&CORE::foo() for nullary functions

This commit makes nullary subs in the CORE package callable with
ampersand syntax and through references--except for wantarray, which
is more complicated and will have its own commit.

It does this by creating an op tree like this:

$ ./perl -Ilib -MO=Concise,CORE::times -e 'BEGIN{\&CORE::times}'
CORE::times:
3  <1> leavesub[1 ref] K/REFC,1 ->(end)
-     <@> lineseq K ->3
1        <$> coreargs(IV 310) v ->2
2        <0> tms ->3
-e syntax OK

The coreargs op checks to make sure there are no arguments, for now.
The 310 is the op number for times (OP_TMS).

There is no nextstate op, because we want to inherit hints from
the caller.

The __FILE__, __LINE__ and __PACKAGE__ directives are implemented
like this:

$ ./perl -Ilib -MO=Concise,CORE::__FILE__ -e 'BEGIN{\&CORE::__FILE__}'
CORE::__FILE__:
7  <1> leavesub[1 ref] K/REFC,1 ->(end)
-     <@> lineseq K ->7
1        <$> coreargs(PV "__FILE__") v ->2
6        <2> lslice K/2 ->7
-           <1> ex-list lK ->4
2              <0> pushmark s ->3
3              <$> const(IV 1) s ->4
-           <1> ex-list lK ->6
4              <0> pushmark s ->5
5              <0> caller[t1] l ->6
-e syntax OK

The lslice op and its children are equivalent to (caller)[1].

1 files changed, 28 insertions, 0 deletions

diff --git a/pp.c b/pp.c
index b1520bad6a..7bf6d6eda2 100644
--- a/pp.c
+++ b/pp.c
@@ -5967,6 +5967,34 @@ PP(pp_boolkeys)
 PP(pp_coreargs)
 {
     dSP;
+    int opnum = SvIOK(cSVOP_sv) ? (int)SvUV(cSVOP_sv) : 0;
+    AV * const at_ = GvAV(PL_defgv);
+    I32 minargs = 0, maxargs = 0, numargs = AvFILLp(at_)+1;
+    I32 oa = opnum ? PL_opargs[opnum] >> OASHIFT : 0;
+    const char *err = NULL;
+
+    /* Count how many args there are. */
+    while (oa) {
+	maxargs++;
+	oa >>= 4;
+    }
+
+    if(numargs < minargs) err = "Not enough";
+    else if(numargs > maxargs) err = "Too many";
+    if (err)
+	/* diag_listed_as: Too many arguments for %s */
+	Perl_croak(aTHX_
+	  "%s arguments for %s", err,
+	   opnum ? OP_DESC(PL_op->op_next) : SvPV_nolen_const(cSVOP_sv)
+	);
+
+    /* Reset the stack pointer.  Without this, we end up returning our own
+       arguments in list context, in addition to the values we are supposed
+       to return.  nextstate usually does this on sub entry, but we need
+       to run the next op with the caller’s hints, so we cannot have a
+       nextstate. */
+    SP = PL_stack_base + cxstack[cxstack_ix].blk_oldsp;
+
     RETURN;
 }