IC: bugfix

compiler/nifbackend.nim

@@ -198,8 +198,16 @@ proc ownsRuntimeRoutine(s: PSym; modPos: int): bool =
   {sfForward, sfImportc, sfCompileTime, sfError} * s.flags == {} and
   s.typ != nil and not signatureHasMetaType(s.typ) and
   s.ast != nil and s.ast.safeLen > bodyPos and
-  s.ast[genericParamsPos].kind == nkEmpty and
-  s.ast[bodyPos].kind != nkEmpty
+  s.ast[genericParamsPos].kind == nkEmpty
+  # NOTE: an `nkEmpty` body is NOT a disqualifier. A concrete, owned, non-
+  # forward/-importc/-magic routine whose body folds to nothing is still a real
+  # definition the owner must emit (`void f(void){}`), exactly as whole-program
+  # cgen does — else a cross-module caller links to nothing. This bites e.g.
+  # Nimbus' `extras.incInternalErrors`, a plain `proc` whose sole statement is a
+  # metrics-counter `.inc()` that the `metrics` library expands to a no-op when
+  # the importing tool (ncli) builds with `-u:metrics`; the body is then a bare
+  # `nkEmpty`, but `state_transition_epoch` still calls it. Forward declarations
+  # (the other empty-body case) carry `sfForward` and are excluded above.
 
 proc generateCodeForModule(g: ModuleGraph; precomp: PrecompiledModule) =
   ## Generate C code for a single module.

tests/ic/memptycaller.nim

@@ -0,0 +1,9 @@
+# Calls `emptyOwned` from a DIFFERENT module than the one that owns it, so the
+# reference at link time must resolve to a definition the owning module emits.
+
+import memptyowned
+
+proc callEmpty*(cond: bool) =
+  if cond:
+    emptyOwned()
+  echo "called ", cond

tests/ic/memptyowned.nim

@@ -0,0 +1,12 @@
+# Helper for `temptyowned`: exports a concrete proc whose body folds to
+# nothing (an `nkEmpty` body, like Nimbus' `extras.incInternalErrors` when the
+# metrics counter's `.inc()` expands to a no-op under `-u:metrics`). The proc is
+# NOT called within its own module — only `memptycaller` references it — so the
+# per-module backend's owned-routine seeding is the ONLY thing that can emit it.
+
+template maybe*(x: untyped) =
+  when false:
+    x
+
+proc emptyOwned*() =
+  maybe(echo "unreachable")

tests/ic/temptyowned.nim

@@ -0,0 +1,18 @@
+discard """
+output: '''called false
+done'''
+"""
+
+# Regression test for the per-module IC backend dropping an owned routine whose
+# body folds to `nkEmpty`. `memptyowned.emptyOwned` is a real, concrete, owned
+# proc with an empty body; `memptycaller.callEmpty` references it across a module
+# boundary. The owning module's `ownsRuntimeRoutine` seeding used to reject any
+# routine with an `nkEmpty` body, so nobody emitted `emptyOwned` -> undefined
+# reference at link (mirrors Nimbus `ncli` linking `extras.incInternalErrors`).
+# Whole-program cgen always emits it as `void emptyOwned(void){}`; the per-module
+# backend must too.
+
+import memptycaller
+
+callEmpty(false)
+echo "done"