Add {.unpreservable.} pragma

src/preserves.nim

@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: ISC
 
 import bigints
-import std/[base64, endians, json, hashes, macros, sets, streams, tables]
+import std/[base64, endians, json, hashes, macros, sets, streams, tables, typetraits]
 
 
 type
@@ -245,9 +245,6 @@ iterator fields*(prs: Preserve): Preserve =
   ## Iterate the fields of a record value.
   for i in 0..<prs.record.high: yield prs.record[i]
 
-proc distinctBase(T: typedesc): typedesc {.magic: "TypeTrait".}
-template distinctBase[T](a: T): untyped = distinctBase(type(a))(a)
-
 proc symbol*(s: string): Preserve {.inline.} =
   ## Symbol constructor.
   Preserve(kind: pkSymbol, symbol: s)
@@ -463,6 +460,9 @@ template record*(label: string) {.pragma.}
   ##  # <foobar 1 2>
   ## ```
 
+template unpreservable*() {.pragma.}
+  ## Pragma to forbid a type from being converted by `toPreserve`.
+
 proc toPreserve*[T](x: T): Preserve =
   ## Serializes `x` to Preserves; uses `toPreserveHook(x: A)` if it's in scope to
   ## customize serialization.
@@ -483,7 +483,8 @@ proc toPreserve*[T](x: T): Preserve =
   elif T is float64:
     result = Preserve(kind: pkDouble, double: x)
   elif T is object | tuple:
-    when T.hasCustomPragma(record):
+    when T.hasCustomPragma(unpreservable): {.error.}
+    elif T.hasCustomPragma(record):
       result = Preserve(kind: pkRecord)
       for _, f in x.fieldPairs: result.record.add(toPreserve(f))
       result.record.add(symbol(T.getCustomPragmaVal(record)))