diff --git a/compiler/sem.nim b/compiler/sem.nim
index 0739c6e162c7..b24f974ef7ed 100644
--- a/compiler/sem.nim
+++ b/compiler/sem.nim
@@ -18,7 +18,7 @@ import
   evaltempl, patterns, parampatterns, sempass2, linter, semmacrosanity,
   lowerings, plugins/active, lineinfos, int128,
   isolation_check, typeallowed, modulegraphs, enumtostr, concepts, astmsgs,
-  extccomp, layeredtable
+  extccomp, layeredtable, semderefs
 
 import vtables
 import std/[strtabs, math, tables, intsets, strutils, packedsets]
@@ -821,6 +821,7 @@ proc semStmtAndGenerateGenerics(c: PContext, n: PNode): PNode =
   else:
     result = n
   result = semStmt(c, result, {})
+  result = injectDerefs(c.config, result)
   when false:
     # Code generators are lazy now and can deal with undeclared procs, so these
     # steps are not required anymore and actually harmful for the upcoming
diff --git a/compiler/semderefs.nim b/compiler/semderefs.nim
new file mode 100644
index 000000000000..4b71cd910866
--- /dev/null
+++ b/compiler/semderefs.nim
@@ -0,0 +1,273 @@
+import ast, lineinfos, msgs, options, renderer
+import std/[strutils]
+
+when defined(nimPreviewSlimSystem):
+  import std/assertions
+
+type
+  Expects = enum
+    WantT
+    WantTButSkipDeref
+    WantVarT
+    WantVarTResult
+    WantMutableT  # `var openArray` does need derefs but mutable locations regardless
+    WantForwarding
+
+  CurrentRoutine = object
+    returnExpects: Expects
+    firstParamKind: TTypeKind
+    firstParam: PSym
+    resultSym: PSym
+    routineSym: PSym
+
+  Context = object
+    config: ConfigRef
+    r: CurrentRoutine
+
+proc tr(c: var Context; n: PNode; e: Expects): PNode
+
+proc trSons(c: var Context; n: PNode; e: Expects): PNode =
+  for i in 0 ..< n.safeLen:
+    n[i] = tr(c, n[i], e)
+
+  result = n
+
+proc validBorrowsFrom(c: var Context; n: PNode): bool =
+  # --------------------------------------------------------------------------
+  # We need to borrow from a location that is derived from the proc's
+  # first parameter.
+  # See RFC #7373
+  # --------------------------------------------------------------------------
+  result = false
+  var n = n
+  var someIndirection = false
+  while true:
+    case n.kind
+    of nkDotExpr, nkCheckedFieldExpr, nkBracketExpr, nkObjUpConv, nkObjDownConv:
+      n = n[0]
+    of nkHiddenStdConv, nkHiddenSubConv, nkConv:
+      n = n[1]
+    of nkHiddenDeref, nkHiddenAddr, nkDerefExpr, nkAddr:
+      n = n[0]
+      someIndirection = true
+    of nkStmtList, nkStmtListExpr:
+      if n.len > 0 and n.typ != nil: n = n.lastSon
+      else: break
+    of nkCallKinds:
+      if n.len > 1:
+        let fnType = n[0].typ
+        n = n[1]
+        if fnType != nil and fnType.len > 1:
+          let firstParamType = fnType[1]
+          let mightForward = firstParamType.kind in {tyVar, tyLent}
+          if not mightForward:
+            someIndirection = true
+      else:
+        break
+    else:
+      break
+
+  if n.kind == nkSym:
+    if n.sym.kind == skParam and n.sym == c.r.firstParam:
+      result = c.r.firstParamKind in {tyVar, tyLent} or someIndirection
+    else:
+      # Allow borrow from global or captured variables for backwards compatibility.
+      result = n.sym.owner != c.r.routineSym
+  else:
+    result = false
+
+proc skipToRoot(n: PNode): PNode =
+  var n = n
+  while true:
+    case n.kind
+    of nkDotExpr, nkCheckedFieldExpr, nkBracketExpr, nkObjUpConv, nkObjDownConv:
+      n = n[0]
+    of nkHiddenStdConv, nkHiddenSubConv, nkConv, nkCast:
+      n = n[1]
+    of nkCallKinds:
+      if n.len > 1:
+        n = n[1]
+      else:
+        break
+    else:
+      break
+  result = n
+
+proc borrowsFromReadonly(c: var Context; n: PNode; allowLet = false): bool =
+  let n = skipToRoot(n)
+  if n.kind == nkSym:
+    let tk = n.sym.typ.kind
+    case n.sym.kind:
+    of skConst:
+      result = true
+    of skLet:
+      if allowLet:
+        result = tk == tyLent
+      else:
+        result = tk notin {tyVar, tyLent}
+    of skVar:
+      result = tk == tyLent
+    of skParam:
+      result = tk notin {tyVar, tyLent, tySink}
+    else:
+      result = false
+  elif n.kind in nkLiterals + {nkBracket, nkObjConstr}:
+    result = true
+  else:
+    result = false
+
+proc wantMutable(e: Expects): bool {.inline.} =
+  e in {WantVarT, WantVarTResult, WantMutableT}
+
+proc trProcDecl(c: var Context; n: PNode): PNode =
+  if isGenericRoutine(n):
+    result = n
+  else:
+    var r = CurrentRoutine(returnExpects: WantT)
+    if n[namePos].kind == nkSym:
+      r.routineSym = n[namePos].sym
+      if n[namePos].sym.typ != nil:
+        let fnType = n[namePos].sym.typ
+        if fnType.len > 1:
+          r.firstParamKind = fnType[1].kind
+    if n[paramsPos].safeLen > 1 and n[paramsPos][1].kind == nkIdentDefs:
+      let firstParam = n[paramsPos][1]
+      if firstParam[0].kind == nkSym:
+        r.firstParam = firstParam[0].sym
+    if n.len > resultPos and n[resultPos].kind == nkSym:
+      r.resultSym = n[resultPos].sym
+      if n[resultPos].sym.typ != nil and n[resultPos].sym.typ.kind in {tyVar, tyLent}:
+        r.returnExpects = WantVarTResult
+
+    swap c.r, r
+    result = n
+    result[bodyPos] = tr(c, n[bodyPos], c.r.returnExpects)
+    swap c.r, r
+
+proc trCallArgs(c: var Context; n: PNode; fnType: PType): PNode =
+  result = n
+  var p = 1
+  for i in 1 ..< n.len:
+    var e = WantT
+    if p >= fnType.len:
+      if tfVarargs in fnType.flags:
+        discard "call proc with varargs pragma"
+      else:
+        assert false
+    else:
+      let param = fnType[p]
+      let pk = param.kind
+      if pk in {tyVar, tyLent}:
+        e = WantVarT
+      if pk != tyVarargs:
+        # do not advance formal parameter when it is tyVarargs
+        inc p
+    result[i] = tr(c, n[i], e)
+
+proc firstArgIsMutable(c: var Context; n: PNode): bool =
+  assert n.kind in nkCallKinds
+
+  if n.len > 1:
+    result = not borrowsFromReadonly(c, n[1])
+  else:
+    result = false
+
+proc cannotPassToVar(c: var Context; info: TLineInfo; arg: PNode) =
+  localError(c.config, info, "cannot pass '$1' to var/out T parameter" % [renderTree(arg, {renderNoComments})])
+
+proc trCall(c: var Context; n: PNode; e: Expects): PNode =
+  result = n
+  assert n.len > 0
+  let fnType = if n[0].kind == nkOpenSymChoice: n[0][0].typ else: n[0].typ
+  let retType = if fnType != nil and fnType.len > 0: fnType[0] else: nil
+  result[0] = tr(c, n[0], WantT)
+
+  if fnType == nil:
+    discard
+  elif retType != nil and retType.kind in {tyVar, tyLent}:
+    if e in {WantT, WantForwarding}:
+      result = trCallArgs(c, n, fnType)
+    elif e in {WantVarTResult, WantTButSkipDeref} or firstArgIsMutable(c, n):
+      result = trCallArgs(c, n, fnType)
+    else:
+      cannotPassToVar c, n.info, n
+      result = n
+  elif e.wantMutable:
+    cannotPassToVar c, n.info, n
+  else:
+    result = trCallArgs(c, n, fnType)
+
+type
+  LvalueStatus = enum
+    Valid
+    InvalidBorrow
+    LocationIsConst
+
+proc trAsgn(c: var Context; n: PNode): PNode =
+  var e = WantT
+  var err = Valid
+  if n[0].kind == nkSym and n[0].sym.kind == skResult and n[0].sym == c.r.resultSym:
+    e = c.r.returnExpects
+    if c.r.returnExpects == WantVarTResult:
+      if not validBorrowsFrom(c, n[1]):
+        err = InvalidBorrow
+        discard
+  case err:
+  of InvalidBorrow:
+    localError c.config, n.info, "cannot borrow from " & renderTree(n[1], {renderNoComments})
+  else:
+    result = tr(c, n[1], e)
+  result = n
+
+proc trLocation(c: var Context; n: PNode; e: Expects): PNode =
+  let typ = n.typ
+  if typ != nil:
+    let k = typ.kind
+    if k in {tyVar, tyLent}:
+      if e.wantMutable:
+        # Consider `fvar(returnsVar(someLet))`: We must not allow this.
+        if borrowsFromReadonly(c, n):
+          cannotPassToVar c, n.info, n
+          result = n
+        else:
+          result = trSons(c, n, WantT)
+      else:
+        result = trSons(c, n, WantT)
+    elif e.wantMutable:
+      if e == WantVarTResult:
+        if n.kind == nkSym:
+          result = n
+        else:
+          result = trSons(c, n, WantT)
+      elif borrowsFromReadonly(c, n):
+        cannotPassToVar c, n.info, n
+        result = n
+      else:
+        result = trSons(c, n, WantT)
+    else:
+      result = trSons(c, n, WantT)
+  else:
+    result = n
+
+proc tr(c: var Context; n: PNode; e: Expects): PNode =
+  case n.kind
+  of nkSym:
+    result = trLocation(c, n, e)
+  of nkLiterals:
+    if e.wantMutable:
+      cannotPassToVar c, n.info, n
+    result = n
+  of nkCallKinds:
+    result = trCall(c, n, e)
+  of nkDotExpr, nkCheckedFieldExpr, nkBracketExpr:
+    result = trLocation(c, n, e)
+  of nkAsgn:
+    result = trAsgn(c, n)
+  of nkProcDef, nkFuncDef, nkMethodDef, nkConverterDef, nkIteratorDef:
+    result = trProcDecl(c, n)
+  else:
+    result = trSons(c, n, e)
+
+proc injectDerefs*(config: ConfigRef; n: PNode): PNode =
+  var c  = Context(config: config, r: CurrentRoutine())
+  result = tr(c, n, WantT)