make substr-rw work like substr, but make substr 2.5x as slow

gistfile1.txt

diff --git a/src/core/Str.pm b/src/core/Str.pm
index 0cb832a..57bf2cf 100644
--- a/src/core/Str.pm
+++ b/src/core/Str.pm
@@ -1556,64 +1556,79 @@ sub chrs(*@c) returns Str:D {
     @c.map({.chr}).join;
 }
 
-sub substr(Str() \what, \from, $chars?) {
-    my str $str  = nqp::unbox_s(what);
-    my int $max  = nqp::chars($str);
-    my int $from = nqp::unbox_i(
-      nqp::istype(from, Callable) ?? (from)(nqp::p6box_i($max)) !! from.Int
+sub SUBSTR-SANITY(Str \what, \start, \chars, \from, \length) {
+    my str $str = nqp::unbox_s(what);
+    my int $max = nqp::chars($str);
+    from = nqp::unbox_i(
+      nqp::istype(start, Callable) ?? (start)(nqp::p6box_i($max)) !! start.Int
     );
 
-    if $from < 0 {
-        if nqp::istype(from, Callable) || -$from > $max {
+    if from < 0 {
+        if nqp::istype(from, Callable) || -from > $max {
             X::OutOfRange.new(
-              :what<Start argument to substr>,:got($from),:range("0..$max"),
+              :what<Start argument to substr>,:got(from),:range("0..$max"),
             ).fail;
         }
         else {
             X::OutOfRange.new(
               :what<Start argument to substr>,:got(from),:range<0..Inf>,
-              :comment("use *$from if you want to index relative to the end")
+              :comment("use *{from} if you want to index relative to the end")
             ).fail;
         }
     }
-    elsif $from > $max {
+    elsif from > $max {
         X::OutOfRange.new(
-          :what<Start of substr>,:got($from),:range("0..$max"),
+          :what<Start of substr>,:got(from),:range("0..$max"),
         ).fail;
     }
 
-    my int $length = nqp::unbox_i(
-      $chars.defined
-        ?? $chars === Inf
-          ?? $max - $from
-          !! nqp::istype($chars,Callable)
-            ?? $chars($max - $from)
-            !! (nqp::istype($chars,Int) ?? $chars !! $chars.Int)
-        !! $max - $from
+    length = nqp::unbox_i(
+      chars.defined
+        ?? chars === Inf
+          ?? $max - from
+          !! nqp::istype(chars,Callable)
+            ?? (chars)($max - from)
+            !! (nqp::istype(chars,Int) ?? chars !! chars.Int)
+        !! $max - from
     );
     X::OutOfRange.new(
-      :what<Length argument to substr>,:got($chars),:range<0..Inf>,
-      :comment("use *$length if you want to index relative to the end")
-    ).fail if $length < 0;
+      :what<Length argument to substr>,:got(chars),:range<0..Inf>,
+      :comment("use *{length} if you want to index relative to the end")
+    ).fail if length < 0;
+    1;
+}
 
-    nqp::p6box_s(nqp::substr($str, $from, $length));
+sub substr(Str() \what, \from, $chars?) {
+
+    # should really be int, but \ then doesn't work for rw access
+    my $r := SUBSTR-SANITY( what, from, $chars, my Int $from, my Int $length);
+    return $r.defined
+      ?? nqp::p6box_s(nqp::substr(
+           nqp::unbox_s(what),nqp::unbox_i($from),nqp::unbox_i($length)
+         ))
+      !! $r;
 }
 
-sub substr-rw(Str() $s is rw, $from, $length?) {
-    my str $str   = nqp::unbox_s($s);
-    my int $chars = nqp::unbox_i(
-      nqp::defined($length) ?? $length !! $from - nqp::chars($str)
-    );
-    my str $substr = nqp::substr($str,$from,$chars);
+sub substr-rw(Str() \what, \from, $chars?) {
+
+    # should really be int, but \ then doesn't work for rw access
+    my $r := SUBSTR-SANITY( what, from, $chars, my Int $from, my Int $length);
+    return $r unless $r.defined;
+
     Proxy.new(
-        FETCH   => sub ($) { nqp::p6box_s($substr) },
-        STORE   => sub ($, $new) {
-            $s = nqp::p6box_s(
-              nqp::substr($str,0,$from)
-              ~ nqp::unbox_s($new)
-              ~ nqp::substr($str,$from + $chars)
-            );
-        }
+      FETCH => sub ($) {
+          nqp::p6box_s(nqp::substr(
+            nqp::unbox_s(what), nqp::unbox_i($from), nqp::unbox_i($length)
+          ));
+      },
+      STORE => sub ($, $new) {
+          my $str = nqp::unbox_s(what);
+          what = nqp::p6box_s(
+            nqp::substr($str,0,$from)
+            ~ nqp::unbox_s($new)
+            ~ nqp::substr($str,$from + $length)
+          );
+      },
     );
 }