Fix capacity for const and shallow [backport] (#22705)

(cherry picked from commit b542be1e7d)

lib/system.nim

@@ -457,10 +457,6 @@ when not defined(js) and not defined(nimSeqsV2):
       data: UncheckedArray[char]
     NimString = ptr NimStringDesc
 
-when notJSnotNims and not defined(nimSeqsV2):
-  template space(s: PGenericSeq): int {.dirty.} =
-    s.reserved and not (seqShallowFlag or strlitFlag)
-
 when notJSnotNims:
   include "system/hti"
 
@@ -1068,6 +1064,10 @@ const
   hasThreadSupport = compileOption("threads") and not defined(nimscript)
   hasSharedHeap = defined(boehmgc) or defined(gogc) # don't share heaps; every thread has its own
 
+when notJSnotNims and not defined(nimSeqsV2):
+  template space(s: PGenericSeq): int =
+    s.reserved and not (seqShallowFlag or strlitFlag)
+
 when hasThreadSupport and defined(tcc) and not compileOption("tlsEmulation"):
   # tcc doesn't support TLS
   {.error: "`--tlsEmulation:on` must be used when using threads with tcc backend".}

lib/system/seqs_v2.nim

@@ -150,7 +150,7 @@ func capacity*[T](self: seq[T]): int {.inline.} =
     assert lst.capacity == 42
 
   let sek = cast[ptr NimSeqV2[T]](unsafeAddr self)
-  result = if sek.p != nil: (sek.p.cap and not strlitFlag) else: 0
+  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

@@ -211,4 +211,4 @@ func capacity*(self: string): int {.inline.} =
     assert str.capacity == 42
 
   let str = cast[ptr NimStringV2](unsafeAddr self)
-  result = if str.p != nil: str.p.cap else: 0
+  result = if str.p != nil: str.p.cap and not strlitFlag else: 0

lib/system/sysstr.nim

@@ -350,7 +350,7 @@ func capacity*(self: string): int {.inline.} =
     assert str.capacity == 42
 
   let str = cast[NimString](self)
-  result = if str != nil: str.reserved else: 0
+  result = if str != nil: str.space else: 0
 
 func capacity*[T](self: seq[T]): int {.inline.} =
   ## Returns the current capacity of the seq.
@@ -361,4 +361,4 @@ func capacity*[T](self: seq[T]): int {.inline.} =
     assert lst.capacity == 42
 
   let sek = cast[PGenericSeq](self)
-  result = if sek != nil: (sek.reserved and not strlitFlag) else: 0
+  result = if sek != nil: sek.space else: 0