fixes #25849; fixes #25872; Iteration on elements of array (#25860)

fixes #25849
fixes https://github.com/nim-lang/Nim/issues/25872

compiler/transf.nim

@@ -22,7 +22,7 @@ import std / tables
 
 import
   options, ast, astalgo, trees, msgs,
-  idents, renderer, types, semfold, magicsys, cgmeth,
+  idents, renderer, types, semfold, magicsys, cgmeth, parampatterns,
   lowerings, liftlocals,
   modulegraphs, lineinfos
 
@@ -675,7 +675,7 @@ type
     paDirectMapping, paFastAsgn, paFastAsgnTakeTypeFromArg
     paVarAsgn, paComplexOpenarray, paViaIndirection
 
-proc putArgInto(arg: PNode, formal: PType): TPutArgInto =
+proc putArgInto(arg: PNode, formal: PType; borrowedFirstArg = false): TPutArgInto =
   # This analyses how to treat the mapping "formal <-> arg" in an
   # inline context.
   if formal.kind == tyTypeDesc: return paDirectMapping
@@ -726,6 +726,13 @@ proc putArgInto(arg: PNode, formal: PType): TPutArgInto =
     if skipTypes(formal, abstractInst).kind in {tyVar, tyLent}: result = paVarAsgn
     else: result = paFastAsgn
 
+  if borrowedFirstArg and result == paDirectMapping and parampatterns.exprRoot(arg) == nil and
+           parampatterns.isAssignable(nil, arg) == arNone:
+    # Inline iterators like `items(array)` borrow from the first argument.
+    # If that argument is just a transient expression, materialize it so the
+    # lifted closure keeps the backing storage alive across yields.
+    result = paFastAsgnTakeTypeFromArg
+
 proc findWrongOwners(c: PTransf, n: PNode) =
   if n.kind == nkVarSection:
     let x = n[0][0]
@@ -824,13 +831,16 @@ proc transformFor(c: PTransf, n: PNode): PNode =
   if iter.kind != skIterator: return result
   # generate access statements for the parameters (unless they are constant)
   pushTransCon(c, newC)
+  let borrowedIterResult =
+    iter.typ != nil and iter.typ.returnType != nil and
+    skipTypes(iter.typ.returnType, abstractInst).kind in {tyLent, tyVar}
   for i in 1..<call.len:
     var arg = transform(c, call[i])
     let ff = skipTypes(iter.typ, abstractInst)
     # can happen for 'nim check':
     if i >= ff.n.len: return result
     var formal = ff.n[i].sym
-    let pa = putArgInto(arg, formal.typ)
+    let pa = putArgInto(arg, formal.typ, borrowedIterResult and i == 1)
     case pa
     of paDirectMapping:
       newC.mapping[formal.itemId] = arg

lib/system/iterators.nim

@@ -3,7 +3,7 @@
 when defined(nimPreviewSlimSystem):
   import std/assertions
 
-when not defined(nimNoLentIterators):
+when (not defined(nimNoLentIterators)) and not defined(js) and not defined(nimscript):
   template lent2(T): untyped = lent T
 else:
   template lent2(T): untyped = T
@@ -37,7 +37,7 @@ iterator mitems*[T](a: var openArray[T]): var T {.inline.} =
     yield a[i]
     unCheckedInc(i)
 
-iterator items*[IX, T](a: array[IX, T]): T {.inline.} =
+iterator items*[IX, T](a: array[IX, T]): lent2 T {.inline.} =
   ## Iterates over each item of `a`.
   when a.len > 0:
     var i = low(IX)

tests/lent/titems_array_lent.nim

@@ -0,0 +1,38 @@
+discard """
+  targets: "c cpp js"
+"""
+
+template sameAddress(a, b): bool =
+  when defined(js):
+    a == b
+  else:
+    a.unsafeAddr == b.unsafeAddr
+
+proc main() =
+  block:
+    let a = [10, 11, 12]
+    for ai in items(a):
+      doAssert sameAddress(ai, a[0])
+      break
+
+  block:
+    let a = [[1, 2], [1, 2], [1, 2]]
+    for ai in items(a):
+      doAssert sameAddress(ai[0], a[0][0])
+      break
+
+  block:
+    let s = @[(1, 2), (3, 4), (5, 6)]
+    doAssert (3, 4) in s
+
+main()
+
+static:
+  main()
+
+block: # issue #25849
+  static:
+    const key = "NIM_TESTS_TOSENV_KEY"
+    for val in ["val", "", "\xc3\x86"]:
+      let s = @[(key, "val"), (key, ""), (key, "\xc3\x86")]
+      doAssert (key, val) in s