templateutils: remove cpp workaround and enable lent only in newer Nim

Author: nitely

stew/lentutils.nim

@@ -0,0 +1,23 @@
+# stew
+# Copyright 2026 Status Research & Development GmbH
+# Licensed under either of
+#
+#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+#  * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+#
+# at your option. This file may not be copied, modified, or distributed except according to those terms.
+
+## Enable `lent` usage in newer Nim versions where it's known to work well.
+
+# Based on https://github.com/arnetheduck/nim-results/blob/df8113dda4c2d74d460a8fa98252b0b771bf1f27/results.nim#L380
+const useLent* =
+  (NimMajor, NimMinor, NimPatch) >= (2, 2, 0) or
+  (defined(gcRefc) and ((NimMajor, NimMinor, NimPatch) >= (2, 0, 8)))
+
+when useLent:
+  template maybeLent*(T: untyped): untyped =
+    ## Generate `lent T` in newer Nim versions, generate `T` otherwise.
+    lent T
+else:
+  template maybeLent*(T: untyped): untyped =
+    T

stew/templateutils.nim

@@ -1,19 +1,19 @@
-type CppVar[T] = distinct ptr T
+# stew
+# Copyright 2026 Status Research & Development GmbH
+# Licensed under either of
+#
+#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+#  * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+#
+# at your option. This file may not be copied, modified, or distributed except according to those terms.
 
-iterator evalTemplateParamOnceImpl[T](x: T): lent T =
-  yield x
+import ./lentutils
 
-when defined(cpp):
-  # TODO `nim cpp` miscompiles iterators returning `var`,
-  #      so we need to emulate them in terms of pointers:
-  iterator evalTemplateParamOnceImpl[T](x: var T): CppVar[T] =
-    yield CppVar[T](addr(x))
+iterator evalTemplateParamOnceImpl[T](x: T): maybeLent T =
+  yield x
 
-  template stripCppVar[T](p: CppVar[T]): var T =
-    ((ptr T)(p))[]
-else:
-  iterator evalTemplateParamOnceImpl[T](x: var T): var T =
-    yield x
+iterator evalTemplateParamOnceImpl[T](x: var T): var T =
+  yield x
 
 template evalTemplateParamOnce*(templateParam, newName, blk: untyped) =
   ## This can be used in templates to avoid the problem of multiple
@@ -43,11 +43,5 @@ template evalTemplateParamOnce*(templateParam, newName, blk: untyped) =
   ##  Both limitations will be lifted in a future implementation based on
   ##  view types.
   block:
-    for paramAddr in evalTemplateParamOnceImpl(templateParam):
-      template newName: auto =
-        when paramAddr is CppVar:
-          stripCppVar(paramAddr)
-        else:
-          paramAddr
-
+    for newName in evalTemplateParamOnceImpl(templateParam):
       blk

tests/all_tests.nim

@@ -27,6 +27,7 @@ import
   test_interval_set,
   test_io2,
   test_keyed_queue,
+  test_lentutils,
   test_macros,
   test_objects,
   test_ptrops,

tests/test_lentutils.nim

@@ -0,0 +1,40 @@
+# stew
+# Copyright 2026 Status Research & Development GmbH
+# Licensed under either of
+#
+#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+#  * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+#
+# at your option. This file may not be copied, modified, or distributed except according to those terms.
+
+{.used.}
+
+import
+  unittest2,
+  ../stew/lentutils
+
+iterator lenter[T](x: T): maybeLent T =
+  yield x
+
+suite "Lent utils":
+  test "iterator yields a lent type if lent is enabled":
+    let s = @[1, 2, 3]
+    for x in lenter(s):
+      when useLent:
+        check unsafeAddr(s) == unsafeAddr(x)
+      else:
+        check unsafeAddr(s) != unsafeAddr(x)
+
+  # sanity check
+  when (NimMajor, NimMinor, NimPatch) >= (2, 2, 4):
+    test "iterator always yields a lent type in newer Nim":
+      let s = @[1, 2, 3]
+      for x in lenter(s):
+        check unsafeAddr(s) == unsafeAddr(x)
+
+  # sanity check
+  when (NimMajor, NimMinor, NimPatch) < (2, 0, 8):
+    test "iterator never yields a lent type in older Nim":
+      let s = @[1, 2, 3]
+      for x in lenter(s):
+        check unsafeAddr(s) != unsafeAddr(x)

tests/test_templateutils.nim

@@ -10,6 +10,8 @@ import
   unittest2,
   ../stew/templateutils
 
+from ../stew/lentutils import useLent
+
 var computations = newSeq[string]()
 var templateParamAddresses = newSeq[pointer]()
 
@@ -66,7 +68,10 @@ test "Template utils":
 
   check computations == ["call", "var", "let", "accessor"]
 
-  check:
-    templateParamAddresses[1] == addr s1
-    templateParamAddresses[2] == unsafeAddr s2
-    templateParamAddresses[3] == addr o.accessSeq
+  check templateParamAddresses[1] == addr s1
+  check templateParamAddresses[3] == addr o.accessSeq
+
+  # let symbols need lent to avoid copying;
+  # they are still computed once though
+  when useLent:
+    check templateParamAddresses[2] == unsafeAddr s2