1 files changed, 27 insertions, 10 deletions
diff --git a/compiler/parser.nim b/compiler/parser.nim
index 9f4897665..2c514ee74 100644
--- a/compiler/parser.nim
+++ b/compiler/parser.nim
@@ -16,20 +16,37 @@
 
 
 # In fact the grammar is generated from this file:
-when isMainModule:
+when isMainModule or defined(nimTestGrammar):
   # Leave a note in grammar.txt that it is generated:
   #| # This file is generated by compiler/parser.nim.
-  import pegs
+  import std/pegs
   when defined(nimPreviewSlimSystem):
     import std/syncio
-  var outp = open("doc/grammar.txt", fmWrite)
-  for line in lines("compiler/parser.nim"):
-    if line =~ peg" \s* '#| ' {.*}":
-      outp.write matches[0], "\L"
-  outp.close
-
-  import ".." / tools / grammar_nanny
-  checkGrammarFile()
+
+  proc writeGrammarFile(x: string) =
+    var outp = open(x, fmWrite)
+    for line in lines("compiler/parser.nim"):
+      if line =~ peg" \s* '#| ' {.*}":
+        outp.write matches[0], "\L"
+    outp.close
+
+  when defined(nimTestGrammar):
+    import std/os
+    from ../testament/lib/stdtest/specialpaths import buildDir
+    const newGrammarText = buildDir / "grammar.txt"
+
+    if not dirExists(buildDir):
+      createDir(buildDir)
+
+    writeGrammarFile(newGrammarText)
+
+    proc checkSameGrammar*() =
+      doAssert sameFileContent(newGrammarText, "doc/grammar.txt"),
+              "execute 'nim r compiler.nim' to keep grammar.txt up-to-date"
+  else:
+    writeGrammarFile("doc/grammar.txt")
+    import ".." / tools / grammar_nanny
+    checkGrammarFile()
 
 import
   llstream, lexer, idents, strutils, ast, msgs, options, lineinfos,