out parameters: enforce that 'out' is only used as a parameter (#20510)

* out parameters: enforce that 'out' is only used as a parameter

* make tables.nim use 'out' parameters

* better backwards compat

compiler/typeallowed.nim

@@ -65,6 +65,8 @@ proc typeAllowedAux(marker: var IntSet, typ: PType, kind: TSymKind,
     elif t.kind == tyLent and ((kind != skResult and views notin c.features) or
                               kind == skParam): # lent can't be used as parameters.
       result = t
+    elif isOutParam(t) and kind != skParam:
+      result = t
     else:
       var t2 = skipTypes(t[0], abstractInst-{tyTypeDesc, tySink})
       case t2.kind

lib/core/macros.nim

@@ -75,7 +75,7 @@ type
     nnkTupleTy, nnkTupleClassTy, nnkTypeClassTy, nnkStaticTy,
     nnkRecList, nnkRecCase, nnkRecWhen,
     nnkRefTy, nnkPtrTy, nnkVarTy,
-    nnkConstTy, nnkMutableTy,
+    nnkConstTy, nnkOutTy,
     nnkDistinctTy,
     nnkProcTy,
     nnkIteratorTy,         # iterator type
@@ -125,6 +125,9 @@ type
 
   TNimSymKinds* {.deprecated.} = set[NimSymKind]
 
+const
+  nnkMutableTy* {.deprecated.} = nnkOutTy
+
 type
   NimIdent* {.deprecated.} = object of RootObj
     ## Represents a Nim identifier in the AST. **Note**: This is only

lib/pure/collections/hashcommon.nim

@@ -13,6 +13,7 @@
 when defined(nimPreviewSlimSystem):
   import std/assertions
 
+import std / outparams
 
 const
   growthFactor = 2
@@ -78,5 +79,5 @@ template rawGetImpl() {.dirty.} =
   genHashImpl(key, hc)
   rawGetKnownHCImpl()
 
-proc rawGet[X, A](t: X, key: A, hc: var Hash): int {.inline.} =
+proc rawGet[X, A](t: X, key: A, hc: var Hash): int {.inline, outParamsAt: [3].} =
   rawGetImpl()

lib/pure/collections/tableimpl.nim

@@ -23,7 +23,7 @@ template rawInsertImpl() {.dirty.} =
   data[h].val = val
   data[h].hcode = hc
 
-proc rawGetDeep[X, A](t: X, key: A, hc: var Hash): int {.inline.} =
+proc rawGetDeep[X, A](t: X, key: A, hc: var Hash): int {.inline, outParamsAt: [3].} =
   rawGetDeepImpl()
 
 proc rawInsert[X, A, B](t: var X, data: var KeyValuePairSeq[A, B],

lib/std/outparams.nim

@@ -0,0 +1,38 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2022 Nim contributors
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+## `outParamsAt` macro for easy writing code that works with both 2.0 and 1.x.
+
+import macros
+
+macro outParamsAt*(positions: static openArray[int]; n: untyped): untyped =
+  ## Use this macro to annotate `out` parameters in a portable way.
+  runnableExamples:
+    proc p(x: var int) {.outParamsAt: [1].} =
+      discard "x is really an 'out int' if the Nim compiler supports 'out' parameters"
+
+  result = n
+  when defined(nimHasOutParams):
+    var p = n.params
+    for po in positions:
+      p[po][^2].expectKind nnkVarTy
+      p[po][^2] = newTree(nnkOutTy, p[po][^2][0])
+
+when isMainModule:
+  {.experimental: "strictDefs".}
+
+  proc main(x: var int) {.outParamsAt: [1].} =
+    x = 3
+
+  proc us =
+    var x: int
+    main x
+    echo x
+
+  us()