7 files changed, 127 insertions, 6 deletions
diff --git a/compiler/ast.nim b/compiler/ast.nim
index b57ee542d..d8fe6b1c0 100644
--- a/compiler/ast.nim
+++ b/compiler/ast.nim
@@ -658,7 +658,7 @@ type
     mSwap, mIsNil, mArrToSeq,
     mNewString, mNewStringOfCap, mParseBiggestFloat,
     mMove, mWasMoved, mDestroy,
-    mDefault, mUnown, mAccessEnv, mReset,
+    mDefault, mUnown, mIsolate, mAccessEnv, mReset,
     mArray, mOpenArray, mRange, mSet, mSeq, mVarargs,
     mRef, mPtr, mVar, mDistinct, mVoid, mTuple,
     mOrdinal,
diff --git a/compiler/ccgexprs.nim b/compiler/ccgexprs.nim
index dc9617fe2..003b3e240 100644
--- a/compiler/ccgexprs.nim
+++ b/compiler/ccgexprs.nim
@@ -2209,7 +2209,7 @@ proc genMagicExpr(p: BProc, e: PNode, d: var TLoc, op: TMagic) =
   of mCharToStr: genDollar(p, e, d, "#nimCharToStr($1)")
   of mFloatToStr: genDollar(p, e, d, "#nimFloatToStr($1)")
   of mCStrToStr: genDollar(p, e, d, "#cstrToNimstr($1)")
-  of mStrToStr, mUnown: expr(p, e[1], d)
+  of mStrToStr, mUnown, mIsolate: expr(p, e[1], d)
   of mEnumToStr:
     if optTinyRtti in p.config.globalOptions:
       genEnumToStr(p, e, d)
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
+
diff --git a/compiler/jsgen.nim b/compiler/jsgen.nim
index 57a6b8094..d876531e9 100644
--- a/compiler/jsgen.nim
+++ b/compiler/jsgen.nim
@@ -653,7 +653,7 @@ proc arithAux(p: PProc, n: PNode, r: var TCompRes, op: TMagic) =
     useMagic(p, "nimFloatToString")
     applyFormat "cstrToNimstr(nimFloatToString($1))"
   of mCStrToStr: applyFormat("cstrToNimstr($1)", "cstrToNimstr($1)")
-  of mStrToStr, mUnown: applyFormat("$1", "$1")
+  of mStrToStr, mUnown, mIsolate: applyFormat("$1", "$1")
   else:
     assert false, $op
 
diff --git a/compiler/sem.nim b/compiler/sem.nim
index a657939a5..1f98b4f87 100644
--- a/compiler/sem.nim
+++ b/compiler/sem.nim
@@ -16,7 +16,8 @@ import
   procfind, lookups, pragmas, passes, semdata, semtypinst, sigmatch,
   intsets, transf, vmdef, vm, idgen, aliases, cgmeth, lambdalifting,
   evaltempl, patterns, parampatterns, sempass2, linter, semmacrosanity,
-  lowerings, plugins/active, rod, lineinfos, strtabs, int128
+  lowerings, plugins/active, rod, lineinfos, strtabs, int128,
+  isolation_check
 
 from modulegraphs import ModuleGraph, PPassContext, onUse, onDef, onDefResolveForward
 
diff --git a/compiler/semmagic.nim b/compiler/semmagic.nim
index e7a964d81..6aa440aa9 100644
--- a/compiler/semmagic.nim
+++ b/compiler/semmagic.nim
@@ -558,6 +558,9 @@ proc magicsAfterOverloadResolution(c: PContext, n: PNode,
     let constructed = result[1].typ.base
     if constructed.requiresInit:
       message(c.config, n.info, warnUnsafeDefault, typeToString(constructed))
+  of mIsolate:
+    if not checkIsolate(n[1]):
+      localError(c.config, n.info, "expression cannot be isolated: " & $n[1])
+    result = n
   else:
     result = n
-
diff --git a/compiler/vmgen.nim b/compiler/vmgen.nim
index 52b7b6eb3..4ef510d20 100644
--- a/compiler/vmgen.nim
+++ b/compiler/vmgen.nim
@@ -997,7 +997,7 @@ proc genMagic(c: PCtx; n: PNode; dest: var TDest; m: TMagic) =
     c.genNarrow(n[1], d)
     c.genAsgnPatch(n[1], d)
     c.freeTemp(d)
-  of mOrd, mChr, mArrToSeq, mUnown: c.gen(n[1], dest)
+  of mOrd, mChr, mArrToSeq, mUnown, mIsolate: c.gen(n[1], dest)
   of mNew, mNewFinalize:
     unused(c, n, dest)
     c.genNew(n)