Make capacity work with refc [backport] (#22697)

followup of #19771.

(cherry picked from commit 325341866f)

lib/system/seqs_v2.nim

@@ -140,8 +140,6 @@ proc newSeq[T](s: var seq[T], len: Natural) =
   setLen(s, len)
 
 
-template capacityImpl(sek: NimSeqV2): int =
-  if sek.p != nil: (sek.p.cap and not strlitFlag) else: 0
 
 func capacity*[T](self: seq[T]): int {.inline.} =
   ## Returns the current capacity of the seq.
@@ -151,9 +149,8 @@ func capacity*[T](self: seq[T]): int {.inline.} =
     lst.add "Nim"
     assert lst.capacity == 42
 
-  {.cast(noSideEffect).}:
-    let sek = unsafeAddr self
-    result = capacityImpl(cast[ptr NimSeqV2[T]](sek)[])
+  let sek = cast[ptr NimSeqV2[T]](unsafeAddr self)
+  result = if sek.p != nil: (sek.p.cap and not strlitFlag) else: 0
 
 
 {.pop.}  # See https://github.com/nim-lang/Nim/issues/21401

lib/system/strs_v2.nim

@@ -177,9 +177,6 @@ proc prepareMutation*(s: var string) {.inline.} =
     nimPrepareStrMutationV2(cast[ptr NimStringV2](s)[])
 
 
-template capacityImpl(str: NimStringV2): int =
-  if str.p != nil: str.p.cap else: 0
-
 func capacity*(self: string): int {.inline.} =
   ## Returns the current capacity of the string.
   # See https://github.com/nim-lang/RFCs/issues/460
@@ -188,6 +185,5 @@ func capacity*(self: string): int {.inline.} =
     str.add "Nim"
     assert str.capacity == 42
 
-  {.cast(noSideEffect).}:
-    let str = unsafeAddr self
-    result = capacityImpl(cast[ptr NimStringV2](str)[])
+  let str = cast[ptr NimStringV2](unsafeAddr self)
+  result = if str.p != nil: str.p.cap else: 0

lib/system/sysstr.nim

@@ -340,3 +340,25 @@ proc setLengthSeqV2(s: PGenericSeq, typ: PNimType, newLen: int): PGenericSeq {.
       result = s
       zeroMem(dataPointer(result, elemAlign, elemSize, result.len), (newLen-%result.len) *% elemSize)
     result.len = newLen
+
+func capacity*(self: string): int {.inline.} =
+  ## Returns the current capacity of the string.
+  # See https://github.com/nim-lang/RFCs/issues/460
+  runnableExamples:
+    var str = newStringOfCap(cap = 42)
+    str.add "Nim"
+    assert str.capacity == 42
+
+  let str = cast[NimString](self)
+  result = if str != nil: str.reserved else: 0
+
+func capacity*[T](self: seq[T]): int {.inline.} =
+  ## Returns the current capacity of the seq.
+  # See https://github.com/nim-lang/RFCs/issues/460
+  runnableExamples:
+    var lst = newSeqOfCap[string](cap = 42)
+    lst.add "Nim"
+    assert lst.capacity == 42
+
+  let sek = cast[PGenericSeq](self)
+  result = if sek != nil: (sek.reserved and not strlitFlag) else: 0