debugger source lines: use PVIV instead of PVMG for source lines

These were saved as PVMG but as bulk88 suggested in
#23171 (comment)
we only need PVIV, since the source lines don't need magic,
aren't blessed and store an integer, not an NV.

So create them as PVIV instead.

If it turns out we do need PVMG instead for some future use, simply
remove the test added here, it's simply to confirm we don't need
PVMG here.

Author: tonycoz

pp_ctl.c

@@ -3735,7 +3735,7 @@ S_save_lines(pTHX_ AV *array, SV *sv)
 
     while (s && s < send) {
         const char *t;
-        SV * const tmpstr = newSV_type(SVt_PVMG);
+        SV * const tmpstr = newSV_type(SVt_PVIV);
 
         t = (const char *)memchr(s, '\n', send - s);
         if (t)

t/op/debug.t

@@ -61,4 +61,64 @@ EXPECT
                   );
 }
 
+SKIP:
+{
+    # Historically lines were stored as PVMG, but we don't need
+    # magic on these lines.
+    #
+    # This checks that none of these lines get upgraded, ie. that
+    # we don't need them to be PVMG
+    #
+    # If this test fails perhaps we do need to make them PVMG
+    # and toke.c:S_update_debugger_info and pp_ctl.c:S_save_lines
+    # can be switched back to using SVt_PVMG and this test
+    # removed.
+    #
+    # See https://github.com/Perl/perl5/pull/23171#issuecomment-2780007725
+    skip_if_miniperl("need B");
+    local $ENV{PERL5DB} = 'sub DB::DB {}';
+    fresh_perl_is(<<'CODE', <<'EXPECT',
+use B;
+
+sub _do_eval {
+  eval $_[0] or die $!;
+}
+
+_do_eval(<<'EVAL');
+
+sub some_code {
+   print "Hello";
+}
+
+1;
+EVAL
+
+# check if any lines have been upgraded from PVIV
+my @files = grep /^_</, keys %::;
+for my $f (@files) {
+    my $lineno = 0;
+
+    for my $l (@{$f}) {
+        if ($l) {
+            my $b = B::svref_2object(\$l);
+            if (ref $b ne "B::PVIV") {
+               print "Not PVIV for $f:$lineno: $l\n";
+               last
+            }
+        }
+        ++$lineno;
+    }
+}
+print "Done\n";
+CODE
+Done
+EXPECT
+                  {
+                      switches => [ '-d' ],
+                      stderr => 1,
+                  },
+                  "saved lines are all PVIV"
+                  );
+}
+
 done_testing();

toke.c

@@ -2012,10 +2012,11 @@ S_update_debugger_info(pTHX_ SV *orig_sv, const char *const buf, STRLEN len)
     AV *av = CopFILEAVx(PL_curcop);
     if (av) {
         SV * sv;
-        if (PL_parser->preambling == NOLINE) sv = newSV_type(SVt_PVMG);
+        if (PL_parser->preambling == NOLINE)
+            sv = newSV_type(SVt_PVIV);
         else {
             sv = *av_fetch(av, 0, 1);
-            SvUPGRADE(sv, SVt_PVMG);
+            SvUPGRADE(sv, SVt_PVIV);
         }
         if (!SvPOK(sv)) SvPVCLEAR(sv);
         if (orig_sv)