Emit experimental::builtin warnings from the builtin function callche…

…cker

builtin.c

@@ -50,6 +50,10 @@ static OP *ck_builtin_const(pTHX_ OP *entersubop, GV *namegv, SV *ckobj)
 {
     const struct BuiltinFuncDescriptor *builtin = NUM2PTR(const struct BuiltinFuncDescriptor *, SvUV(ckobj));
 
+    Perl_ck_warner_d(aTHX_
+            packWARN(WARN_EXPERIMENTAL__BUILTIN),
+            "Built-in function '%s' is experimental", builtin->name);
+
     SV *prototype = newSVpvs("");
     SAVEFREESV(prototype);
 
@@ -145,6 +149,10 @@ static OP *ck_builtin_func1(pTHX_ OP *entersubop, GV *namegv, SV *ckobj)
 {
     const struct BuiltinFuncDescriptor *builtin = NUM2PTR(const struct BuiltinFuncDescriptor *, SvUV(ckobj));
 
+    Perl_ck_warner_d(aTHX_
+            packWARN(WARN_EXPERIMENTAL__BUILTIN),
+            "Built-in function '%s' is experimental", builtin->name);
+
     SV *prototype = newSVpvs("$");
     SAVEFREESV(prototype);
 

pod/perldiag.pod

@@ -638,6 +638,13 @@ the warning gets raised.
 iterate over %ENV, it encountered a logical name or symbol definition
 which was too long, so it was truncated to the string shown.
 
+=item Built-in function '%s' is experimental
+
+(S experimental::builtin) A call is being made to a function in the
+C<builtin::> namespace, which is currently experimental. The existence
+or nature of the function may be subject to change in a future version
+of Perl.
+
 =item Callback called exit
 
 (F) A subroutine invoked from an external package via call_sv()