Performance: make readDollarExpr fail early if no $

ShellCheck/Parser.hs

@@ -1328,7 +1328,12 @@ prop_readDollarExpression1 = isOk readDollarExpression "$(((1) && 3))"
 prop_readDollarExpression2 = isWarning readDollarExpression "$(((1)) && 3)"
 prop_readDollarExpression3 = isWarning readDollarExpression "$((\"$@\" &); foo;)"
 readDollarExpression :: Monad m => SCParser m Token
-readDollarExpression = arithmetic <|> readDollarExpansion <|> readDollarBracket <|> readDollarBraceCommandExpansion <|> readDollarBraced <|> readDollarVariable
+readDollarExpression = do
+    -- The grammar should have been designed along the lines of readDollarExpr = char '$' >> stuff, but
+    -- instead, each subunit parses its own $. This results in ~7 1-3 char lookaheads instead of one 1-char.
+    -- Instead of optimizing the grammar, here's a green cut that decreases shellcheck runtime by 10%:
+    lookAhead $ char '$'
+    arithmetic <|> readDollarExpansion <|> readDollarBracket <|> readDollarBraceCommandExpansion <|> readDollarBraced <|> readDollarVariable
   where
     arithmetic = readAmbiguous "$((" readDollarArithmetic readDollarExpansion (\pos ->
         parseNoteAt pos WarningC 1102 "Shells disambiguate $(( differently or not at all. If the first $( should start command substitution, add a space after it.")