[Parse] Emit error on prefix operator containing /

When forward slash regex is enabled, start emitting
an error on prefix operators containing the
`/` character.

Author: hamishknight

include/swift/AST/DiagnosticsParse.def

@@ -94,6 +94,9 @@ ERROR(forbidden_extended_escaping_string,none,
 ERROR(regex_literal_parsing_error,none,
       "%0", (StringRef))
 
+ERROR(prefix_slash_not_allowed,none,
+      "prefix operator may not contain '/'", ())
+
 //------------------------------------------------------------------------------
 // MARK: Lexer diagnostics
 //------------------------------------------------------------------------------

lib/Parse/ParseDecl.cpp

@@ -8528,6 +8528,13 @@ Parser::parseDeclOperator(ParseDeclOptions Flags, DeclAttributes &Attributes) {
                                    Tok.getRawText().front() == '!'))
       diagnose(Tok, diag::postfix_operator_name_cannot_start_with_unwrap);
 
+  // Prefix operators may not contain the `/` character when `/.../` regex
+  // literals are enabled.
+  if (Context.LangOpts.EnableBareSlashRegexLiterals) {
+    if (Attributes.hasAttribute<PrefixAttr>() && Tok.getText().contains("/"))
+      diagnose(Tok, diag::prefix_slash_not_allowed);
+  }
+
   // A common error is to try to define an operator with something in the
   // unicode plane considered to be an operator, or to try to define an
   // operator like "not".  Analyze and diagnose this specifically.

test/StringProcessing/Parse/forward-slash-regex.swift

@@ -0,0 +1,5 @@
+// RUN: %target-typecheck-verify-swift -enable-bare-slash-regex
+
+prefix operator /  // expected-error {{prefix operator may not contain '/'}}
+prefix operator ^/ // expected-error {{prefix operator may not contain '/'}}
+prefix operator /^/ // expected-error {{prefix operator may not contain '/'}}