Introduce {.noRewrite.} expr pragma disabling TR

Term rewriting macros/templates are currently greedy and they will rewrite as
long as there is a match. So there was no way to ensure some rewrite happens
only once, eg. when rewriting term to same term plus extra content.

With new macro we can actually prevent further rewriting on marked expr or
stmts, eg. with given example echo(...) will be rewritten just once:

  template pwnEcho{echo(x)}(x: expr) =
    {.noRewrite.}: echo("pwned!")

  echo "ab"

compiler/ast.nim

@@ -423,6 +423,7 @@ type
                 # but unfortunately it has measurable impact for compilation
                 # efficiency
     nfTransf,   # node has been transformed
+    nfNoRewrite # node should not be transformed anymore
     nfSem       # node has been checked for semantics
     nfLL        # node has gone through lambda lifting
     nfDotField  # the call can use a dot operator

compiler/patterns.nim

@@ -130,7 +130,9 @@ proc matchNested(c: PPatternContext, p, n: PNode, rpn: bool): bool =
 
 proc matches(c: PPatternContext, p, n: PNode): bool =
   # hidden conversions (?)
-  if isPatternParam(c, p):
+  if nfNoRewrite in n.flags:
+    result = false
+  elif isPatternParam(c, p):
     result = bindOrCheck(c, p.sym, n)
   elif n.kind == nkSym and p.kind == nkIdent:
     result = p.ident.id == n.sym.name.id

compiler/pragmas.nim

@@ -37,7 +37,7 @@ const
     wImportc, wExportc, wNodecl, wMagic, wDeprecated, wBorrow, wExtern,
     wImportCpp, wImportObjC, wError, wDiscardable, wGensym, wInject, wRaises,
     wTags, wLocks, wGcSafe}
-  exprPragmas* = {wLine, wLocks}
+  exprPragmas* = {wLine, wLocks, wNoRewrite}
   stmtPragmas* = {wChecks, wObjChecks, wFieldChecks, wRangechecks,
     wBoundchecks, wOverflowchecks, wNilchecks, wAssertions, wWarnings, wHints,
     wLinedir, wStacktrace, wLinetrace, wOptimization, wHint, wWarning, wError,
@@ -859,6 +859,8 @@ proc singlePragma(c: PContext, sym: PSym, n: PNode, i: int,
             c.module.flags.incl sfExperimental
           else:
             localError(it.info, "'experimental' pragma only valid as toplevel statement")
+        of wNoRewrite:
+          noVal(it)
         else: invalidPragma(it)
       else: invalidPragma(it)
   else: processNote(c, it)

compiler/semstmts.nim

@@ -1268,6 +1268,8 @@ proc semPragmaBlock(c: PContext, n: PNode): PNode =
     of wLocks:
       result = n
       result.typ = n.sons[1].typ
+    of wNoRewrite:
+      incl(result.flags, nfNoRewrite)
     else: discard
 
 proc semStaticStmt(c: PContext, n: PNode): PNode =

compiler/wordrecg.nim

@@ -55,7 +55,7 @@ type
     wFloatchecks, wNanChecks, wInfChecks,
     wAssertions, wPatterns, wWarnings,
     wHints, wOptimization, wRaises, wWrites, wReads, wSize, wEffects, wTags,
-    wDeadCodeElim, wSafecode, wNoForward,
+    wDeadCodeElim, wSafecode, wNoForward, wNoRewrite,
     wPragma,
     wCompileTime, wNoInit,
     wPassc, wPassl, wBorrow, wDiscardable,
@@ -139,7 +139,7 @@ const
 
     "assertions", "patterns", "warnings", "hints",
     "optimization", "raises", "writes", "reads", "size", "effects", "tags",
-    "deadcodeelim", "safecode", "noforward",
+    "deadcodeelim", "safecode", "noforward", "norewrite",
     "pragma",
     "compiletime", "noinit",
     "passc", "passl", "borrow", "discardable", "fieldchecks",