basic 'lent T' test works

compiler/ccgexprs.nim

@@ -414,7 +414,7 @@ proc genDeepCopy(p: BProc; dest, src: TLoc) =
     else:
       linefmt(p, cpsStmts, "$1 = $2;$n", rdLoc(dest), rdLoc(src))
   of tyPointer, tyChar, tyBool, tyEnum, tyCString,
-     tyInt..tyUInt64, tyRange, tyVar:
+     tyInt..tyUInt64, tyRange, tyVar, tyLent:
     linefmt(p, cpsStmts, "$1 = $2;$n", rdLoc(dest), rdLoc(src))
   else: internalError("genDeepCopy: " & $ty.kind)
 
@@ -699,7 +699,7 @@ proc genDeref(p: BProc, e: PNode, d: var TLoc; enforceDeref=false) =
       case typ.kind
       of tyRef:
         d.storage = OnHeap
-      of tyVar:
+      of tyVar, tyLent:
         d.storage = OnUnknown
         if tfVarIsPtr notin typ.flags and p.module.compileToCpp and
             e.kind == nkHiddenDeref:
@@ -1359,9 +1359,9 @@ proc genOf(p: BProc, x: PNode, typ: PType, d: var TLoc) =
   var r = rdLoc(a)
   var nilCheck: Rope = nil
   var t = skipTypes(a.t, abstractInst)
-  while t.kind in {tyVar, tyPtr, tyRef}:
-    if t.kind != tyVar: nilCheck = r
-    if t.kind != tyVar or not p.module.compileToCpp:
+  while t.kind in {tyVar, tyLent, tyPtr, tyRef}:
+    if t.kind notin {tyVar, tyLent}: nilCheck = r
+    if t.kind notin {tyVar, tyLent} or not p.module.compileToCpp:
       r = rfmt(nil, "(*$1)", r)
     t = skipTypes(t.lastSon, typedescInst)
   if not p.module.compileToCpp:
@@ -1723,7 +1723,7 @@ proc genRangeChck(p: BProc, n: PNode, d: var TLoc, magic: string) =
         rope(magic)]), a.storage)
 
 proc genConv(p: BProc, e: PNode, d: var TLoc) =
-  let destType = e.typ.skipTypes({tyVar, tyGenericInst, tyAlias, tySink})
+  let destType = e.typ.skipTypes({tyVar, tyLent, tyGenericInst, tyAlias, tySink})
   if sameBackendType(destType, e.sons[1].typ):
     expr(p, e.sons[1], d)
   else:
@@ -1799,7 +1799,7 @@ proc genMagicExpr(p: BProc, e: PNode, d: var TLoc, op: TMagic) =
                                                "$# = #subInt64($#, $#);$n"]
     const fun: array[mInc..mDec, string] = ["$# = #addInt($#, $#);$n",
                                              "$# = #subInt($#, $#);$n"]
-    let underlying = skipTypes(e.sons[1].typ, {tyGenericInst, tyAlias, tySink, tyVar, tyRange})
+    let underlying = skipTypes(e.sons[1].typ, {tyGenericInst, tyAlias, tySink, tyVar, tyLent, tyRange})
     if optOverflowCheck notin p.options or underlying.kind in {tyUInt..tyUInt64}:
       binaryStmt(p, e, d, opr[op])
     else:

compiler/parampatterns.nim

@@ -222,7 +222,10 @@ proc isAssignable*(owner: PSym, n: PNode; isUnsafeAddr=false): TAssignableResult
     elif compareTypes(n.typ, n.sons[1].typ, dcEqIgnoreDistinct):
       # types that are equal modulo distinction preserve l-value:
       result = isAssignable(owner, n.sons[1], isUnsafeAddr)
-  of nkHiddenDeref, nkDerefExpr, nkHiddenAddr:
+  of nkHiddenDeref:
+    if n[0].typ.kind == tyLent: result = arDiscriminant
+    else: result = arLValue
+  of nkDerefExpr, nkHiddenAddr:
     result = arLValue
   of nkObjUpConv, nkObjDownConv, nkCheckedFieldExpr:
     result = isAssignable(owner, n.sons[0], isUnsafeAddr)

compiler/semexprs.nim

@@ -951,7 +951,7 @@ proc semSym(c: PContext, n: PNode, sym: PSym, flags: TExprFlags): PNode =
     else:
       result = semMacroExpr(c, n, n, s, flags)
   of skTemplate:
-    if efNoEvaluateGeneric in flags and s.ast[genericParamsPos].len > 0 or 
+    if efNoEvaluateGeneric in flags and s.ast[genericParamsPos].len > 0 or
        sfCustomPragma in sym.flags:
       markUsed(n.info, s, c.graph.usageSym)
       styleCheckUse(n.info, s)
@@ -1296,7 +1296,7 @@ proc propertyWriteAccess(c: PContext, n, nOrig, a: PNode): PNode =
     #fixAbstractType(c, result)
     #analyseIfAddressTakenInCall(c, result)
 
-proc takeImplicitAddr(c: PContext, n: PNode): PNode =
+proc takeImplicitAddr(c: PContext, n: PNode; isLent: bool): PNode =
   case n.kind
   of nkHiddenAddr, nkAddr: return n
   of nkHiddenDeref, nkDerefExpr: return n.sons[0]
@@ -1307,7 +1307,7 @@ proc takeImplicitAddr(c: PContext, n: PNode): PNode =
   if valid != arLValue:
     if valid == arLocalLValue:
       localError(n.info, errXStackEscape, renderTree(n, {renderNoComments}))
-    else:
+    elif not isLent:
       localError(n.info, errExprHasNoAddress)
   result = newNodeIT(nkHiddenAddr, n.info, makePtrType(c, n.typ))
   result.add(n)
@@ -1315,9 +1315,9 @@ proc takeImplicitAddr(c: PContext, n: PNode): PNode =
 proc asgnToResultVar(c: PContext, n, le, ri: PNode) {.inline.} =
   if le.kind == nkHiddenDeref:
     var x = le.sons[0]
-    if x.typ.kind == tyVar and x.kind == nkSym and x.sym.kind == skResult:
+    if x.typ.kind in {tyVar, tyLent} and x.kind == nkSym and x.sym.kind == skResult:
       n.sons[0] = x # 'result[]' --> 'result'
-      n.sons[1] = takeImplicitAddr(c, ri)
+      n.sons[1] = takeImplicitAddr(c, ri, x.typ.kind == tyLent)
       x.typ.flags.incl tfVarIsPtr
       #echo x.info, " setting it for this type ", typeToString(x.typ), " ", n.info
 
@@ -1473,18 +1473,18 @@ proc semYieldVarResult(c: PContext, n: PNode, restype: PType) =
     if t.kind == tyVar: t.flags.incl tfVarIsPtr # bugfix for #4048, #4910, #6892
     if n.sons[0].kind in {nkHiddenStdConv, nkHiddenSubConv}:
       n.sons[0] = n.sons[0].sons[1]
-    n.sons[0] = takeImplicitAddr(c, n.sons[0])
+    n.sons[0] = takeImplicitAddr(c, n.sons[0], t.kind == tyLent)
   of tyTuple:
     for i in 0..<t.sonsLen:
       var e = skipTypes(t.sons[i], {tyGenericInst, tyAlias, tySink})
       if e.kind in {tyVar, tyLent}:
         if e.kind == tyVar: e.flags.incl tfVarIsPtr # bugfix for #4048, #4910, #6892
         if n.sons[0].kind == nkPar:
-          n.sons[0].sons[i] = takeImplicitAddr(c, n.sons[0].sons[i])
+          n.sons[0].sons[i] = takeImplicitAddr(c, n.sons[0].sons[i], e.kind == tyLent)
         elif n.sons[0].kind in {nkHiddenStdConv, nkHiddenSubConv} and
              n.sons[0].sons[1].kind == nkPar:
           var a = n.sons[0].sons[1]
-          a.sons[i] = takeImplicitAddr(c, a.sons[i])
+          a.sons[i] = takeImplicitAddr(c, a.sons[i], false)
         else:
           localError(n.sons[0].info, errXExpected, "tuple constructor")
   else: discard

tests/lent/tbasic_lent_check.nim

@@ -0,0 +1,17 @@
+discard """
+  cmd: "nim c --newRuntime $file"
+  output: "1"
+"""
+
+proc viewInto(a: array[4, string]): lent string =
+  result = a[0]
+
+proc passToVar(x: var string) =
+  discard
+
+proc main =
+  let x = ["1", "2", "3", "4"]
+  echo viewInto(x)
+  doAssert(not compiles(passToVar(viewInto(x))))
+
+main()