#[] to be aliased again
#6687 Expand MRB_SYM/MRB_GVSYM support for symbols with special characters
#6698 Bigint tests fail on architectures other than x86_64 and i386
#6701 Heap-use-after-free in mrb_vm_exec involving mruby-rational / mruby-bigint
#6702 mruby-bigint doesn't compile in C++ project
#6704 Heap-buffer-overflow in mrb_vm_exec via malformed source code
#6705 Can't get outer class of an object in C
#6713 mruby-polarssl not work
#6720 Random float range: different behavior from CRuby
#6722 RBreak size overflow on 32-bit platforms with MRB_NO_BOXING
#6740 %w()/%i() register pressure with large literals
#6741 case/in without else should raise NoMatchingPatternError
#6760 mrb_gc_unregister() not removing all matching entries

Merged Pull Requests

#6418 Add ls-lint with GitHub Actions
#6492 fix a typo, update specs
#6493 Fix TYPO in memory.md
#6495 Remove MRB_ENDIAN_LOHI() that is no longer in use
#6497 gha: update build.yml try windows-2025 image
#6498 Clean up and standardize the pre-commit config
#6501 Update pre-commit Node.js version to v22.14.0 LTS
#6502 pre-commit: update prettier to the latest version
#6503 misc: fix typos
#6505 mrbgems: fix spelling
#6510 Fixed class method visibility via module_function
#6511 Exclude the external project "lrama" from pre-commit
#6513 mruby 3.4.0 released
#6517 core/codegen.c: remove unneeded duplicate semicolon
#6518 Change mrbc_args.flags bit width from 2 to 3
#6519 Add tools/lrama to .prettierignore
#6520 pre-commit: autoupdate and update node LTS version
#6521 Add codespell config file .codespellrc
#6522 gha: label more files
#6523 add rand(Range) and unify implementations of Random#rand and Kernel#rand
#6524 Fix Kernel#p when no argument
#6525 Skip adding empty input to mirb history
#6526 Add build config for Luckfox Pico embedded SBC
#6528 misc: fix spelling
#6530 Revert "class.c (find_visibility_scope): when callinfo returns, *ep == NULL; #6512"
#6531 Improve method table performance by rehashing at 75% load factor
#6532 Reverted method table optimizations to prioritize memory savings
#6533 Fix calling extended callback
#6534 Add descriptive comment to mrb_read_float function
#6535 Added descriptive comments for functions/macros in src/mempool.c
#6536 Add descriptive comments to public functions in src/debug.c
#6537 Updated comments in cdump.c to remove the @brief tag
#6539 Add descriptive comments for functions in src/load.c
#6540 Add descriptive comments to MRB_API functions in object.c
#6541 Add descriptive comments for MRB_API functions in src/array.c
#6542 Add descriptive comments to MRB_API functions in src/symbol.c
#6543 Add descriptive comments to several functions in src/dump.c
#6544 Fix build strings that must be mutable
#6545 Add descriptive comments for MRB_API functions in src/class.c
#6548 Add descriptive comments to MRB_API functions in src/etc.c
#6549 Add descriptive comments to kernel functions
#6550 Add descriptive comments for MRB_API functions in src/proc.c
#6551 Add descriptive comments for MRB_API functions in src/state.c
#6552 Fix: Correct placement of comments in src/variable.c
#6553 Add descriptive comments for MRB_API functions in src/vm.c
#6555 mrb_mt_foreach() needs to update the pointer at each loop
#6556 iv_foreach() needs to update the pointer at each loop
#6560 Refactor: Improve Set GC marking and freeing
#6561 pre-commit updates and fix prettier entrypoint
#6562 misc: fix spelling word case
#6563 pre-commit add rubocop with one rule spaces for indentation
#6564 Remove jumanjihouse pre-commit hooks no longer maintained
#6565 Rubocop: fix target Ruby version; add two more cops; fix lint error
#6566 Removed unreferenced variables in CrossBuild#run_bintest
#6567 Avoid array object creation in cmd_bin method in bintest
#6568 mruby-bin-debugger depends on mruby-bin-mrbc in bintest
#6569 sed s/Mruby/MRuby/g
#6571 Update limitations.md to add behavior on small hash
#6572 Add Claude Code GitHub Workflow
#6573 pre-commit fixes and updates
#6574 Support initializing structs via keyword arguments
#6575 Fix typo in file time methods
#6581 Merge mrb_obj_iv_inspect() into mrb_obj_inspect()
#6582 Stricter type tag in mrb_obj_alloc()
#6583 Add fallback to local build_config.rb before using default configuration
#6585 Fix typo in mruby3.2 docs
#6586 Makefile: refactor add docs and add command line help target
#6587 Add Set#hash tests
#6588 Add CodeQL Analysis for GitHub Actions
#6589 Add pre-commit hook check-zip-file-is-not-committed
#6591 mruby-eval fix license link in README
#6593 README: Add Contributors Avatars, Star History, Table of Contents
#6598 Fix heap buffer overflow in #method_missing
#6599 pre-commit: run markdown-link-check, oxipng, prettier manually
#6600 dreamcast_shelf build config: update to use KallistiOS wrappers
#6601 fix: skip local build_config.rb when working in MRUBY_ROOT
#6602 Improved iseq annotations for new and !=
#6604 pre-commit config updates
#6607 fix bigint on raspberry pi
#6610 Extract golden ratio prime into constant
#6614 Fix uninitialized variable in io_gets causing segmentation fault
#6617 Fix various minor problems and speed up build
#6618 Stop generating unnecessary C++ files in mruby-bin-mruby
#6621 Set up all GEMS before mruby core tasks definition
#6624 Fixed wrong MRuby::Build.current at the top level of mrbgem.rake
#6628 Revert File.absolute_path logic
#6629 pre-commit update
#6631 Revert "Rakefile: make the whole thing parallel unless SERIAL=1"
#6633 Fix a heap-buffer-overflow in str strip! methods
#6643 Fix crash caused by an incorrect node type check in codegen_masgn
#6651 Address stack-use-after-return in the mruby bigint implementation
#6653 Improve HAL-related components for MinGW
#6655 Preventing Memory Leaks in Array#__combination_init
#6656 Fix integer overflow in allocation size calculation
#6663 Added the kh_is_end() macro function
#6665 Fixed use-after-free with Set#join
#6670 Arranging VM dispatch macros
#6673 Adjust broken license links; clean up Markdown
#6677 gha: run pre-commit with --color=always
#6678 Put ls-lint and pre-commit in separate workflow files
#6679 pre-commit autoupdate; update node and prettier
#6681 Add Cosmopolitan Libc build configuration
#6689 docs: fix pre-commit manual hooks; fix link
#6694 Fix mirb build under Cosmopolitan
#6695 Dependabot: add a cooldown period for new releases
#6696 Fix parse error with required kwargs and omitted parens
#6699 Fix mruby-task for PicoRuby Integration
#6700 Fix float/double pack/unpack on s390x
#6706 Refactor task class to use symbol IDs
#6708 initialize_copy and respond_to_missing? defined as private
#6709 Add the MRB_ENSURE() macro
#6711 Fix out of bounds read and write in IO.select
#6714 Fix OP_DEBUG operand type and add NULL check for debug_op_hook
#6716 Fixes identity for proc object
#6717 Fix mruby-task: wrapping by critical section and setting initial task receiver
#6718 Add installation instructions for conda and Homebrew
#6723 Add RInteger and RFloat to RVALUE
#6727 Language documentation: update wording of "overloading" section
#6729 Simplifying dependency addition for gensym task
#6730 Simplifying presym file generation actions
#6733 Include mruby/presym.h for all source files
#6734 Chunk array literals at 64 elements to reduce register pressure
#6735 Prevent full recompilation without changes to presym file
#6739 Fix MSYS2 build error with drive letters
#6743 Chunk %w() and %i() literals to reduce register pressure
#6744 Raise NoMatchingPatternError in case/in without else
#6747 Correctly handle empty hash as default named argument
#6749 Fix microcontroller profile
#6750 Fix out-of-bounds read and divide-by-zero in Array#product
#6752 Fix attr_reader-generated methods accepting extra arguments
#6753 Further optimize Array#product
#6754 Mark attr_reader procs as noarg
#6755 Reload ci after mrb_hash_delete_key() in keyword argument handling
#6756 Avoid impact of object modifications caused by mrb_vm_exec() calls
#6758 Don't assign result of mrb_funcall() directly to regs
#6759 Define mrb_bigint_p() always
#6761 Fix mrb_gc_unregister() to remove all matching entries
#6762 Write generated test C files atomically to avoid build race condition
#6765 Fix Lazy#flat_map to handle non-enumerable block return values
#6767 Allow compound statement in parenthesized argument context
#6780 Fix String#prepend with self-referencing arguments
#6781 Protect sprintf format string from mutation during callbacks
#6783 Pin GitHub Actions workflows to commit hashes

Security Fixes

Buffer overflow in bigint uadd (3f2611e)
Stack buffer overflow in Montgomery reduction (edce0a3)
Buffer overflow in pack_uu encoding (2993302)
Buffer overflow in IO#ungetc (01ab2ff)
Heap-buffer-overflow in pattern alternation codegen (eea9e30)
Out of bounds read and write in IO.select (44831711)
Off-by-one in bounds check for symbol names and pool strings in load.c (b3b8c01)
Use-after-free in Set operations (a6b55e7)
Use-after-free in Array set operations (729b84c)
Use-after-free in Set#join (0e653eb)
Use-after-realloc in Array#sort! (eb39897)
Heap-use-after-free in insertion_sort (099d2c47)
Integer overflow in str_check_length (6afff1c3)
Integer overflow in Integer#lcm (070bef24)
Heap buffer overflow in #method_missing (550d10a)
Out-of-bounds read and divide-by-zero in Array#product (8441eaf)
Heap buffer overflow in String#prepend with self-referencing arguments (18ba026)
Use-after-free in sprintf via to_s callback mutating format string (48fc422)
Multiple memory leak fixes in bigint, Set, Array, and Task gems

User visible changes in `mruby4.0` from `mruby3.4`

User visible changes in mruby4.0 from mruby3.4

The language

Pattern Matching

Other Language Changes

Changes in C API

ROM Method Tables

GC and Memory

Build & Configuration

Changes in mrbgems

New Gems

mruby-bin-mirb Improvements

mruby-bigint Improvements

Other Gem Changes

Compiler Improvements

VM Optimizations

Fixed GitHub Issues

Merged Pull Requests

Security Fixes

User visible changes in `mruby4.0` from `mruby3.4`