Make ensureSpine on Clash number types synthesizable

Fixes #3021

Author: christiaanb

changelog/2025-09-30T14_03_21+02_00_fix_3021

@@ -0,0 +1 @@
+FIXED: Clash will no longer error out when converting `ensureSpine` on Clash number types to HDL [#3021](https://github.com/clash-lang/clash-compiler/issues/3021)

clash-prelude/src/Clash/Sized/Internal/BitVector.hs

@@ -298,6 +298,7 @@ instance ShowX Bit where
 
 instance NFDataX Bit where
   deepErrorX = errorX
+  ensureSpine = unpack# . xToBV . pack#
   rnfX = rwhnfX
   hasUndefined bv = isLeft (isX bv) || unsafeMask# bv /= 0
 

clash-prelude/src/Clash/Sized/Internal/Index.hs

@@ -557,6 +557,7 @@ instance ShowX (Index n) where
 
 instance NFDataX (Index n) where
   deepErrorX = errorX
+  ensureSpine = id
   rnfX = rwhnfX
 
 -- | None of the 'Read' class' methods are synthesizable.

clash-prelude/src/Clash/Sized/Internal/Signed.hs

@@ -194,6 +194,7 @@ data Signed (n :: Nat) =
 
 instance NFDataX (Signed n) where
   deepErrorX = errorX
+  ensureSpine = id
   rnfX = rwhnfX
 
 -- See: https://github.com/clash-lang/clash-compiler/pull/2511

clash-prelude/src/Clash/Sized/Internal/Unsigned.hs

@@ -233,6 +233,7 @@ instance ShowX (Unsigned n) where
 
 instance NFDataX (Unsigned n) where
   deepErrorX = errorX
+  ensureSpine = id
   rnfX = rwhnfX
 
 -- | None of the 'Read' class' methods are synthesizable.

tests/Main.hs

@@ -671,6 +671,8 @@ runClashTest = defaultMain
         , runTest "T2988" def{hdlSim=[]}
         , outputTest "T3008" def{hdlSim=[]}
         , outputTest "T3011" def{hdlSim=[]}
+        , let _opts = def { hdlTargets = [VHDL], hdlLoad = [], hdlSim = []}
+           in runTest "T3021" _opts
         ] <>
         if compiledWith == Cabal then
           -- This tests fails without environment files present, which are only

tests/shouldwork/Issues/T3021.hs

@@ -0,0 +1,7 @@
+module T3021 where
+
+import Clash.Prelude
+
+topEntity ::
+  (Unsigned 8, Signed 8, Index 8, Bit) -> (Unsigned 8, Signed 8, Index 8, Bit)
+topEntity = ensureSpine