Catch wrong attribute usage in a regex.

Regexes are methods on Cursor (or some subclass of it), meaning that
an attribute access inside of them is relative to that. Rather than
failing silently or exploding noisily at runtime, catch the problem at
compile time. The typed exception's message explains the issue and
offers a suggestion; wording tweaks welcome.

src/Perl6/Grammar.nqp

@@ -4923,7 +4923,25 @@ grammar Perl6::QGrammar is HLL::Grammar does STD {
     }
 }
 
-grammar Perl6::RegexGrammar is QRegex::P6Regex::Grammar does STD {
+my role CursorPackageNibbler {
+    method nibble-in-cursor($parent) {
+        my $*PACKAGE := $*W.find_symbol(['Cursor']);
+        my %*ATTR_USAGES;
+        my $cur := nqp::findmethod($parent, 'nibbler')(self);
+        for %*ATTR_USAGES {
+            my $name := $_.key;
+            my $node := $_.value[0].node;
+            $node.CURSOR.typed_sorry('X::Attribute::Regex', symbol => $name);
+        }
+        $cur
+    }
+}
+
+grammar Perl6::RegexGrammar is QRegex::P6Regex::Grammar does STD does CursorPackageNibbler {
+    method nibbler() {
+        self.nibble-in-cursor(QRegex::P6Regex::Grammar)
+    }
+
     method throw_unrecognized_metachar ($metachar) {
         self.typed_sorry('X::Syntax::Regex::UnrecognizedMetachar', :$metachar);
     }
@@ -5016,7 +5034,11 @@ grammar Perl6::RegexGrammar is QRegex::P6Regex::Grammar does STD {
     }
 }
 
-grammar Perl6::P5RegexGrammar is QRegex::P5Regex::Grammar does STD {
+grammar Perl6::P5RegexGrammar is QRegex::P5Regex::Grammar does STD does CursorPackageNibbler {
+    method nibbler() {
+        self.nibble-in-cursor(QRegex::P5Regex::Grammar)
+    }
+
     token rxstopper { <stopper> }
 
     token p5metachar:sym<(?{ })> {

src/core/Exception.pm

@@ -583,6 +583,13 @@ my class X::Attribute::Undeclared is X::Undeclared {
     }
 }
 
+my class X::Attribute::Regex is X::Undeclared {
+    method message() {
+        "Attribute $.symbol not available inside of a regex, since regexes are methods on Cursor.\n" ~
+            "Consider storing the attribute in a lexical, and using that in the regex.";
+    }
+}
+
 my class X::Undeclared::Symbols does X::Comp {
     has %.post_types;
     has %.unk_types;

src/core/core_prologue.pm

@@ -5,6 +5,7 @@ my class Pair { ... }
 my class Whatever { ... }
 my class HyperWhatever { ... }
 my class WhateverCode { ... }
+my class Cursor { ... }
 
 # Stub these or we can't use any sigil other than $.
 my role Positional { ... }