Quote do now works with result in block (nim-lang#7343)

* Fix result not being able to use in quote do

This fixes the annoying issue of not be able to use result inside a
quote do block. It works by a simple trick. The quote do mechanic is
based on dynamically creating a template and immediately calling it with
the arguments found within the quote do block. Since this is called in
the scope of the macro the result variable is shadowed. This trick works
by changing all occurences of result (which shouldn't cause any issues
as result isn't used for anything else for the same reason) to another
name and then passing in an IdentNode with result as a named parameter
with that name.

Note that currently this just replaces it with a fixed named variable
"res" which should be changed to a non-colliding, dynamically created
name.

* Fix hard coded parameter "res" to anonymous symbol

This fixes the hard coded parameter "res" to be an anonymous symbol
instead so it won't collide with other parts of the argument list.

* Add test case for result in quote do block

A simple test case based on GitHub issue nim-lang#7323 on how you can't put
result in a quote do block. This test verifies that it actually works
correctly now.

* Add test for explicit capturing of result

* Rebased against devel

compiler/semexprs.nim

@@ -1812,6 +1812,7 @@ proc processQuotations(c: PContext; n: var PNode, op: string,
     ids.add n
     return
 
+
   if n.kind == nkPrefix:
     checkSonsLen(n, 2, c.config)
     if n[0].kind == nkIdent:
@@ -1822,6 +1823,9 @@ proc processQuotations(c: PContext; n: var PNode, op: string,
         n.sons[0] = newIdentNode(getIdent(c.cache, examinedOp.substr(op.len)), n.info)
   elif n.kind == nkAccQuoted and op == "``":
     returnQuote n[0]
+  elif n.kind == nkIdent:
+    if n.ident.s == "result":
+      n = ids[0]
 
   for i in 0 ..< n.safeLen:
     processQuotations(c, n.sons[i], op, quotes, ids)
@@ -1833,15 +1837,18 @@ proc semQuoteAst(c: PContext, n: PNode): PNode =
   var
     quotedBlock = n[^1]
     op = if n.len == 3: expectString(c, n[1]) else: "``"
-    quotes = newSeq[PNode](1)
+    quotes = newSeq[PNode](2)
       # the quotes will be added to a nkCall statement
-      # leave some room for the callee symbol
-    ids = newSeq[PNode]()
+      # leave some room for the callee symbol and the result symbol
+    ids = newSeq[PNode](1)
       # this will store the generated param names
+      # leave some room for the result symbol
 
   if quotedBlock.kind != nkStmtList:
     localError(c.config, n.info, errXExpected, "block")
 
+  # This adds a default first field to pass the result symbol
+  ids[0] = newAnonSym(c, skParam, n.info).newSymNode
   processQuotations(c, quotedBlock, op, quotes, ids)
 
   var dummyTemplate = newProcNode(
@@ -1860,6 +1867,8 @@ proc semQuoteAst(c: PContext, n: PNode): PNode =
 
   var tmpl = semTemplateDef(c, dummyTemplate)
   quotes[0] = tmpl[namePos]
+  # This adds a call to newIdentNode("result") as the first argument to the template call
+  quotes[1] = newNode(nkCall, n.info, @[newIdentNode(getIdent(c.cache, "newIdentNode"), n.info), newStrNode(nkStrLit, "result")])
   result = newNode(nkCall, n.info, @[
      createMagic(c.graph, "getAst", mExpandToAst).newSymNode,
     newNode(nkCall, n.info, quotes)])

tests/macros/tquotedo.nim

@@ -0,0 +1,17 @@
+import macros
+
+macro mac(): untyped =
+  quote do:
+    proc test(): int =
+      (proc(): int = result = 123)()
+
+mac()
+echo test()
+
+macro foobar(arg: untyped): untyped =
+  result = arg
+  result.add quote do:
+    `result`
+
+foobar:
+  echo "Hallo Welt"