Empty check in shallow [backport] (#9676)

lib/system.nim

@@ -3963,6 +3963,7 @@ proc shallow*[T](s: var seq[T]) {.noSideEffect, inline.} =
   ## marks a sequence `s` as `shallow`:idx:. Subsequent assignments will not
   ## perform deep copies of `s`. This is only useful for optimization
   ## purposes.
+  if s.len == 0: return
   when not defined(JS) and not defined(nimscript):
     var s = cast[PGenericSeq](s)
     s.reserved = s.reserved or seqShallowFlag

tests/seq/tseq.nim

@@ -0,0 +1,205 @@
+discard """
+  output: '''
+Hithere, what's your name?Hathere, what's your name?
+fA13msg1falsefB14msg2truefC15msg3false
+Zip: [{"Field0": 1, "Field1": 2}, {"Field0": 3, "Field1": 4}, {"Field0": 5, "Field1": 6}]
+Filter Iterator: 3
+Filter Iterator: 5
+Filter Iterator: 7
+Filter: [3, 5, 7]
+FilterIt: [1, 3, 7]
+Concat: [1, 3, 5, 7, 2, 4, 6]
+Deduplicate: [1, 2, 3, 4, 5, 7]
+@[()]
+@[1, 42, 3]
+@[1, 42, 3]
+2345623456
+'''
+"""
+
+block tseq2:
+  proc `*`(a, b: seq[int]): seq[int] =
+    # allocate a new sequence:
+    newSeq(result, len(a))
+    # multiply two int sequences:
+    for i in 0..len(a)-1: result[i] = a[i] * b[i]
+
+  assert(@[1, 2, 3] * @[1, 2, 3] == @[1, 4, 9])
+
+
+
+block tseqcon:
+  const nestedFixed = true
+
+  type
+    TRec {.final.} = object
+      x, y: int
+      s: string
+      seq: seq[string]
+    TRecSeq = seq[TRec]
+
+  proc test() =
+    var s, b: seq[string]
+    s = @[]
+    add(s, "Hi")
+    add(s, "there, ")
+    add(s, "what's your name?")
+
+    b = s # deep copying here!
+    b[0][1] = 'a'
+
+    for i in 0 .. len(s)-1:
+      write(stdout, s[i])
+    for i in 0 .. len(b)-1:
+      write(stdout, b[i])
+
+  when nestedFixed:
+    proc nested() =
+      var
+        s: seq[seq[string]]
+      for i in 0..10_000: # test if the garbage collector
+        # now works with sequences
+        s = @[
+          @["A", "B", "C", "D"],
+          @["E", "F", "G", "H"],
+          @["I", "J", "K", "L"],
+          @["M", "N", "O", "P"]]
+
+  test()
+  when nestedFixed:
+    nested()
+  echo ""
+
+
+
+import os
+block tseqcon2:
+  proc rec_dir(dir: string): seq[string] =
+    result = @[]
+    for kind, path in walk_dir(dir):
+      if kind == pcDir:
+        add(result, rec_dir(path))
+      else:
+        add(result, path)
+
+
+
+block tseqtuple:
+  type
+    TMsg = tuple[
+      file: string,
+      line: int,
+      msg: string,
+      err: bool]
+
+  var s: seq[TMsg] = @[]
+
+  s.add(("fA", 13, "msg1", false))
+  s.add(("fB", 14, "msg2", true))
+  s.add(("fC", 15, "msg3", false))
+
+  for file, line, msg, err in items(s):
+    stdout.write(file)
+    stdout.write($line)
+    stdout.write(msg)
+    stdout.write($err)
+  echo ""
+
+
+import sequtils, marshal
+block tsequtils:
+  proc testFindWhere(item : int) : bool =
+    if item != 1: return true
+
+  var seq1: seq[int] = @[]
+
+  seq1.add(1)
+  seq1.add(3)
+  seq1.add(5)
+  seq1.add(7)
+
+  var seq2: seq[int] = @[2, 4, 6]
+  var final = zip(seq1, seq2)
+
+  echo "Zip: ", $$(final)
+
+  #Test findWhere as a iterator
+
+  for itms in filter(seq1, testFindWhere):
+    echo "Filter Iterator: ", $$(itms)
+
+
+  #Test findWhere as a proc
+
+  var fullseq: seq[int] = filter(seq1, testFindWhere)
+
+  echo "Filter: ", $$(fullseq)
+
+  #Test findIt as a template
+
+  var finditval: seq[int] = filterIt(seq1, it!=5)
+
+  echo "FilterIt: ", $$(finditval)
+
+  var concatseq = concat(seq1,seq2)
+  echo "Concat: ", $$(concatseq)
+
+  var seq3 = @[1,2,3,4,5,5,5,7]
+  var dedupseq = deduplicate(seq3)
+  echo "Deduplicate: ", $$(dedupseq)
+  # bug #4973
+  type
+    SomeObj = object
+    OtherObj = object
+      field: SomeObj
+
+  let aSeq = @[OtherObj(field: SomeObj())]
+  let someObjSeq = aSeq.mapIt(it.field)
+  echo someObjSeq
+
+
+
+block tshallowseq:
+  proc xxx() =
+    var x: seq[int] = @[1, 2, 3]
+    var y: seq[int]
+    system.shallowCopy(y, x)
+    y[1] = 42
+    echo y
+    echo x
+  xxx()
+
+
+block tshallowemptyseq:
+  proc test() =
+    var nilSeq: seq[int] = @[]
+    var emptySeq: seq[int] = newSeq[int]()
+    block:
+      var t = @[1,2,3]
+      shallow(nilSeq)
+      t = nilSeq
+      doAssert t == @[]
+    block:
+      var t = @[1,2,3]
+      shallow(emptySeq)
+      t = emptySeq
+      doAssert t == @[]
+    block:
+      var t = @[1,2,3]
+      shallowCopy(t, nilSeq)
+      doAssert t == @[]
+    block:
+      var t = @[1,2,3]
+      shallowCopy(t, emptySeq)
+      doAssert t == @[]
+  test()
+
+
+import strutils
+block ttoseq:
+  for x in toSeq(countup(2, 6)):
+    stdout.write(x)
+  for x in items(toSeq(countup(2, 6))):
+    stdout.write(x)
+  var y: type("a b c".split)
+  y = "xzy"