Flatten vstrings modified in place

A substitution forces its target to a string upon successful substitu-
tion, even if the substitution did nothing:

$ ./perl -Ilib -le '$a = *f; $a =~ s/f/f/; print ref \$a'
SCALAR

Notice that $a is no longer a glob after s///.

But vstrings are different:

$ ./perl -Ilib -le '$a = v102; $a =~ s/f/f/; print ref \$a'
VSTRING

I fixed this in 5.16 (1e6bda93) for those cases where the vstring ends
up with a value that doesn’t correspond to the actual string:

$ ./perl -Ilib -le '$a = v102; $a =~ s/f/o/; print ref \$a'
SCALAR

It works through vstring set-magic, that does the check and removes
the magic if it doesn’t match.

I did it that way because I couldn’t think of any other way to fix
bug #29070, and I didn’t realise at the time that I hadn’t fixed
all the bugs.

By making SvTHINKFIRST true on a vstring, we force it through
sv_force_normal before any in-place string operations.  We can also
make sv_force_normal handle vstrings as well.  This fixes all the lin-
gering-vstring-magic bugs in just two lines, making the vstring set-
magic (which is also slow) redundant.  It also allows the special case
in sv_setsv_flags to be removed.

Or at least that was what I had hoped.

It turns out that pp_subst, twists and turns in tortuous ways, and
needs special treatment for things like this.

And do_trans functions wasn’t checking SvTHINKFIRST when arguably it
should have.

I tweaked sv_2pv{utf8,byte} to avoid copying magic variables that do
not need copying.

1 files changed, 1 insertions, 2 deletions

diff --git a/regen/mg_vtable.pl b/regen/mg_vtable.pl
index f96c7a0fa5..5fcdc4c78f 100644
--- a/regen/mg_vtable.pl
+++ b/regen/mg_vtable.pl
@@ -84,7 +84,7 @@ my %mg =
 		    unknown_to_sv_magic => 1 },
      vec => { char => 'v', vtable => 'vec', value_magic => 1,
 	      desc => 'vec() lvalue' },
-     vstring => { char => 'V', value_magic => 1, vtable => 'vstring',
+     vstring => { char => 'V', value_magic => 1,
 		  desc => 'SV was vstring literal' },
      utf8 => { char => 'w', vtable => 'utf8', value_magic => 1,
 	       desc => 'Cached UTF-8 information' },
@@ -142,7 +142,6 @@ my %sig =
 		    cond => '#ifdef USE_LOCALE_COLLATE'},
      'hintselem' => {set => 'sethint', clear => 'clearhint'},
      'hints' => {clear => 'clearhints'},
-     'vstring' => {set => 'setvstring'},
      'checkcall' => {copy => 'copycallchecker'},
 );