implement `ensureMove` (#22339)

* implement `ensureMove`

* use an additional flag

* improve some logics

* progress: fixes discard ensureMove

* forbids nested expressions

* improve error messages

* checkpoint

* fixes cursor

* ADD MORE TESTS

* fixes cursorinference again

* tiny cleanup

* improve error messages

* fixes docs

* implement comments add more tests

* fixes js

14 files changed, 255 insertions, 4 deletions

diff --git a/compiler/ast.nim b/compiler/ast.nim
index 706c0d38f..539b6e954 100644
--- a/compiler/ast.nim
+++ b/compiler/ast.nim
@@ -690,7 +690,7 @@ type
     mIsPartOf, mAstToStr, mParallel,
     mSwap, mIsNil, mArrToSeq, mOpenArrayToSeq,
     mNewString, mNewStringOfCap, mParseBiggestFloat,
-    mMove, mWasMoved, mDup, mDestroy, mTrace,
+    mMove, mEnsureMove, mWasMoved, mDup, mDestroy, mTrace,
     mDefault, mUnown, mFinished, mIsolate, mAccessEnv, mAccessTypeField, mReset,
     mArray, mOpenArray, mRange, mSet, mSeq, mVarargs,
     mRef, mPtr, mVar, mDistinct, mVoid, mTuple,
diff --git a/compiler/ccgexprs.nim b/compiler/ccgexprs.nim
index 2b9f4221f..712b874d9 100644
--- a/compiler/ccgexprs.nim
+++ b/compiler/ccgexprs.nim
@@ -2622,6 +2622,8 @@ proc genMagicExpr(p: BProc, e: PNode, d: var TLoc, op: TMagic) =
   of mAccessTypeField: genAccessTypeField(p, e, d)
   of mSlice: genSlice(p, e, d)
   of mTrace: discard "no code to generate"
+  of mEnsureMove:
+    expr(p, e[1], d)
   else:
     when defined(debugMagics):
       echo p.prc.name.s, " ", p.prc.id, " ", p.prc.flags, " ", p.prc.ast[genericParamsPos].kind
diff --git a/compiler/condsyms.nim b/compiler/condsyms.nim
index c68050449..1146bed14 100644
--- a/compiler/condsyms.nim
+++ b/compiler/condsyms.nim
@@ -157,3 +157,4 @@ proc initDefines*(symbols: StringTableRef) =
   defineSymbol("nimHasSendable")
   defineSymbol("nimAllowNonVarDestructor")
   defineSymbol("nimHasQuirky")
+  defineSymbol("nimHasEnsureMove")
diff --git a/compiler/injectdestructors.nim b/compiler/injectdestructors.nim
index d7f4e38d2..590012806 100644
--- a/compiler/injectdestructors.nim
+++ b/compiler/injectdestructors.nim
@@ -36,6 +36,7 @@ type
     body: PNode
     otherUsage: TLineInfo
     inUncheckedAssignSection: int
+    inEnsureMove: int
 
   Scope = object # we do scope-based memory management.
     # a scope is comparable to an nkStmtListExpr like
@@ -332,6 +333,9 @@ proc genCopyNoCheck(c: var Con; dest, ri: PNode; a: TTypeAttachedOp): PNode =
   assert ri.typ != nil
 
 proc genCopy(c: var Con; dest, ri: PNode; flags: set[MoveOrCopyFlag]): PNode =
+  if c.inEnsureMove > 0:
+    localError(c.graph.config, ri.info, errFailedMove, "cannot move '" & $ri &
+                                                      "', which introduces an implicit copy")
   let t = dest.typ
   if tfHasOwned in t.flags and ri.kind != nkNilLit:
     # try to improve the error message here:
@@ -400,7 +404,7 @@ proc genDefaultCall(t: PType; c: Con; info: TLineInfo): PNode =
 
 proc destructiveMoveVar(n: PNode; c: var Con; s: var Scope): PNode =
   # generate: (let tmp = v; reset(v); tmp)
-  if not hasDestructor(c, n.typ):
+  if (not hasDestructor(c, n.typ)) and c.inEnsureMove == 0:
     assert n.kind != nkSym or not hasDestructor(c, n.sym.typ)
     result = copyTree(n)
   else:
@@ -419,7 +423,8 @@ proc destructiveMoveVar(n: PNode; c: var Con; s: var Scope): PNode =
 
     result.add v
     let nn = skipConv(n)
-    c.genMarkCyclic(result, nn)
+    if hasDestructor(c, n.typ):
+      c.genMarkCyclic(result, nn)
     let wasMovedCall = c.genWasMoved(nn)
     result.add wasMovedCall
     result.add tempAsNode
@@ -462,6 +467,9 @@ proc passCopyToSink(n: PNode; c: var Con; s: var Scope): PNode =
       message(c.graph.config, n.info, hintPerformance,
         ("passing '$1' to a sink parameter introduces an implicit copy; " &
         "if possible, rearrange your program's control flow to prevent it") % $n)
+    if c.inEnsureMove > 0:
+      localError(c.graph.config, n.info, errFailedMove,
+        ("cannot move '$1', passing '$1' to a sink parameter introduces an implicit copy") % $n)
   else:
     if c.graph.config.selectedGC in {gcArc, gcOrc, gcAtomicArc}:
       assert(not containsManagedMemory(n.typ))
@@ -765,7 +773,15 @@ proc p(n: PNode; c: var Con; s: var Scope; mode: ProcessMode; tmpFlags = {sfSing
       result = passCopyToSink(n, c, s)
     elif n.kind in {nkBracket, nkObjConstr, nkTupleConstr, nkClosure, nkNilLit} +
          nkCallKinds + nkLiterals:
-      result = p(n, c, s, consumed)
+      if n.kind in nkCallKinds and n[0].kind == nkSym:
+        if n[0].sym.magic == mEnsureMove:
+          inc c.inEnsureMove
+          result = p(n[1], c, s, sinkArg)
+          dec c.inEnsureMove
+        else:
+          result = p(n, c, s, consumed)
+      else:
+        result = p(n, c, s, consumed)
     elif ((n.kind == nkSym and isSinkParam(n.sym)) or isAnalysableFieldAccess(n, c.owner)) and
         isLastRead(n, c, s) and not (n.kind == nkSym and isCursor(n)):
       # Sinked params can be consumed only once. We need to reset the memory
@@ -837,6 +853,12 @@ proc p(n: PNode; c: var Con; s: var Scope; mode: ProcessMode; tmpFlags = {sfSing
       if mode == normal and isRefConstr:
         result = ensureDestruction(result, n, c, s)
     of nkCallKinds:
+      if n[0].kind == nkSym and n[0].sym.magic == mEnsureMove:
+        inc c.inEnsureMove
+        result = p(n[1], c, s, sinkArg)
+        dec c.inEnsureMove
+        return
+
       let inSpawn = c.inSpawn
       if n[0].kind == nkSym and n[0].sym.magic == mSpawn:
         c.inSpawn.inc
@@ -1069,6 +1091,11 @@ proc genFieldAccessSideEffects(c: var Con; s: var Scope; dest, ri: PNode; flags:
   result = newTree(nkStmtList, v, snk, c.genWasMoved(newAccess))
 
 proc moveOrCopy(dest, ri: PNode; c: var Con; s: var Scope, flags: set[MoveOrCopyFlag] = {}): PNode =
+  var ri = ri
+  var isEnsureMove = 0
+  if ri.kind in nkCallKinds and ri[0].kind == nkSym and ri[0].sym.magic == mEnsureMove:
+    ri = ri[1]
+    isEnsureMove = 1
   if sameLocation(dest, ri):
     # rule (self-assignment-removal):
     result = newNodeI(nkEmpty, dest.info)
@@ -1103,13 +1130,17 @@ proc moveOrCopy(dest, ri: PNode; c: var Con; s: var Scope, flags: set[MoveOrCopy
         else:
           result = c.genSink(s, dest, destructiveMoveVar(ri, c, s), flags)
       else:
+        inc c.inEnsureMove, isEnsureMove
         result = c.genCopy(dest, ri, flags)
+        dec c.inEnsureMove, isEnsureMove
         result.add p(ri, c, s, consumed)
         c.finishCopy(result, dest, isFromSink = false)
     of nkBracket:
       # array constructor
       if ri.len > 0 and isDangerousSeq(ri.typ):
+        inc c.inEnsureMove, isEnsureMove
         result = c.genCopy(dest, ri, flags)
+        dec c.inEnsureMove, isEnsureMove
         result.add p(ri, c, s, consumed)
         c.finishCopy(result, dest, isFromSink = false)
       else:
@@ -1127,7 +1158,9 @@ proc moveOrCopy(dest, ri: PNode; c: var Con; s: var Scope, flags: set[MoveOrCopy
         let snk = c.genSink(s, dest, ri, flags)
         result = newTree(nkStmtList, snk, c.genWasMoved(ri))
       else:
+        inc c.inEnsureMove, isEnsureMove
         result = c.genCopy(dest, ri, flags)
+        dec c.inEnsureMove, isEnsureMove
         result.add p(ri, c, s, consumed)
         c.finishCopy(result, dest, isFromSink = false)
     of nkHiddenSubConv, nkHiddenStdConv, nkConv, nkObjDownConv, nkObjUpConv, nkCast:
@@ -1145,7 +1178,9 @@ proc moveOrCopy(dest, ri: PNode; c: var Con; s: var Scope, flags: set[MoveOrCopy
         let snk = c.genSink(s, dest, ri, flags)
         result = newTree(nkStmtList, snk, c.genWasMoved(ri))
       else:
+        inc c.inEnsureMove, isEnsureMove
         result = c.genCopy(dest, ri, flags)
+        dec c.inEnsureMove, isEnsureMove
         result.add p(ri, c, s, consumed)
         c.finishCopy(result, dest, isFromSink = false)
 
diff --git a/compiler/jsgen.nim b/compiler/jsgen.nim
index ce1fdb1a5..8be4d9d07 100644
--- a/compiler/jsgen.nim
+++ b/compiler/jsgen.nim
@@ -2405,6 +2405,8 @@ proc genMagic(p: PProc, n: PNode, r: var TCompRes) =
     genMove(p, n, r)
   of mDup:
     genDup(p, n, r)
+  of mEnsureMove:
+    gen(p, n[1], r)
   else:
     genCall(p, n, r)
     #else internalError(p.config, e.info, 'genMagic: ' + magicToStr[op]);
diff --git a/compiler/lineinfos.nim b/compiler/lineinfos.nim
index 37adc5660..785b10197 100644
--- a/compiler/lineinfos.nim
+++ b/compiler/lineinfos.nim
@@ -44,6 +44,7 @@ type
     errRstSandboxedDirective,
     errProveInit, # deadcode
     errGenerated,
+    errFailedMove,
     errUser,
     # warnings
     warnCannotOpenFile = "CannotOpenFile", warnOctalEscape = "OctalEscape",
@@ -128,6 +129,7 @@ const
     errRstSandboxedDirective: "disabled directive: '$1'",
     errProveInit: "Cannot prove that '$1' is initialized.",  # deadcode
     errGenerated: "$1",
+    errFailedMove: "$1",
     errUser: "$1",
     warnCannotOpenFile: "cannot open '$1'",
     warnOctalEscape: "octal escape sequences do not exist; leading zero is ignored",
diff --git a/compiler/semmagic.nim b/compiler/semmagic.nim
index ad7e9821b..97a320774 100644
--- a/compiler/semmagic.nim
+++ b/compiler/semmagic.nim
@@ -666,5 +666,9 @@ proc magicsAfterOverloadResolution(c: PContext, n: PNode,
     result = n
     if result.typ != nil and expectedType != nil and result.typ.kind == tySequence and expectedType.kind == tySequence and result.typ[0].kind == tyEmpty:
       result.typ = expectedType # type inference for empty sequence # bug #21377
+  of mEnsureMove:
+    result = n
+    if isAssignable(c, n[1]) notin {arLValue, arLocalLValue}:
+      localError(c.config, n.info, "'" & $n[1] & "'" & " is not a mutable location; it cannot be moved")
   else:
     result = n
diff --git a/compiler/varpartitions.nim b/compiler/varpartitions.nim
index 6598ef508..6290b311f 100644
--- a/compiler/varpartitions.nim
+++ b/compiler/varpartitions.nim
@@ -707,6 +707,10 @@ proc traverse(c: var Partitions; n: PNode) =
     let L = if parameters != nil: parameters.len else: 0
     let m = getMagic(n)
 
+    if m == mEnsureMove and n[1].kind == nkSym:
+      # we know that it must be moved so it cannot be a cursor
+      noCursor(c, n[1].sym)
+
     for i in 1..<n.len:
       let it = n[i]
       if i < L:
diff --git a/compiler/vmgen.nim b/compiler/vmgen.nim
index 50331b971..3a09345ee 100644
--- a/compiler/vmgen.nim
+++ b/compiler/vmgen.nim
@@ -1390,6 +1390,8 @@ proc genMagic(c: PCtx; n: PNode; dest: var TDest; m: TMagic) =
   of mRunnableExamples:
     discard "just ignore any call to runnableExamples"
   of mDestroy, mTrace: discard "ignore calls to the default destructor"
+  of mEnsureMove:
+    gen(c, n[1], dest)
   of mMove:
     let arg = n[1]
     let a = c.genx(arg)
diff --git a/lib/system.nim b/lib/system.nim
index fc8476b40..25133fca5 100644
--- a/lib/system.nim
+++ b/lib/system.nim
@@ -156,6 +156,16 @@ proc move*[T](x: var T): T {.magic: "Move", noSideEffect.} =
   {.cast(raises: []), cast(tags: []).}:
     `=wasMoved`(x)
 
+when defined(nimHasEnsureMove):
+  proc ensureMove*[T](x: T): T {.magic: "EnsureMove", noSideEffect.} =
+    ## Ensures that `x` is moved to the new location, otherwise it gives
+    ## an error at the compile time.
+    runnableExamples:
+      var x = "Hello"
+      let y = ensureMove(x)
+      doAssert y == "Hello"
+    discard "implemented in injectdestructors"
+
 type
   range*[T]{.magic: "Range".}         ## Generic type to construct range types.
   array*[I, T]{.magic: "Array".}      ## Generic type to construct
diff --git a/tests/system/tensuremove.nim b/tests/system/tensuremove.nim
new file mode 100644
index 000000000..980f2ea58
--- /dev/null
+++ b/tests/system/tensuremove.nim
@@ -0,0 +1,130 @@
+discard """
+  target: "c js"
+  matrix: "--cursorinference:on; --cursorinference:off"
+"""
+
+block:
+  type
+    X = object
+      s: string
+
+  proc `=copy`(x: var X, y: X) =
+    x.s = "copied " & y.s
+
+  proc `=sink`(x: var X, y: X) =
+    `=destroy`(x)
+    wasMoved(x)
+    x.s = "moved " & y.s
+
+  proc consume(x: sink X) =
+    discard x.s
+
+  proc main =
+    var x = X(s: "abcdefg")
+    consume(ensureMove x)
+
+  static: main()
+  main()
+
+block:
+  type
+    String = object
+      id: string
+
+  proc hello =
+    var s = String(id: "1")
+    var m = ensureMove s
+    doAssert m.id == "1"
+
+  hello()
+
+block:
+  type
+    String = object
+      id: string
+
+  proc hello =
+    var n = "1"
+    var s = String(id: ensureMove n)
+    var m = ensureMove s
+    doAssert m.id == "1"
+
+  hello()
+
+block:
+  type
+    String = object
+      id: string
+
+  proc hello =
+    var n = "1"
+    var s = [ensureMove n]
+    var m = ensureMove s
+    doAssert m[0] == "1"
+
+  hello()
+
+block:
+  type
+    String = object
+      id: string
+
+  proc hello =
+    var n = "1"
+    var s = @[ensureMove n]
+    var m = ensureMove s
+    doAssert m[0] == "1"
+
+  hello()
+
+block:
+  type
+    String = object
+      id: string
+
+  proc hello =
+    var s = String(id: "1")
+    var m = ensureMove s.id
+    doAssert m == "1"
+
+  hello()
+
+block:
+  proc foo =
+    var x = 1
+    let y = ensureMove x # move
+    when not defined(js):
+      doAssert (y, x) == (1, 0) # (1, 0)
+  foo()
+
+block:
+  proc foo =
+    var x = 1
+    let y = ensureMove x # move
+    doAssert y == 1
+  foo()
+
+block:
+  proc foo =
+    var x = @[1, 2, 3]
+    let y = ensureMove x[0] # move
+    doAssert y == 1
+    when not defined(js):
+      doAssert x == @[0, 2, 3]
+  foo()
+
+block:
+  proc foo =
+    var x = [1, 2, 3]
+    let y = ensureMove x[0] # move
+    doAssert y == 1
+    when not defined(js):
+      doAssert x == @[0, 2, 3]
+  foo()
+
+block:
+  proc foo =
+    var x = @["1", "2", "3"]
+    let y = ensureMove x[0] # move
+    doAssert y == "1"
+  foo()
diff --git a/tests/system/tensuremove1.nim b/tests/system/tensuremove1.nim
new file mode 100644
index 000000000..b7e19c4fb
--- /dev/null
+++ b/tests/system/tensuremove1.nim
@@ -0,0 +1,16 @@
+discard """
+  errormsg: "cannot move 's', which introduces an implicit copy"
+  matrix: "--cursorinference:on; --cursorinference:off"
+"""
+
+type
+  String = object
+    id: string
+
+proc hello =
+  var s = String(id: "1")
+  var m = ensureMove s
+  discard m
+  discard s
+
+hello()
\ No newline at end of file
diff --git a/tests/system/tensuremove2.nim b/tests/system/tensuremove2.nim
new file mode 100644
index 000000000..1fcbc1c0f
--- /dev/null
+++ b/tests/system/tensuremove2.nim
@@ -0,0 +1,15 @@
+discard """
+  errormsg: "'if true: s else: String()' is not a mutable location; it cannot be moved"
+"""
+
+type
+  String = object
+    id: string
+
+proc hello =
+  var s = String(id: "1")
+  var m = ensureMove(if true: s else: String())
+  discard m
+  discard s
+
+hello()
\ No newline at end of file
diff --git a/tests/system/tensuremove3.nim b/tests/system/tensuremove3.nim
new file mode 100644
index 000000000..eca032673
--- /dev/null
+++ b/tests/system/tensuremove3.nim
@@ -0,0 +1,28 @@
+discard """
+  errormsg: "cannot move 'x', passing 'x' to a sink parameter introduces an implicit copy"
+  matrix: "--cursorinference:on; --cursorinference:off"
+"""
+
+block:
+  type
+    X = object
+      s: string
+
+  proc `=copy`(x: var X, y: X) =
+    x.s = "copied " & y.s
+
+  proc `=sink`(x: var X, y: X) =
+    `=destroy`(x)
+    wasMoved(x)
+    x.s = "moved " & y.s
+
+  proc consume(x: sink X) =
+    discard x.s
+
+  proc main =
+    var s = "abcdefg"
+    var x = X(s: ensureMove s)
+    consume(ensureMove x)
+    discard x
+
+  main()
\ No newline at end of file