don't require an implementation for procs marked with .error; activate the move optimizer for destructors

compiler/destroyer.nim

@@ -181,7 +181,7 @@ proc isHarmlessVar*(s: PSym; c: Con): bool =
       if c.g[i].sym == s:
         if defsite < 0: defsite = i
         else: return false
-    of use:
+    of use, useWithinCall:
       if c.g[i].sym == s:
         if defsite < 0: return false
         for j in defsite .. i:
@@ -190,10 +190,11 @@ proc isHarmlessVar*(s: PSym; c: Con): bool =
         # if we want to die after the first 'use':
         if usages > 1: return false
         inc usages
-    of useWithinCall:
-      if c.g[i].sym == s: return false
+    #of useWithinCall:
+    #  if c.g[i].sym == s: return false
     of goto, fork:
       discard "we do not perform an abstract interpretation yet"
+  result = usages <= 1
 
 template interestingSym(s: PSym): bool =
   s.owner == c.owner and s.kind in InterestingSyms and hasDestructor(s.typ)
@@ -222,26 +223,35 @@ proc patchHead(s: PSym) =
   if sfFromGeneric in s.flags:
     patchHead(s.ast[bodyPos])
 
-template genOp(opr, opname) =
+proc checkForErrorPragma(c: Con; t: PType; ri: PNode; opname: string) =
+  var m = "'" & opname & "' is not available for type <" & typeToString(t) & ">"
+  if opname == "=" and ri != nil:
+    m.add "; requires a copy because it's not the last read of '"
+    m.add renderTree(ri)
+    m.add '\''
+  localError(c.graph.config, ri.info, errGenerated, m)
+
+template genOp(opr, opname, ri) =
   let op = opr
   if op == nil:
     globalError(c.graph.config, dest.info, "internal error: '" & opname & "' operator not found for type " & typeToString(t))
   elif op.ast[genericParamsPos].kind != nkEmpty:
     globalError(c.graph.config, dest.info, "internal error: '" & opname & "' operator is generic")
   patchHead op
+  if sfError in op.flags: checkForErrorPragma(c, t, ri, opname)
   result = newTree(nkCall, newSymNode(op), newTree(nkHiddenAddr, dest))
 
-proc genSink(c: Con; t: PType; dest: PNode): PNode =
+proc genSink(c: Con; t: PType; dest, ri: PNode): PNode =
   let t = t.skipTypes({tyGenericInst, tyAlias, tySink})
-  genOp(if t.sink != nil: t.sink else: t.assignment, "=sink")
+  genOp(if t.sink != nil: t.sink else: t.assignment, "=sink", ri)
 
-proc genCopy(c: Con; t: PType; dest: PNode): PNode =
+proc genCopy(c: Con; t: PType; dest, ri: PNode): PNode =
   let t = t.skipTypes({tyGenericInst, tyAlias, tySink})
-  genOp(t.assignment, "=")
+  genOp(t.assignment, "=", ri)
 
 proc genDestroy(c: Con; t: PType; dest: PNode): PNode =
   let t = t.skipTypes({tyGenericInst, tyAlias, tySink})
-  genOp(t.destructor, "=destroy")
+  genOp(t.destructor, "=destroy", nil)
 
 proc addTopVar(c: var Con; v: PNode) =
   c.topLevelVars.add newTree(nkIdentDefs, v, c.emptyNode, c.emptyNode)
@@ -291,33 +301,33 @@ proc genMagicCall(n: PNode; c: var Con; magicname: string; m: TMagic): PNode =
 
 proc moveOrCopy(dest, ri: PNode; c: var Con): PNode =
   if ri.kind in constrExprs:
-    result = genSink(c, dest.typ, dest)
+    result = genSink(c, dest.typ, dest, ri)
     # watch out and no not transform 'ri' twice if it's a call:
     let ri2 = copyNode(ri)
     recurse(ri, ri2)
     result.add ri2
-  elif ri.kind == nkSym and isHarmlessVar(ri.sym, c):
+  elif ri.kind == nkSym and ri.sym.kind != skParam and isHarmlessVar(ri.sym, c):
     # Rule 3: `=sink`(x, z); wasMoved(z)
-    var snk = genSink(c, dest.typ, dest)
+    var snk = genSink(c, dest.typ, dest, ri)
     snk.add p(ri, c)
     result = newTree(nkStmtList, snk, genMagicCall(ri, c, "wasMoved", mWasMoved))
   elif ri.kind == nkSym and isSinkParam(ri.sym):
-    result = genSink(c, dest.typ, dest)
+    result = genSink(c, dest.typ, dest, ri)
     result.add destructiveMoveSink(ri, c)
   else:
-    result = genCopy(c, dest.typ, dest)
+    result = genCopy(c, dest.typ, dest, ri)
     result.add p(ri, c)
 
 proc passCopyToSink(n: PNode; c: var Con): PNode =
   result = newNodeIT(nkStmtListExpr, n.info, n.typ)
   let tmp = getTemp(c, n.typ, n.info)
   if hasDestructor(n.typ):
-    var m = genCopy(c, n.typ, tmp)
+    var m = genCopy(c, n.typ, tmp, n)
     m.add p(n, c)
     result.add m
     message(c.graph.config, n.info, hintPerformance,
-      "passing '$1' to a sink parameter introduces an implicit copy; " &
-      "use 'move($1)' to prevent it" % $n)
+      ("passing '$1' to a sink parameter introduces an implicit copy; " &
+      "use 'move($1)' to prevent it") % $n)
   else:
     result.add newTree(nkAsgn, tmp, p(n, c))
   result.add tmp
@@ -331,6 +341,7 @@ proc destructiveMoveVar(n: PNode; c: var Con): PNode =
   result = newNodeIT(nkStmtListExpr, n.info, n.typ)
 
   var temp = newSym(skLet, getIdent(c.graph.cache, "blitTmp"), c.owner, n.info)
+  temp.typ = n.typ
   var v = newNodeI(nkLetSection, n.info)
   let tempAsNode = newSymNode(temp)
 
@@ -410,7 +421,7 @@ proc p(n: PNode; c: var Con): PNode =
       discard "produce temp creation"
       result = newNodeIT(nkStmtListExpr, n.info, n.typ)
       let tmp = getTemp(c, n.typ, n.info)
-      var sinkExpr = genSink(c, n.typ, tmp)
+      var sinkExpr = genSink(c, n.typ, tmp, n)
       sinkExpr.add n
       result.add sinkExpr
       result.add tmp

compiler/dfa.nim

@@ -442,4 +442,5 @@ proc dataflowAnalysis*(s: PSym; body: PNode; conf: ConfigRef) =
 proc constructCfg*(s: PSym; body: PNode): ControlFlowGraph =
   ## constructs a control flow graph for ``body``.
   var c = Con(code: @[], blocks: @[])
+  gen(c, body)
   shallowCopy(result, c.code)

compiler/pragmas.nim

@@ -961,6 +961,7 @@ proc singlePragma(c: PContext, sym: PSym, n: PNode, i: var int,
           # ``proc p() {.error}`` and ``proc p() = {.error: "msg".}``
           if it.kind in nkPragmaCallKinds: discard getStrLitNode(c, it)
           incl(sym.flags, sfError)
+          excl(sym.flags, sfForward)
         else:
           let s = expectStrLit(c, it)
           recordPragma(c, it, "error", s)

compiler/semstmts.nim

@@ -1677,7 +1677,7 @@ proc semProcAux(c: PContext, n: PNode, kind: TSymKind,
   else:
     if s.kind == skMethod: semMethodPrototype(c, s, n)
     if proto != nil: localError(c.config, n.info, errImplOfXexpected % proto.name.s)
-    if {sfImportc, sfBorrow} * s.flags == {} and s.magic == mNone:
+    if {sfImportc, sfBorrow, sfError} * s.flags == {} and s.magic == mNone:
       incl(s.flags, sfForward)
     elif sfBorrow in s.flags: semBorrow(c, n, s)
   sideEffectsCheck(c, s)

tests/destructor/tprevent_assign.nim

@@ -0,0 +1,33 @@
+discard """
+  errormsg: "'=' is not available for type <Foo>; requires a copy because it's not the last read of 'otherTree'"
+  line: 29
+"""
+
+type
+  Foo = object
+    x: int
+
+proc `=destroy`(f: var Foo) = f.x = 0
+proc `=`(a: var Foo; b: Foo) {.error.} # = a.x = b.x
+proc `=sink`(a: var Foo; b: Foo) = a.x = b.x
+
+proc createTree(x: int): Foo =
+  Foo(x: x)
+
+proc take2(a, b: sink Foo) =
+  echo a.x, " ", b.x
+
+proc allowThis() =
+  # all these temporary lets are harmless:
+  let otherTree = createTree(44)
+  let b = otherTree
+  let c = b
+  take2(createTree(34), c)
+
+proc preventThis() =
+  let otherTree = createTree(44)
+  let b = otherTree
+  take2(createTree(34), otherTree)
+
+allowThis()
+preventThis()