deprecate the ':all' feature bundle

As discussed in Perl/PPCs#60, `:all` is very unlikely to be useful.

Since it will enable all features a future Perl might support, it will
also enable some that might be incompatible with the code as written
(see `bitwise`, for example, which changes the meaning of the existing
bitwise operators).

With the addition of "negative" features in v5.32, `:all` became even
less useful, since it would re-enable features deemed undesirable in
modern Perl.

As this point in time, `:all` is effectively a footgun.

If the keyword `all` (from PPC0027) is added as a feature, there's an
extra risk of confusion between `use feature 'all'` and `use feature ':all'`.

This patch makes `:all` warn. We should consider removing that bundle
entirely in the future.

Author: book

lib/feature.pm

@@ -546,7 +546,7 @@ sub longest {
 
 __END__
 package feature;
-our $VERSION = '1.92';
+our $VERSION = '1.93';
 
 FEATURES
 
@@ -1175,6 +1175,8 @@ sub __common {
         my $name = shift;
         if (substr($name, 0, 1) eq ":") {
             my $v = substr($name, 1);
+            carp('Feature bundle ":all" is deprecated')
+              if $v eq 'all';
             if (!exists $feature_bundle{$v}) {
                 $v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/;
                 if (!exists $feature_bundle{$v}) {
@@ -1215,6 +1217,11 @@ sub unknown_feature_bundle {
             $feature, $^V));
 }
 
+sub carp {
+    require Carp;
+    Carp::carp(@_);
+}
+
 sub croak {
     require Carp;
     Carp::croak(@_);

regen/feature.pl

@@ -100,6 +100,7 @@ Assigning non-zero to $[ is no longer possible at - line 5.
 no feature ':all'; # turns array_base (and everything else) off
 $[ = 1;
 EXPECT
+Feature bundle ":all" is deprecated at - line 2.
 Assigning non-zero to $[ is no longer possible at - line 3.
 ########
 # NAME $^H accidentally enabling all features

t/lib/feature/bundle

@@ -442,7 +442,7 @@ like runperl(
 # This used to fail an assertion, but only as a standalone script
 is runperl(switches => ['-lXMfeature=:all'],
            prog     => 'state sub x {}; undef &x; print defined &x',
-           stderr   => 1), "\n", 'undefining state sub';
+           stderr   => 1), "Feature bundle \":all\" is deprecated at -e line 0.\n\n", 'undefining state sub';
 {
   state sub x { is +(caller 0)[3], 'x', 'state sub name in caller' }
   x
@@ -475,7 +475,7 @@ is runperl(switches => ['-lXMfeature=:all'],
                           x()
                         }
                         x()',
-           stderr   => 1), "42\n",
+           stderr   => 1), "Feature bundle \":all\" is deprecated at -e line 0.\n42\n",
   'closure behaviour of state sub in predeclared package sub';
 
 # -------------------- my -------------------- #
@@ -830,7 +830,7 @@ pass "pad taking ownership once more of packagified my-sub";
 # This used to fail an assertion, but only as a standalone script
 is runperl(switches => ['-lXMfeature=:all'],
            prog     => 'my sub x {}; undef &x; print defined &x',
-           stderr   => 1), "\n", 'undefining my sub';
+           stderr   => 1), "Feature bundle \":all\" is deprecated at -e line 0.\n\n", 'undefining my sub';
 {
   my sub x { is +(caller 0)[3], 'x', 'my sub name in caller' }
   x