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, 0 insertions, 1 deletions

diff --git a/embed.fnc b/embed.fnc
index dd4daefe61..b3f757c1bf 100644
--- a/embed.fnc
+++ b/embed.fnc
@@ -775,7 +775,6 @@ p	|int	|magic_settaint	|NN SV* sv|NN MAGIC* mg
 p	|int	|magic_setuvar	|NN SV* sv|NN MAGIC* mg
 p	|int	|magic_setvec	|NN SV* sv|NN MAGIC* mg
 p	|int	|magic_setutf8	|NN SV* sv|NN MAGIC* mg
-p	|int	|magic_setvstring|NN SV* sv|NN MAGIC* mg
 p	|int	|magic_set_all_env|NN SV* sv|NN MAGIC* mg
 p	|U32	|magic_sizepack	|NN SV* sv|NN MAGIC* mg
 p	|int	|magic_wipepack	|NN SV* sv|NN MAGIC* mg