fix #15851 (#15852)

* fix #15851

* {.cast(noSideEffect).}

lib/pure/httpcore.nim

@@ -126,8 +126,10 @@ func newHttpHeaders*(keyValuePairs:
   new result
   result.table = newTable[string, seq[string]]()
   result.isTitleCase = titleCase
+
   for pair in keyValuePairs:
     let key = result.toCaseInsensitive(pair.key)
+    {.cast(noSideEffect).}:
       if key in result.table:
         result.table[key].add(pair.val)
       else:
@@ -148,6 +150,7 @@ func `[]`*(headers: HttpHeaders, key: string): HttpHeaderValues =
   ##
   ## To access multiple values of a key, use the overloaded ``[]`` below or
   ## to get all of them access the ``table`` field directly.
+  {.cast(noSideEffect).}:
     return headers.table[headers.toCaseInsensitive(key)].HttpHeaderValues
 
 converter toString*(values: HttpHeaderValues): string =
@@ -157,6 +160,7 @@ func `[]`*(headers: HttpHeaders, key: string, i: int): string =
   ## Returns the ``i``'th value associated with the given key. If there are
   ## no values associated with the key or the ``i``'th value doesn't exist,
   ## an exception is raised.
+  {.cast(noSideEffect).}:
     return headers.table[headers.toCaseInsensitive(key)][i]
 
 proc `[]=`*(headers: HttpHeaders, key, value: string) =

tests/effects/tstrict_funcs.nim

@@ -2,7 +2,7 @@ discard """
   cmd: "nim c --experimental:strictFuncs --experimental:views $file"
 """
 
-import tables, streams, nre, parsecsv, uri
+import tables, streams, nre, parsecsv, uri, httpcore
 
 type
   Contig2Reads = TableRef[string, seq[string]]