arkham: handle the comesfrom statement tag

src/arkham/analyser.nim

@@ -523,6 +523,19 @@ proc analyse(c: var Context; n: var Cursor) =
         if frame.sawCall: c.loopStack[^1].sawCall = true
         for pnm in frame.usedParams: c.loopStack[^1].usedParams.incl pnm
     else:
+      when declared(ComesfromS):
+        if n.stmtKind == ComesfromS:
+          # `(comesfrom SYM S*)`: debug-info marker, walks like `stmts` but
+          # opens no scope frame. The leading child is the origin SYMBOL, not a
+          # statement, so `analyseChildren` would analyse it as a symbol *use*
+          # and record a read of the expanded routine at this position.
+          #
+          # Guarded so arkham still compiles against a nimony that predates the
+          # tag (nim-lang/nimony#2240); drop the `when` once that has merged.
+          n.into:
+            skip n                        # the origin symbol
+            while n.hasMore: analyse(c, n)
+          return
       analyseChildren(c, n)             # if/case/ret/... : recurse
   else:
     inc n

src/arkham/codegen_a64.nim

@@ -4702,7 +4702,22 @@ proc genStmt2(g: var CodeGen; c: Cursor) =
           g.ovfReg2 = rB
           g.ovfBridges = @[rA, rB]
       while cc.hasMore: skip cc
-  else: raiseAssert "arkham a64n: genStmt2 " & $c.stmtKind
+  else:
+    when declared(ComesfromS):
+      if c.stmtKind == ComesfromS:
+        # `(comesfrom SYM S*)` — statements produced by expanding SYM (a
+        # template today). Debug-info only: no scope and no code of its own, so
+        # it behaves as `stmts` once the leading origin SYMBOL is stepped over.
+        # That first child is an operand, not a statement.
+        #
+        # Guarded so arkham still compiles against a nimony that predates the
+        # tag (nim-lang/nimony#2240); drop the `when` once that has merged.
+        var cc = c
+        cc.into:
+          skip cc                               # the origin symbol
+          while cc.hasMore: (g.genStmt2(cc); skip cc)
+        return
+    raiseAssert "arkham a64n: genStmt2 " & $c.stmtKind
 
 # ── proc emission / driver (pure-emit path) ──────────────────────────────────
 

src/arkham/codegen_x64.nim

@@ -4590,7 +4590,25 @@ proc genStmt2(g: var CodeGen; c: Cursor) =
         g.genStore2(opCur, memLoc(lhsCur, ScalarSlot))
       g.noFoldPos = -1
       while cc.hasMore: skip cc
-  else: raiseAssert "arkham x64n: genStmt2 " & $c.stmtKind
+  else:
+    when declared(ComesfromS):
+      if c.stmtKind == ComesfromS:
+        # `(comesfrom SYM S*)` — statements produced by expanding SYM (a
+        # template today). Debug-info only: no scope and no code of its own, so
+        # it behaves as `stmts` once the leading origin SYMBOL is stepped over.
+        # That first child is an operand, not a statement.
+        #
+        # Guarded so arkham still compiles against a nimony that predates the
+        # tag (nim-lang/nimony#2240); drop the `when` once that has merged.
+        var cc = c
+        cc.into:
+          skip cc                               # the origin symbol
+          while cc.hasMore:
+            var nx = cc; skip nx
+            g.tailStmt = myTail and not nx.hasMore
+            g.genStmt2(cc); skip cc
+        return
+    raiseAssert "arkham x64n: genStmt2 " & $c.stmtKind
 
 # ── fused value core: unconverted-proc stubs (die as each case lands) ────────
 proc emitBin2(g: var CodeGen; c: Cursor; dest: var Location) =
@@ -6838,6 +6856,19 @@ proc asmStmt(g: var CodeGen; c: Cursor) =
       g.retLabelUsed2 = true
       g.emJmp(g.retLabel2)
   else:
+    when declared(ComesfromS):
+      if c.stmtKind == ComesfromS:
+        # See `genStmt2`: transparent debug-info marker, first child is an
+        # operand. Guarded so arkham still compiles against a nimony that
+        # predates the tag (nim-lang/nimony#2240).
+        var cc = c
+        cc.into:
+          skip cc                               # the origin symbol
+          while cc.hasMore:
+            var nx = cc; skip nx
+            g.tailStmt = myTail and not nx.hasMore
+            g.asmStmt(cc); skip cc
+        return
     lengError c, "`" & $c.stmtKind & "` is not allowed in an `.assembler` proc", g.asmInfo
 
 proc genAsmProc(g: var CodeGen; info: ProcInfo) =

src/arkham/register_allocator.nim

@@ -527,6 +527,20 @@ proc walk(b: var Builder; n: var Cursor) =
     n.into:
       while n.hasMore: walk(b, n)
   else:
+    when declared(ComesfromS):
+      if n.stmtKind == ComesfromS:
+        # `(comesfrom SYM S*)`: debug-info marker, walks like `stmts`. The
+        # leading child is the origin SYMBOL, not a statement, so the generic
+        # recursion below would count it as a use of the expanded routine here.
+        #
+        # Guarded so arkham still compiles against a nimony that predates the
+        # tag (nim-lang/nimony#2240); drop the `when` once that has merged.
+        n.into:
+          skip n                              # the origin symbol
+          while n.hasMore:
+            walk(b, n)
+            flushFree(b, b.posOf(n))
+        return
     if n.kind == TagLit:
       n.into:
         while n.hasMore: walk(b, n)          # recurse (var decls may nest)