Only check isomorphism of types in debug compiles (#7801)

* Only check isomorpism of types in debug compiles

`default(T)` must create a T in the compiler VM which is slow

Author: arnetheduck

beacon_chain/libnimbus_lc/libnimbus_lc.nim

@@ -9,6 +9,7 @@
 
 import
   std/[json, sequtils, times],
+  stew/objects,
   eth/common/eth_types_rlp,
   eth/common/keys,
   eth/p2p/discoveryv5/random2,
@@ -957,7 +958,7 @@ proc ETHLightClientHeaderCopyExecutionHash(
   root.toUnmanagedPtr()
 
 type ExecutionPayloadHeader =
-  typeof(default(lcDataFork.LightClientHeader).execution)
+  typeof(declval(lcDataFork.LightClientHeader).execution)
 
 func ETHLightClientHeaderGetExecution(
     header: ptr lcDataFork.LightClientHeader

beacon_chain/spec/datatypes/base.nim

@@ -884,19 +884,22 @@ func getSizeofSig(x: auto, n: int = 0): seq[(string, int, int)] =
 template isomorphicCast*[T](x: auto): T =
   # Each of these pairs of types has ABI-compatible memory representations.
   type U = typeof(x)
+
   static: doAssert (T is ref) == (U is ref)
   when T is ref:
-    type
-      TT = typeof default(typeof T)[]
-      UU = typeof default(typeof U)[]
-    static:
-      doAssert sizeof(TT) == sizeof(UU)
-      doAssert getSizeofSig(TT()) == getSizeofSig(UU())
+    when defined(debug):
+      type
+        TT = pointerBase(T)
+        UU = pointerBase(U)
+      static:
+        doAssert sizeof(TT) == sizeof(UU)
+        doAssert getSizeofSig(TT()) == getSizeofSig(UU())
     cast[T](x)
   else:
-    static:
-      doAssert getSizeofSig(T()) == getSizeofSig(U())
-      doAssert sizeof(T) == sizeof(U)
+    when defined(debug): # 10s+ compile time due to `default(T)` and `replace`!
+      static:
+        doAssert sizeof(T) == sizeof(U)
+        doAssert getSizeofSig(T()) == getSizeofSig(U())
     cast[ptr T](unsafeAddr x)[]
 
 func prune*(cache: var StateCache, epoch: Epoch) =

beacon_chain/spec/state_transition.nim

@@ -43,10 +43,12 @@
 import
   chronicles,
   results,
+  stew/objects,
   ../extras,
-  "."/[
-    beaconstate, eth2_merkleization, forks, helpers, signatures,
-    state_transition_block, state_transition_epoch, validator]
+  ./[
+    beaconstate, eth2_merkleization, forks, helpers, signatures, state_transition_block,
+    state_transition_epoch, validator,
+  ]
 
 export results, extras, state_transition_block
 
@@ -377,7 +379,7 @@ proc makeBeaconBlockWithRewards*(
   ## the block is to be created.
   type
     MaybeBlindedBeaconBlock = consensusFork.BeaconBlock(type(execution_payload))
-    MaybeBlindedBlockBody = typeof(default(MaybeBlindedBeaconBlock).body)
+    MaybeBlindedBlockBody = typeof(declval(MaybeBlindedBeaconBlock).body)
 
   # https://github.com/ethereum/consensus-specs/blob/v1.5.0-beta.2/specs/phase0/validator.md#preparing-for-a-beaconblock
   var blck = MaybeBlindedBeaconBlock(