initialisation of simple aggregate state variables

[perl5.git] / pod / perldelta.pod

=encoding utf8

=head1 NAME

[ this is a template for a new perldelta file.  Any text flagged as XXX needs
to be processed before release. ]

perldelta - what is new for perl v5.27.6

=head1 DESCRIPTION

This document describes differences between the 5.27.5 release and the 5.27.6
release.

If you are upgrading from an earlier release such as 5.27.4, first read
L<perl5275delta>, which describes differences between 5.27.4 and 5.27.5.

=head1 Notice

XXX Any important notices here

=head1 Core Enhancements

XXX New core language features go here.  Summarize user-visible core language
enhancements.  Particularly prominent performance optimisations could go
here, but most should go in the L</Performance Enhancements> section.

[ List each enhancement as a =head2 entry ]

=head2 Initialisation of aggregate state variables

A persistent lexical array or hash variable can now be initialized,
by an expression such as C<state @a = qw(x y z)>.  Initialization of a
list of persistent lexical variables is still not possible.

=head1 Security

XXX Any security-related notices go here.  In particular, any security
vulnerabilities closed should be noted here rather than in the
L</Selected Bug Fixes> section.

[ List each security issue as a =head2 entry ]

=head1 Incompatible Changes

XXX For a release on a stable branch, this section aspires to be:

    There are no changes intentionally incompatible with 5.XXX.XXX
    If any exist, they are bugs, and we request that you submit a
    report.  See L</Reporting Bugs> below.

[ List each incompatible change as a =head2 entry ]

=head1 Deprecations

XXX Any deprecated features, syntax, modules etc. should be listed here.

=head2 Module removals

XXX Remove this section if inapplicable.

The following modules will be removed from the core distribution in a
future release, and will at that time need to be installed from CPAN.
Distributions on CPAN which require these modules will need to list them as
prerequisites.

The core versions of these modules will now issue C<"deprecated">-category
warnings to alert you to this fact.  To silence these deprecation warnings,
install the modules in question from CPAN.

Note that these are (with rare exceptions) fine modules that you are encouraged
to continue to use.  Their disinclusion from core primarily hinges on their
necessity to bootstrapping a fully functional, CPAN-capable Perl installation,
not usually on concerns over their design.

=over

=item XXX

XXX Note that deprecated modules should be listed here even if they are listed
as an updated module in the L</Modules and Pragmata> section.

=back

[ List each other deprecation as a =head2 entry ]

=head1 Performance Enhancements

XXX Changes which enhance performance without changing behaviour go here.
There may well be none in a stable release.

[ List each enhancement as an =item entry ]

=over 4

=item *

Many string concatenation expressions are now considerably faster, due
to the introduction internally of a C<multiconcat> opcode which combines
multiple concatenations, and optionally a C<=> or C<.=>, into a single
action. For example, apart from retrieving C<$s>, C<$a> and C<$b>, this
whole expression is now handled as a single op:

    $s .= "a=$a b=$b\n"

As a special case, if the LHS of an assign is a lexical variable or
C<my $s>, the op itself handles retrieving the lexical variable, which
is faster.

In general, the more the expression includes a mix of constant strings and
variable expressions, the longer the expression, and the more it mixes
together non-utf8 and utf8 strings, the more marked the performance
improvement. For example on a C<x86_64> system, this code has been
benchmarked running four times faster:

    my $s;
    my $a = "ab\x{100}cde";
    my $b = "fghij";
    my $c = "\x{101}klmn";

    for my $i (1..10_000_000) {
        $s = "\x{100}wxyz";
        $s .= "foo=$a bar=$b baz=$c";
    }

In addition, C<sprintf> expressions which have a constant format
containing only C<%s> and C<%%> format elements, and which have a fixed
number of arguments, are now also optimised into a C<multiconcat> op.

=back

=head1 Modules and Pragmata

XXX All changes to installed files in F<cpan/>, F<dist/>, F<ext/> and F<lib/>
go here.  If Module::CoreList is updated, generate an initial draft of the
following sections using F<Porting/corelist-perldelta.pl>.  A paragraph summary
for important changes should then be added by hand.  In an ideal world,
dual-life modules would have a F<Changes> file that could be cribbed.

The list of new and updated modules is modified automatically as part of
preparing a Perl release, so the only reason to manually add entries here is if
you're summarising the important changes in the module update. (Also, if the
manually-added details don't match the automatically-generated ones, the
release manager will have to investigate the situation carefully.)

[ Within each section, list entries as an =item entry ]

=head2 New Modules and Pragmata

=over 4

=item *

XXX

=back

=head2 Updated Modules and Pragmata

=over 4

=item *

L<Carp> has been upgraded from version 1.43 to 1.44.

If a package on the call stack contains a constant named C<ISA>, Carp no
longer throws a "Not a GLOB reference" error.

=back

=head2 Removed Modules and Pragmata

=over 4

=item *

XXX

=back

=head1 Documentation

XXX Changes to files in F<pod/> go here.  Consider grouping entries by
file and be sure to link to the appropriate page, e.g. L<perlfunc>.

=head2 New Documentation

XXX Changes which create B<new> files in F<pod/> go here.

=head3 L<XXX>

XXX Description of the purpose of the new file here

=head2 Changes to Existing Documentation

We have attempted to update the documentation to reflect the changes
listed in this document.  If you find any we have missed, send email
to L<perlbug@perl.org|mailto:perlbug@perl.org>.

XXX Changes which significantly change existing files in F<pod/> go here.
However, any changes to F<pod/perldiag.pod> should go in the L</Diagnostics>
section.

Additionally, the following selected changes have been made:

=head3 L<perldiag/Variable length lookbehind not implemented in regex m/%s/>

This now gives more ideas as to workarounds to the issue that was
introduced in Perl 5.18 (but not documented explicitly in its perldelta)
for the fact that some Unicode C</i> rules cause a few sequences such as

 (?<!st)

to be considered variable length, and hence disallowed.

=over 4

=item *

XXX Description of the change here

=back

=head1 Diagnostics

The following additions or changes have been made to diagnostic output,
including warnings and fatal error messages.  For the complete list of
diagnostic messages, see L<perldiag>.

XXX New or changed warnings emitted by the core's C<C> code go here.  Also
include any changes in L<perldiag> that reconcile it to the C<C> code.

=head2 New Diagnostics

XXX Newly added diagnostic messages go under here, separated into New Errors
and New Warnings

=head3 New Errors

=over 4

=item *

XXX L<message|perldiag/"message">

=back

=head3 New Warnings

=over 4

=item *

XXX L<message|perldiag/"message">

=back

=head2 Changes to Existing Diagnostics

XXX Changes (i.e. rewording) of diagnostic messages go here

=over 4

=item *

The diagnostic C<Initialization of state variables in list context
currently forbidden> has changed to C<Initialization of state variables
in list currently forbidden>, because list-context initialization of
single aggregate state variables is now permitted.

=item *

XXX Describe change here

=back

=head1 Utility Changes

XXX Changes to installed programs such as F<perlbug> and F<xsubpp> go here.
Most of these are built within the directory F<utils>.

[ List utility changes as a =head2 entry for each utility and =item
entries for each change
Use L<XXX> with program names to get proper documentation linking. ]

=head2 L<XXX>

=over 4

=item *

XXX

=back

=head1 Configuration and Compilation

XXX Changes to F<Configure>, F<installperl>, F<installman>, and analogous tools
go here.  Any other changes to the Perl build process should be listed here.
However, any platform-specific changes should be listed in the
L</Platform Support> section, instead.

[ List changes as an =item entry ].

=over 4

=item New probes

=over 2

=item HAS_BUILTIN_ADD_OVERFLOW

=item HAS_BUILTIN_MUL_OVERFLOW

=item HAS_BUILTIN_SUB_OVERFLOW

=item HAS_THREAD_SAFE_NL_LANGINFO_L

=item HAS_LOCALECONV_L

=item HAS_MBRLEN

=item HAS_MBRTOWC

=item HAS_MEMRCHR

=item HAS_NANOSLEEP

=item HAS_STRNLEN

=item HAS_STRTOLD_L

=item I_WCHAR

=back

=back

=head1 Testing

XXX Any significant changes to the testing of a freshly built perl should be
listed here.  Changes which create B<new> files in F<t/> go here as do any
large changes to the testing harness (e.g. when parallel testing was added).
Changes to existing files in F<t/> aren't worth summarizing, although the bugs
that they represent may be covered elsewhere.

XXX If there were no significant test changes, say this:

Tests were added and changed to reflect the other additions and changes
in this release.

XXX If instead there were significant changes, say this:

Tests were added and changed to reflect the other additions and
changes in this release.  Furthermore, these significant changes were
made:

[ List each test improvement as an =item entry ]

=over 4

=item *

XXX

=back

=head1 Platform Support

XXX Any changes to platform support should be listed in the sections below.

[ Within the sections, list each platform as an =item entry with specific
changes as paragraphs below it. ]

=head2 New Platforms

XXX List any platforms that this version of perl compiles on, that previous
versions did not.  These will either be enabled by new files in the F<hints/>
directories, or new subdirectories and F<README> files at the top level of the
source tree.

=over 4

=item XXX-some-platform

XXX

=back

=head2 Discontinued Platforms

XXX List any platforms that this version of perl no longer compiles on.

=over 4

=item XXX-some-platform

XXX

=back

=head2 Platform-Specific Notes

XXX List any changes for specific platforms.  This could include configuration
and compilation changes or changes in portability/compatibility.  However,
changes within modules for platforms should generally be listed in the
L</Modules and Pragmata> section.

=over 4

=item XXX-some-platform

XXX

=back

=head1 Internal Changes

XXX Changes which affect the interface available to C<XS> code go here.  Other
significant internal changes for future core maintainers should be noted as
well.

[ List each change as an =item entry ]

=over 4

=item *

A new optimisation phase has been added to the compiler,
C<optimize_optree()>, which does a top-down scan of a complete optree
just before the peephole optimiser is run. This phase is not currently
hookable.

=item *

An C<OP_MULTICONCAT> op has been added. At C<optimize_optree()> time, a
chain of C<OP_CONCAT> and C<OP_CONST> ops, together optionally with an
C<OP_STRINGIFY> and/or C<OP_SASSIGN>, are combined into a single
C<OP_MULTICONCAT> op. The op is of type C<UNOP_AUX>, and the aux array
contains the argument count, plus a pointer to a constant string and a set
of segment lengths. For example with

    my $x = "foo=$foo, bar=$bar\n";

the constant string would be C<"foo=, bar=\n"> and the segment lengths
would be (4,6,1). If the string contains characters such as C<\x80>, whose
representation changes under utf8, two sets of strings plus lengths are
precomputed and stored.

=back

=head1 Selected Bug Fixes

XXX Important bug fixes in the core language are summarized here.  Bug fixes in
files in F<ext/> and F<lib/> are best summarized in L</Modules and Pragmata>.

[ List each fix as an =item entry ]

=over 4

=item *

C<stat()>, C<lstat()>, and file test operators now fail if given a
filename containing a nul character, in the same way that C<open()>
already fails.

=item *

The in-place reverse optimisation now correctly strengthens weak
references using the L<C<sv_rvunweaken()>|perlapi/sv_rvunweaken>
API function.

=item *

Fixed a read before buffer when parsing a range starting with C<\N{}>
at the beginning of the character set for the transliteration
operator.  [perl #132245]

=item *

Fixed a leaked SV when parsing an empty C<\N{}> at compile-time.
[perl #132245]

=back

=head1 Known Problems

XXX Descriptions of platform agnostic bugs we know we can't fix go here.  Any
tests that had to be C<TODO>ed for the release would be noted here.  Unfixed
platform specific bugs also go here.

[ List each fix as an =item entry ]

=over 4

=item *

XXX

=back

=head1 Errata From Previous Releases

=over 4

=item *

XXX Add anything here that we forgot to add, or were mistaken about, in
the perldelta of a previous release.

=back

=head1 Obituary

XXX If any significant core contributor has died, we've added a short obituary
here.

=head1 Acknowledgements

XXX Generate this with:

  perl Porting/acknowledgements.pl v5.27.5..HEAD

=head1 Reporting Bugs

If you find what you think is a bug, you might check the perl bug database
at L<https://rt.perl.org/> .  There may also be information at
L<http://www.perl.org/> , the Perl Home Page.

If you believe you have an unreported bug, please run the L<perlbug> program
included with your release.  Be sure to trim your bug down to a tiny but
sufficient test case.  Your bug report, along with the output of C<perl -V>,
will be sent off to perlbug@perl.org to be analysed by the Perl porting team.

If the bug you are reporting has security implications which make it
inappropriate to send to a publicly archived mailing list, then see
L<perlsec/SECURITY VULNERABILITY CONTACT INFORMATION>
for details of how to report the issue.

=head1 Give Thanks

If you wish to thank the Perl 5 Porters for the work we had done in Perl 5,
you can do so by running the C<perlthanks> program:

    perlthanks

This will send an email to the Perl 5 Porters list with your show of thanks.

=head1 SEE ALSO

The F<Changes> file for an explanation of how to view exhaustive details on
what changed.

The F<INSTALL> file for how to build Perl.

The F<README> file for general stuff.

The F<Artistic> and F<Copying> files for copyright information.

=cut