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# NEWS for Ruby 3.3.0
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This document is a list of user-visible feature changes
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since the **3.2.0** release, except for bug fixes.
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Note that each entry is kept to a minimum, see links for details.
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## Language changes
Evaluate multiple assignment left hand side before right hand side
In regular assignment, Ruby evaluates the left hand side before
the right hand side. For example:
```ruby
foo[0] = bar
```
Calls `foo`, then `bar`, then `[]=` on the result of `foo`.
Previously, multiple assignment didn't work this way. If you did:
```ruby
abc.def, foo[0] = bar, baz
```
Ruby would previously call `bar`, then `baz`, then `abc`, then
`def=` on the result of `abc`, then `foo`, then `[]=` on the
result of `foo`.
This change makes multiple assignment similar to single assignment,
changing the evaluation order of the above multiple assignment code
to calling `abc`, then `foo`, then `bar`, then `baz`, then `def=` on
the result of `abc`, then `[]=` on the result of `foo`.
Implementing this is challenging with the stack-based virtual machine.
We need to keep track of all of the left hand side attribute setter
receivers and setter arguments, and then keep track of the stack level
while handling the assignment processing, so we can issue the
appropriate topn instructions to get the receiver. Here's an example
of how the multiple assignment is executed, showing the stack and
instructions:
```
self # putself
abc # send
abc, self # putself
abc, foo # send
abc, foo, 0 # putobject 0
abc, foo, 0, [bar, baz] # evaluate RHS
abc, foo, 0, [bar, baz], baz, bar # expandarray
abc, foo, 0, [bar, baz], baz, bar, abc # topn 5
abc, foo, 0, [bar, baz], baz, abc, bar # swap
abc, foo, 0, [bar, baz], baz, def= # send
abc, foo, 0, [bar, baz], baz # pop
abc, foo, 0, [bar, baz], baz, foo # topn 3
abc, foo, 0, [bar, baz], baz, foo, 0 # topn 3
abc, foo, 0, [bar, baz], baz, foo, 0, baz # topn 2
abc, foo, 0, [bar, baz], baz, []= # send
abc, foo, 0, [bar, baz], baz # pop
abc, foo, 0, [bar, baz] # pop
[bar, baz], foo, 0, [bar, baz] # setn 3
[bar, baz], foo, 0 # pop
[bar, baz], foo # pop
[bar, baz] # pop
```
As multiple assignment must deal with splats, post args, and any level
of nesting, it gets quite a bit more complex than this in non-trivial
cases. To handle this, struct masgn_state is added to keep
track of the overall state of the mass assignment, which stores a linked
list of struct masgn_attrasgn, one for each assigned attribute.
This adds a new optimization that replaces a topn 1/pop instruction
combination with a single swap instruction for multiple assignment
to non-aref attributes.
This new approach isn't compatible with one of the optimizations
previously used, in the case where the multiple assignment return value
was not needed, there was no lhs splat, and one of the left hand side
used an attribute setter. This removes that optimization. Removing
the optimization allowed for removing the POP_ELEMENT and adjust_stack
functions.
This adds a benchmark to measure how much slower multiple
assignment is with the correct evaluation order.
This benchmark shows:
* 4-9% decrease for attribute sets
* 14-23% decrease for array member sets
* Basically same speed for local variable sets
Importantly, it shows no significant difference between the popped
(where return value of the multiple assignment is not needed) and
!popped (where return value of the multiple assignment is needed)
cases for attribute and array member sets. This indicates the
previous optimization, which was dropped in the evaluation
order fix and only affected the popped case, is not important to
performance.
Fixes [Bug #4443]
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## Command line options
* A new `performance` warning category was introduced.
They are not displayed by default even in verbose mode.
Turn them on with `-W:performance` or `Warning[:performance] = true` . [[Feature #19538 ]]
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## Core classes updates
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Note: We're only listing outstanding class updates.
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* Array
* `Array#pack` now raises ArgumentError for unknown directives. [[Bug #19150 ]]
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* Dir
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* `Dir.for_fd` added for returning a Dir object for the directory specified
by the provided directory file descriptor. [[Feature #19347 ]]
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* `Dir.fchdir` added for changing the directory to the directory specified
by the provided directory file descriptor. [[Feature #19347 ]]
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* `Dir#chdir` added for changing the directory to the directory specified
by the provided `Dir` object. [[Feature #19347 ]]
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* MatchData
* MatchData#named_captures now accepts optional `symbolize_names` keyword. [[Feature #19591 ]]
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* String
* `String#unpack` now raises ArgumentError for unknown directives. [[Bug #19150 ]]
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* `String#bytesplice` now accepts new arguments index/length or range of the source string to be copied. [[Feature #19314 ]]
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* ObjectSpace::WeakKeyMap
* New core class to build collections with weak references.
The class use equality semantic to lookup keys like a regular hash,
but it doesn't hold strong references on the keys. [[Feature #18498 ]]
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## Stdlib updates
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The following default gems are updated.
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* RubyGems 3.5.0.dev
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* bigdecimal 3.1.4
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* bundler 2.5.0.dev
* csv 3.2.7
* fiddle 1.1.2
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* fileutils 1.7.1
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* irb 1.6.4
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* nkf 0.1.3
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* optparse 0.4.0.pre.1
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* psych 5.1.0
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* reline 0.3.4
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* stringio 3.0.7
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* strscan 3.0.7
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* syntax_suggest 1.1.0
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* time 0.2.2
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* timeout 0.3.2
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* uri 0.12.1
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The following bundled gems are updated.
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* minitest 5.18.0
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* test-unit 3.5.9
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* rbs 3.1.0
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* typeprof 0.21.7
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* debug 1.8.0
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See GitHub releases like [Logger ](https://github.com/ruby/logger/releases ) or
changelog for details of the default gems or bundled gems.
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## Supported platforms
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## Compatibility issues
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## Stdlib compatibility issues
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### `ext/readline` is retired
* We have `reline` that is pure Ruby implementation compatible with `ext/readline` API. We rely on `reline` in the future. If you need to use `ext/readline` , you can install `ext/readline` via rubygems.org with `gem install readline-ext` .
* We no longer need to install libraries like `libreadline` or `libedit` .
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## C API updates
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## Implementation improvements
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* `defined?(@ivar)` is optimized with Object Shapes.
### YJIT
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* Significant performance improvements over 3.2
* Splat and rest arguments support has been improved.
* Registers are allocated for stack operations of the virtual machine.
* More calls with optional arguments are compiled.
* `Integer#!=` , `String#!=` , `Kernel#block_given?` , `Kernel#is_a?` ,
`Kernel#instance_of?` , `Module#===` are specially optimized.
* Instance variables no longer exit to the interpreter
with megamorphic Object Shapes.
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* Metadata for compiled code uses a lot less memory.
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* Improved code generation on ARM64
* Option to start YJIT in paused mode and then later enable it manually
* `--yjit-pause` and `RubyVM::YJIT.resume`
* This can be used to enable YJIT only once your application is done booting
* Exit tracing option now supports sampling
* `--trace-exits-sample-rate=N`
* Multiple bug fixes
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### RJIT
* Introduced a pure-Ruby JIT compiler RJIT and replaced MJIT.
* RJIT supports only x86\_64 architecture on Unix platforms.
* Unlike MJIT, it doesn't require a C compiler at runtime.
* RJIT exists only for experimental purposes.
* You should keep using YJIT in production.
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[Feature #18498 ]: https://bugs.ruby-lang.org/issues/18498
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[Bug #19150 ]: https://bugs.ruby-lang.org/issues/19150
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[Feature #19314 ]: https://bugs.ruby-lang.org/issues/19314
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[Feature #19347 ]: https://bugs.ruby-lang.org/issues/19347
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[Feature #19538 ]: https://bugs.ruby-lang.org/issues/19538
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[Feature #19591 ]: https://bugs.ruby-lang.org/issues/19591
