Add suggestion for PatternSynonyms parse error (fixes #12429)

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3261

compiler/parser/Lexer.x

@@ -2394,8 +2394,12 @@ srcParseErr options buf len
               $$ ppWhen (token == "=")
                         (text "Perhaps you need a 'let' in a 'do' block?"
                          $$ text "e.g. 'let x = 5' instead of 'x = 5'")
+              $$ ppWhen (not ps_enabled && pattern == "pattern") -- #12429
+                        (text "Perhaps you intended to use PatternSynonyms")
   where token = lexemeToString (offsetBytes (-len) buf) len
+        pattern = lexemeToString (offsetBytes (-len - 8) buf) 7
         th_enabled = extopt LangExt.TemplateHaskell options
+        ps_enabled = extopt LangExt.PatternSynonyms options
 
 -- Report a parse failure, giving the span of the previous token as
 -- the location of the error.  This is the entry point for errors

testsuite/tests/parser/should_fail/T12429.hs

+module X where
+  import Data.Text (pattern Y)
+  x = 3

testsuite/tests/parser/should_fail/T12429.stderr

+
+testsuite/tests/parser/should_fail/T12429.hs:2:29: error:
+    parse error on input ‘Y’
+    Perhaps you intended to use PatternSynonyms
+  |
+2 |   import Data.Text (pattern Y)
+  |                             ^

testsuite/tests/parser/should_fail/all.T

@@ -94,3 +94,4 @@ test('T10196Fail2', normal, compile_fail, [''])
 test('T10498a', normal, compile_fail, [''])
 test('T10498b', normal, compile_fail, [''])
 test('T12051', normal, compile_fail, [''])
+test('T12429', normal, compile_fail, [''])