implement the smartmatch feature and test it

interestingly the TOKEN(0) line here isn't exercised by the test
suite, I haven't tracked down what it would take to exercise it.

The deprecation messages will be removed in an upcoming commit

MANIFEST

@@ -6131,6 +6131,7 @@ t/lib/feature/multidimensional		Tests for enabling/disabling $foo{$x, $y} => $fo
 t/lib/feature/nonesuch			Tests for enabling/disabling nonexistent feature
 t/lib/feature/removed			Tests for enabling/disabling removed feature
 t/lib/feature/say			Tests for enabling/disabling say feature
+t/lib/feature/smartmatch		Tests for enabling/disabling smartmatch feature
 t/lib/feature/switch			Tests for enabling/disabling switch feature
 t/lib/h2ph.h				Test header file for h2ph
 t/lib/h2ph.pht				Generated output from h2ph.h by h2ph, for comparison

t/lib/feature/smartmatch

@@ -0,0 +1,49 @@
+Test no feature smartmatch
+
+__END__
+# NAME default
+my @x = qw(a b c);
+my $y = "b" ~~ @x;
+EXPECT
+Smartmatch is deprecated at - line 2.
+########
+# NAME explicit disable
+no feature "smartmatch";
+my @x = qw(a b c);
+my $y = "b" ~~ @x;
+EXPECT
+OPTION fatal
+syntax error at - line 3, near ""b" ~"
+Execution of - aborted due to compilation errors.
+########
+# NAME explicit enable
+use feature "smartmatch";
+my @x = qw(a b c);
+my $y = "b" ~~ @x;
+EXPECT
+Smartmatch is deprecated at - line 3.
+########
+# NAME explicit disable then enable
+no feature "smartmatch";
+use feature "smartmatch";
+my @x = qw(a b c);
+my $y = "b" ~~ @x;
+EXPECT
+Smartmatch is deprecated at - line 4.
+########
+# NAME implicit disable
+use v5.41;
+my @x = qw(a b c);
+my $y = "b" ~~ @x;
+EXPECT
+OPTION fatal
+syntax error at - line 3, near ""b" ~"
+Execution of - aborted due to compilation errors.
+########
+# NAME implicit disable then enable
+use v5.41;
+use feature "smartmatch";
+my @x = qw(a b c);
+my $y = "b" ~~ @x;
+EXPECT
+Smartmatch is deprecated at - line 4.

toke.c

@@ -6775,7 +6775,8 @@ static int
 yyl_tilde(pTHX_ char *s)
 {
     bool bof;
-    if (s[1] == '~' && (PL_expect == XOPERATOR || PL_expect == XTERMORDORDOR)) {
+    if (FEATURE_SMARTMATCH_IS_ENABLED &&
+        s[1] == '~' && (PL_expect == XOPERATOR || PL_expect == XTERMORDORDOR)) {
         if (!PL_lex_allbrackets && PL_lex_fakeeof >= LEX_FAKEEOF_COMPARE)
             TOKEN(0);
         s += 2;