1 files changed, 117 insertions, 0 deletions
diff --git a/compiler/isolation_check.nim b/compiler/isolation_check.nim
new file mode 100644
index 000000000..a77e8c97a
--- /dev/null
+++ b/compiler/isolation_check.nim
@@ -0,0 +1,117 @@
+#
+#
+#           The Nim Compiler
+#        (c) Copyright 2020 Andreas Rumpf
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+## Implementation of the check that `recover` needs, see
+## https://github.com/nim-lang/RFCs/issues/244 for more details.
+
+import
+  ast, types, renderer, idents, intsets, options, msgs
+
+proc canAlias(arg, ret: PType; marker: var IntSet): bool
+
+proc canAliasN(arg: PType; n: PNode; marker: var IntSet): bool =
+  case n.kind
+  of nkRecList:
+    for i in 0..<n.len:
+      result = canAliasN(arg, n[i], marker)
+      if result: return
+  of nkRecCase:
+    assert(n[0].kind == nkSym)
+    result = canAliasN(arg, n[0], marker)
+    if result: return
+    for i in 1..<n.len:
+      case n[i].kind
+      of nkOfBranch, nkElse:
+        result = canAliasN(arg, lastSon(n[i]), marker)
+        if result: return
+      else: discard
+  of nkSym:
+    result = canAlias(arg, n.sym.typ, marker)
+  else: discard
+
+proc canAlias(arg, ret: PType; marker: var IntSet): bool =
+  if containsOrIncl(marker, ret.id):
+    return false
+
+  if ret.kind in {tyPtr, tyPointer}:
+    # unsafe so we don't care:
+    return false
+  if compareTypes(arg, ret, dcEqIgnoreDistinct):
+    return true
+  case ret.kind
+  of tyObject:
+    if isFinal(ret):
+      result = canAliasN(arg, ret.n, marker)
+      if not result and ret.len > 0 and ret[0] != nil:
+        result = canAlias(arg, ret[0], marker)
+    else:
+      result = true
+  of tyTuple:
+    for i in 0..<ret.len:
+      result = canAlias(arg, ret[i], marker)
+      if result: break
+  of tyArray, tySequence, tyDistinct, tyGenericInst,
+     tyAlias, tyInferred, tySink, tyLent, tyOwned, tyRef:
+    result = canAlias(arg, ret.lastSon, marker)
+  of tyProc:
+    result = ret.callConv == ccClosure
+  else:
+    result = false
+
+proc canAlias(arg, ret: PType): bool =
+  var marker = initIntSet()
+  result = canAlias(arg, ret, marker)
+
+proc checkIsolate*(n: PNode): bool =
+  if types.containsTyRef(n.typ):
+    # XXX Maybe require that 'n.typ' is acyclic. This is not much
+    # worse than the already exisiting inheritance and closure restrictions.
+    case n.kind
+    of nkCharLit..nkNilLit:
+      result = true
+    of nkCallKinds:
+      if n[0].typ.flags * {tfGcSafe, tfNoSideEffect} == {}:
+        return false
+      for i in 1..<n.len:
+        if checkIsolate(n[i]):
+          discard "fine, it is isolated already"
+        else:
+          let argType = n[i].typ
+          if argType != nil and not isCompileTimeOnly(argType) and containsTyRef(argType):
+            if argType.canAlias(n.typ):
+              return false
+      result = true
+    of nkIfStmt, nkIfExpr:
+      for it in n:
+        result = checkIsolate(it.lastSon)
+        if not result: break
+    of nkCaseStmt, nkObjConstr:
+      for i in 1..<n.len:
+        result = checkIsolate(n[i].lastSon)
+        if not result: break
+    of nkBracket, nkTupleConstr, nkPar:
+      for it in n:
+        result = checkIsolate(it)
+        if not result: break
+    of nkHiddenStdConv, nkHiddenSubConv, nkCast, nkConv:
+      result = checkIsolate(n[1])
+    of nkObjUpConv, nkObjDownConv, nkDotExpr:
+      result = checkIsolate(n[0])
+    of nkStmtList, nkStmtListExpr:
+      if n.len > 0:
+        result = checkIsolate(n[^1])
+      else:
+        result = false
+    else:
+      # unanalysable expression:
+      result = false
+  else:
+    # no ref, no cry:
+    result = true
+