RFC-460 implemented (#19771)

* RFC-460 implemented

* RFC-460 implemented

* RFC-460 implemented

* Fix dumb GitHub autoupdate on changelog

changelog.md

@@ -85,6 +85,7 @@
   `replaceChildren`, `replaceWith`, `scrollIntoViewIfNeeded`, `setHTML`,
   `toggleAttribute`, and `matches` to `std/dom`.
 - Added [`jsre.hasIndices`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/hasIndices)
+- Added `capacity` for `string` and `seq` to return the current capacity, see https://github.com/nim-lang/RFCs/issues/460
 
 [//]: # "Deprecations:"
 - Deprecated `selfExe` for Nimscript.

lib/system/seqs_v2.nim

@@ -129,3 +129,19 @@ proc setLen[T](s: var seq[T], newlen: Natural) =
 proc newSeq[T](s: var seq[T], len: Natural) =
   shrink(s, 0)
   setLen(s, len)
+
+
+template capacityImpl(sek: NimSeqV2): int =
+  if sek.p != nil: sek.p.cap 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
+
+  {.cast(noSideEffect).}:
+    let sek = unsafeAddr self
+    result = capacityImpl(cast[ptr NimSeqV2](sek)[])

lib/system/strs_v2.nim

@@ -175,3 +175,19 @@ proc prepareMutation*(s: var string) {.inline.} =
   {.cast(noSideEffect).}:
     let s = unsafeAddr s
     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
+  runnableExamples:
+    var str = newStringOfCap(cap = 42)
+    str.add "Nim"
+    assert str.capacity == 42
+
+  {.cast(noSideEffect).}:
+    let str = unsafeAddr self
+    result = capacityImpl(cast[ptr NimStringV2](str)[])