spec for view types (#15424)

* spec for view types
* spec additions
* refactoring; there are two different kinds of views
* refactorings and spec additions
* enforce that view types are initialized
* enforce borrowing from the first formal parameter
* enforce lifetimes for borrowing of locals
* typo in the manual
* clarify in the implementation what a borrow operation really is

compiler/ast.nim

@@ -1403,9 +1403,6 @@ proc copyType*(t: PType, owner: PSym, keepId: bool): PType =
 
 proc exactReplica*(t: PType): PType = copyType(t, t.owner, true)
 
-template requiresInit*(t: PType): bool =
-  t.flags * {tfRequiresInit, tfNotNil} != {}
-
 proc copySym*(s: PSym): PSym =
   result = newSym(s.kind, s.name, s.owner, s.info, s.options)
   #result.ast = nil            # BUGFIX; was: s.ast which made problems

compiler/semexprs.nim

@@ -1601,7 +1601,7 @@ proc takeImplicitAddr(c: PContext, n: PNode; isLent: bool): PNode =
 proc asgnToResultVar(c: PContext, n, le, ri: PNode) {.inline.} =
   if le.kind == nkHiddenDeref:
     var x = le[0]
-    if (x.typ.kind in {tyVar, tyLent} or isViewType(x.typ)) and x.kind == nkSym and x.sym.kind == skResult:
+    if (x.typ.kind in {tyVar, tyLent} or classifyViewType(x.typ) != noView) and x.kind == nkSym and x.sym.kind == skResult:
       n[0] = x # 'result[]' --> 'result'
       n[1] = takeImplicitAddr(c, ri, x.typ.kind == tyLent)
       x.typ.flags.incl tfVarIsPtr
@@ -1712,8 +1712,8 @@ proc semAsgn(c: PContext, n: PNode; mode=asgnNormal): PNode =
   if le == nil:
     localError(c.config, a.info, "expression has no type")
   elif (skipTypes(le, {tyGenericInst, tyAlias, tySink}).kind notin {tyVar} and
-        isAssignable(c, a) in {arNone, arLentValue}) or
+        isAssignable(c, a) in {arNone, arLentValue}) or (
-      skipTypes(le, abstractVar).kind in {tyOpenArray, tyVarargs}:
+      skipTypes(le, abstractVar).kind in {tyOpenArray, tyVarargs} and views notin c.features):
     # Direct assignment to a discriminant is allowed!
     localError(c.config, a.info, errXCannotBeAssignedTo %
                renderTree(a, {renderNoComments}))

compiler/sempass2.nim

@@ -10,7 +10,7 @@
 import
   intsets, ast, astalgo, msgs, renderer, magicsys, types, idents, trees,
   wordrecg, strutils, options, guards, lineinfos, semfold, semdata,
-  modulegraphs, varpartitions
+  modulegraphs, varpartitions, typeallowed
 
 when defined(useDfa):
   import dfa
@@ -1244,7 +1244,7 @@ proc trackProc*(c: PContext; s: PSym, body: PNode) =
 
   var mutationInfo = MutationInfo()
   if {strictFuncs, views} * c.features != {}:
-    var partitions = computeGraphPartitions(s, body)
+    var partitions = computeGraphPartitions(s, body, g.config)
     if not t.hasSideEffect and t.hasDangerousAssign:
       t.hasSideEffect = varpartitions.hasSideEffect(partitions, mutationInfo)
     if views in c.features:

compiler/typeallowed.nim

@@ -187,42 +187,77 @@ proc typeAllowed*(t: PType, kind: TSymKind; c: PContext; flags: TTypeAllowedFlag
   var marker = initIntSet()
   result = typeAllowedAux(marker, t, kind, c, flags)
 
-proc isViewTypeAux(marker: var IntSet, t: PType): bool
+type
+  ViewTypeKind* = enum
+    noView, immutableView, mutableView
 
-proc isViewTypeNode(marker: var IntSet, n: PNode): bool =
+proc combine(dest: var ViewTypeKind, b: ViewTypeKind) {.inline.} =
+  case dest
+  of noView, mutableView:
+    dest = b
+  of immutableView:
+    if b == mutableView: dest = b
+
+proc classifyViewTypeAux(marker: var IntSet, t: PType): ViewTypeKind
+
+proc classifyViewTypeNode(marker: var IntSet, n: PNode): ViewTypeKind =
   case n.kind
   of nkSym:
-    result = isViewTypeAux(marker, n.typ)
+    result = classifyViewTypeAux(marker, n.typ)
   of nkOfBranch:
-    result = isViewTypeNode(marker, n.lastSon)
+    result = classifyViewTypeNode(marker, n.lastSon)
   else:
+    result = noView
     for child in n:
-      result = isViewTypeNode(marker, child)
+      result.combine classifyViewTypeNode(marker, child)
-      if result: break
+      if result == mutableView: break
 
-proc isViewTypeAux(marker: var IntSet, t: PType): bool =
+proc classifyViewTypeAux(marker: var IntSet, t: PType): ViewTypeKind =
-  if containsOrIncl(marker, t.id): return false
+  if containsOrIncl(marker, t.id): return noView
   case t.kind
-  of tyVar, tyLent, tyVarargs, tyOpenArray:
+  of tyVar:
-    result = true
+    result = mutableView
+  of tyLent, tyVarargs, tyOpenArray:
+    result = immutableView
   of tyGenericInst, tyDistinct, tyAlias, tyInferred, tySink, tyOwned,
-     tyUncheckedArray, tySequence, tyArray, tyRef, tyStatic, tyFromExpr:
+     tyUncheckedArray, tySequence, tyArray, tyRef, tyStatic:
-    result = isViewTypeAux(marker, lastSon(t))
+    result = classifyViewTypeAux(marker, lastSon(t))
+  of tyFromExpr:
+    if t.len > 0:
+      result = classifyViewTypeAux(marker, lastSon(t))
+    else:
+      result = noView
   of tyTuple:
+    result = noView
     for i in 0..<t.len:
-      result = isViewTypeAux(marker, t[i])
+      result.combine classifyViewTypeAux(marker, t[i])
-      if result: break
+      if result == mutableView: break
   of tyObject:
-    result = false
+    result = noView
     if t.n != nil:
-      result = isViewTypeNode(marker, t.n)
+      result = classifyViewTypeNode(marker, t.n)
     if t[0] != nil:
-      result = result or isViewTypeAux(marker, t[0])
+      result.combine classifyViewTypeAux(marker, t[0])
   else:
     # it doesn't matter what these types contain, 'ptr openArray' is not a
     # view type!
-    result = false
+    result = noView
 
-proc isViewType*(t: PType): bool =
+proc classifyViewType*(t: PType): ViewTypeKind =
   var marker = initIntSet()
-  result = isViewTypeAux(marker, t)
+  result = classifyViewTypeAux(marker, t)
+
+proc directViewType*(t: PType): ViewTypeKind =
+  # does classify 't' without looking recursively into 't'.
+  case t.kind
+  of tyVar:
+    result = mutableView
+  of tyLent, tyOpenArray:
+    result = immutableView
+  of abstractInst-{tyTypeDesc}:
+    result = directViewType(t.lastSon)
+  else:
+    result = noView
+
+proc requiresInit*(t: PType): bool =
+  (t.flags * {tfRequiresInit, tfNotNil} != {}) or classifyViewType(t) != noView

compiler/varpartitions.nim

@@ -8,7 +8,8 @@
 #
 
 ## Partition variables into different graphs. Used for
-## Nim's write tracking and also for the cursor inference.
+## Nim's write tracking, borrow checking and also for the
+## cursor inference.
 ## The algorithm is a reinvention / variation of Steensgaard's
 ## algorithm.
 ## The used data structure is "union find" with path compression.
@@ -23,12 +24,27 @@
 ##     borrowed from locations that are connected to a graph
 ##     that is mutated during the liveness of the cursor.
 ##     (We track all possible mutations of a graph.)
+##
+## See https://nim-lang.github.io/Nim/manual_experimental.html#view-types-algorithm
+## for a high-level description of how borrow checking works.
 
-import ast, types, lineinfos, options, msgs, renderer
+import ast, types, lineinfos, options, msgs, renderer, typeallowed
 from trees import getMagic, whichPragma
 from wordrecg import wNoSideEffect
 from isolation_check import canAlias
-from typeallowed import isViewType
+
+type
+  AbstractTime = distinct int
+
+const
+  MaxTime = AbstractTime high(int)
+  MinTime = AbstractTime(-1)
+
+proc `<=`(a, b: AbstractTime): bool {.borrow.}
+proc `<`(a, b: AbstractTime): bool {.borrow.}
+
+proc inc(x: var AbstractTime; diff = 1) {.borrow.}
+proc dec(x: var AbstractTime; diff = 1) {.borrow.}
 
 type
   SubgraphFlag = enum
@@ -56,21 +72,24 @@ type
     flags: set[VarFlag]
     sym: PSym
     reassignedTo: int
-    aliveStart, aliveEnd: int # the range for which the variable is alive.
+    aliveStart, aliveEnd: AbstractTime # the range for which the variable is alive.
+    borrowsFrom: seq[int] # indexes into Partitions.s
 
   MutationInfo* = object
     param: PSym
     mutatedHere, connectedVia: TLineInfo
     flags: set[SubgraphFlag]
-    maxMutation, minConnection: int
+    maxMutation, minConnection: AbstractTime
-    mutations: seq[int]
+    mutations: seq[AbstractTime]
 
   Partitions* = object
-    abstractTime: int
+    abstractTime: AbstractTime
     s: seq[VarIndex]
     graphs: seq[MutationInfo]
     unanalysableMutation, performCursorInference: bool
     inAsgnSource, inConstructor, inNoSideEffectSection: int
+    owner: PSym
+    config: ConfigRef
 
 proc mutationAfterConnection(g: MutationInfo): bool {.inline.} =
   #echo g.maxMutation, " ", g.minConnection, " ", g.param
@@ -108,7 +127,7 @@ proc variableId(c: Partitions; x: PSym): int =
 proc registerResult(c: var Partitions; n: PNode) =
   if n.kind == nkSym:
     c.s.add VarIndex(con: Connection(kind: isEmptyRoot), sym: n.sym, reassignedTo: 0,
-                      aliveStart: high(int), aliveEnd: c.abstractTime)
+                      aliveStart: MaxTime, aliveEnd: c.abstractTime)
 
 proc registerParam(c: var Partitions; n: PNode) =
   assert n.kind == nkSym
@@ -118,7 +137,7 @@ proc registerParam(c: var Partitions; n: PNode) =
                       aliveStart: c.abstractTime, aliveEnd: c.abstractTime)
     c.graphs.add MutationInfo(param: n.sym, mutatedHere: unknownLineInfo,
                           connectedVia: unknownLineInfo, flags: {connectsConstParam},
-                          maxMutation: -1, minConnection: high(int),
+                          maxMutation: MinTime, minConnection: MaxTime,
                           mutations: @[])
   else:
     c.s.add VarIndex(con: Connection(kind: isEmptyRoot), sym: n.sym, reassignedTo: 0,
@@ -152,7 +171,7 @@ proc potentialMutation(v: var Partitions; s: PSym; info: TLineInfo) =
       v.s[r].con = Connection(kind: isRootOf, graphIndex: v.graphs.len)
       v.graphs.add MutationInfo(param: if isConstParam(s): s else: nil, mutatedHere: info,
                             connectedVia: unknownLineInfo, flags: {isMutated},
-                            maxMutation: v.abstractTime, minConnection: high(int),
+                            maxMutation: v.abstractTime, minConnection: MaxTime,
                             mutations: @[v.abstractTime])
     of isRootOf:
       let g = addr v.graphs[v.s[r].con.graphIndex]
@@ -192,7 +211,7 @@ proc connect(v: var Partitions; a, b: PSym; info: TLineInfo) =
     # for now we always make 'rb' the slave and 'ra' the master:
     var rbFlags: set[SubgraphFlag] = {}
     var mutatedHere = unknownLineInfo
-    var mut = 0
+    var mut = AbstractTime 0
     var con = v.abstractTime
     var gb: ptr MutationInfo = nil
     if v.s[rb].con.kind == isRootOf:
@@ -223,6 +242,80 @@ proc connect(v: var Partitions; a, b: PSym; info: TLineInfo) =
     else:
       assert false, "cannot happen"
 
+proc borrowFromConstExpr(n: PNode): bool =
+  case n.kind
+  of nkCharLit..nkNilLit:
+    result = true
+  of nkExprEqExpr, nkExprColonExpr, nkHiddenStdConv, nkHiddenSubConv,
+      nkCast, nkObjUpConv, nkObjDownConv:
+    result = borrowFromConstExpr(n.lastSon)
+  of nkCurly, nkBracket, nkPar, nkTupleConstr, nkObjConstr, nkClosure, nkRange:
+    result = true
+    for i in ord(n.kind == nkObjConstr)..<n.len:
+      if not borrowFromConstExpr(n[i]): return false
+  of nkCallKinds:
+    if getMagic(n) == mArrToSeq:
+      result = true
+      for i in 1..<n.len:
+        if not borrowFromConstExpr(n[i]): return false
+  else: discard
+
+proc pathExpr(node: PNode; owner: PSym): PNode =
+  #[ From the spec:
+
+  - ``source`` itself is a path expression.
+  - Container access like ``e[i]`` is a path expression.
+  - Tuple access ``e[0]`` is a path expression.
+  - Object field access ``e.field`` is a path expression.
+  - ``system.toOpenArray(e, ...)`` is a path expression.
+  - Pointer dereference ``e[]`` is a path expression.
+  - An address ``addr e``, ``unsafeAddr e`` is a path expression.
+  - A type conversion ``T(e)`` is a path expression.
+  - A cast expression ``cast[T](e)`` is a path expression.
+  - ``f(e, ...)`` is a path expression if ``f``'s return type is a view type.
+    Because the view can only have been borrowed from ``e``, we then know
+    that owner of ``f(e, ...)`` is ``e``.
+
+  Returns the owner of the path expression. Returns ``nil``
+  if it is not a valid path expression.
+  ]#
+  var n = node
+  result = nil
+  while true:
+    case n.kind
+    of nkSym:
+      case n.sym.kind
+      of skParam, skTemp, skResult, skForVar:
+        if n.sym.owner == owner: result = n
+      of skVar:
+        if n.sym.owner == owner or sfThread in n.sym.flags: result = n
+      of skLet, skConst:
+        if n.sym.owner == owner or {sfThread, sfGlobal} * n.sym.flags != {}:
+          result = n
+      else:
+        discard
+      break
+    of nkDotExpr, nkDerefExpr, nkBracketExpr, nkHiddenDeref,
+        nkCheckedFieldExpr, nkAddr, nkHiddenAddr:
+      n = n[0]
+    of nkHiddenStdConv, nkHiddenSubConv, nkConv,  nkCast,
+        nkObjUpConv, nkObjDownConv:
+      n = n.lastSon
+    of nkCallKinds:
+      if n.len > 1:
+        if (n.typ != nil and classifyViewType(n.typ) != noView) or getMagic(n) == mSlice:
+          n = n[1]
+        else:
+          break
+      else:
+        break
+    else:
+      break
+  # borrowFromConstExpr(n) is correct here because we need 'node'
+  # stripped off the path suffixes:
+  if result == nil and borrowFromConstExpr(n):
+    result = n
+
 proc allRoots(n: PNode; result: var seq[PSym]; followDotExpr = true) =
   case n.kind
   of nkSym:
@@ -385,13 +478,57 @@ proc pretendOwnsData(c: var Partitions, s: PSym) =
 const
   explainCursors = false
 
+proc borrowFrom(c: var Partitions; dest: PSym; src: PNode) =
+  const
+    url = "; see https://nim-lang.github.io/Nim/manual_experimental.html#view-types-algorithm-path-expressions for details"
+
+  let s = pathExpr(src, c.owner)
+  if s == nil:
+    localError(c.config, src.info, "cannot borrow from " & $src & ", it is not a path expression; " & url)
+  elif s.kind == nkSym:
+    if dest.kind == skResult:
+      if s.sym.kind != skParam or s.sym.position != 0:
+        localError(c.config, src.info, "'result' must borrow from the first parameter")
+
+    let vid = variableId(c, dest)
+    if vid >= 0:
+      var sourceIdx = variableId(c, s.sym)
+      if sourceIdx < 0:
+        sourceIdx = c.s.len
+        c.s.add VarIndex(con: Connection(kind: isEmptyRoot), sym: s.sym, reassignedTo: 0,
+                        aliveStart: MinTime, aliveEnd: MaxTime)
+
+      c.s[vid].borrowsFrom.add sourceIdx
+  else:
+    discard "a valid borrow location that is a deeply constant expression so we have nothing to track"
+
+
+proc borrowingCall(c: var Partitions; destType: PType; n: PNode; i: int) =
+  let v = pathExpr(n[i], c.owner)
+  if v.kind == nkSym:
+    for j in i+1..<n.len:
+      if getMagic(n[j]) == mSlice:
+        borrowFrom(c, v.sym, n[j])
+  else:
+    localError(c.config, n[i].info, "cannot determine the target of the borrow")
+
+proc trackBorrow(c: var Partitions; dest, src: PNode) =
+  if dest.kind == nkSym:
+    let vk = directViewType(dest.typ)
+    if vk != noView:
+      borrowFrom(c, dest.sym, src)
+
 proc deps(c: var Partitions; dest, src: PNode) =
+  if not c.performCursorInference:
+    trackBorrow(c, dest, src)
+
   var targets, sources: seq[PSym]
   allRoots(dest, targets)
   allRoots(src, sources)
 
   proc wrap(t: PType): bool {.nimcall.} = t.kind in {tyRef, tyPtr}
-  let destIsComplex = types.searchTypeFor(dest.typ, wrap) or isViewType(dest.typ)
+
+  let destIsComplex = types.searchTypeFor(dest.typ, wrap)
 
   for t in targets:
     if dest.kind != nkSym and c.inNoSideEffectSection == 0:
@@ -483,6 +620,13 @@ proc traverse(c: var Partitions; n: PNode) =
               for r in roots: potentialMutation(c, r, it.info)
             for r in roots: noCursor(c, r)
 
+            if not c.performCursorInference:
+              # a call like 'result.add toOpenArray()' can also be a borrow
+              # operation. We know 'paramType' is a tyVar and we really care if
+              # 'paramType[0]' is still a view type, this is not a typo!
+              if directViewType(paramType[0]) == noView and classifyViewType(paramType[0]) != noView:
+                borrowingCall(c, paramType[0], n, i)
+
   of nkAddr, nkHiddenAddr:
     traverse(c, n[0])
     when false:
@@ -614,8 +758,8 @@ proc computeLiveRanges(c: var Partitions; n: PNode) =
   else:
     for child in n: computeLiveRanges(c, child)
 
-proc computeGraphPartitions*(s: PSym; n: PNode; cursorInference = false): Partitions =
+proc computeGraphPartitions*(s: PSym; n: PNode; config: ConfigRef; cursorInference = false): Partitions =
-  result = Partitions(performCursorInference: cursorInference)
+  result = Partitions(performCursorInference: cursorInference, owner: s, config: config)
   if s.kind notin {skModule, skMacro}:
     let params = s.typ.n
     for i in 1..<params.len:
@@ -624,8 +768,8 @@ proc computeGraphPartitions*(s: PSym; n: PNode; cursorInference = false): Partit
       registerResult(result, s.ast[resultPos])
 
   computeLiveRanges(result, n)
-  # resart the timer for the second pass:
+  # restart the timer for the second pass:
-  result.abstractTime = 0
+  result.abstractTime = AbstractTime 0
   traverse(result, n)
 
 proc dangerousMutation(g: MutationInfo; v: VarIndex): bool =
@@ -653,14 +797,24 @@ proc cannotBorrow(config: ConfigRef; s: PSym; g: MutationInfo) =
 
 proc checkBorrowedLocations*(par: var Partitions; body: PNode; config: ConfigRef) =
   for i in 0 ..< par.s.len:
-    let s = par.s[i].sym
+    let v = par.s[i].sym
-    if s.kind != skParam and isViewType(s.typ):
+    if v.kind != skParam and classifyViewType(v.typ) != noView:
       let rid = root(par, i)
-      if par.s[rid].con.kind == isRootOf and dangerousMutation(par.graphs[par.s[rid].con.graphIndex], par.s[i]):
+      if rid >= 0:
-        cannotBorrow(config, s, par.graphs[par.s[rid].con.graphIndex])
+        for b in par.s[rid].borrowsFrom:
+          let sid = root(par, b)
+          if sid >= 0:
+            if par.s[sid].con.kind == isRootOf and dangerousMutation(par.graphs[par.s[sid].con.graphIndex], par.s[i]):
+              cannotBorrow(config, v, par.graphs[par.s[sid].con.graphIndex])
+            if par.s[sid].sym.kind != skParam and par.s[sid].aliveEnd < par.s[rid].aliveEnd:
+              localError(config, v.info, "'" & v.name.s & "' borrows from location '" & par.s[sid].sym.name.s &
+                "' which does not live long enough")
+
+      #if par.s[rid].con.kind == isRootOf and dangerousMutation(par.graphs[par.s[rid].con.graphIndex], par.s[i]):
+      #  cannotBorrow(config, s, par.graphs[par.s[rid].con.graphIndex])
 
 proc computeCursors*(s: PSym; n: PNode; config: ConfigRef) =
-  var par = computeGraphPartitions(s, n, true)
+  var par = computeGraphPartitions(s, n, config, true)
   for i in 0 ..< par.s.len:
     let v = addr(par.s[i])
     if v.flags * {ownsData, preventCursor} == {} and v.sym.kind notin {skParam, skResult} and

doc/manual.rst

@@ -655,7 +655,7 @@ Precedence level    Operators                                              First
 ================  =======================================================  ==================  ===============
 
 
-Whether an operator is used a prefix operator is also affected by preceding
+Whether an operator is used as a prefix operator is also affected by preceding
 whitespace (this parsing change was introduced with version 0.13.0):
 
 .. code-block:: nim

doc/manual_experimental.rst

@@ -1814,18 +1814,49 @@ For example:
     # an object reachable from 'n' is potentially mutated
 
 
-The algorithm behind this analysis is currently not documented.
+The algorithm behind this analysis is described in
+the `view types section <#view-types-algorithm>`_.
 
 
 View types
 ==========
 
-A view type is a type that contains one of the following types:
+**Note**:  ``--experimental:views`` is more effective
+with ``--experimental:strictFuncs``.
+
+A view type is a type that is or contains one of the following types:
 
 - ``var T`` (mutable view into ``T``)
 - ``lent T`` (immutable view into ``T``)
 - ``openArray[T]`` (pair of (pointer to array of ``T``, size))
 
+For example:
+
+.. code-block:: nim
+
+  type
+    View1 = var int
+    View2 = openArray[byte]
+    View3 = lent string
+    View4 = Table[openArray[char], int]
+
+
+Exceptions to this rule are types constructed via ``ptr`` or ``proc``.
+For example, the following types are **not** view types:
+
+.. code-block:: nim
+
+  type
+    NotView1 = proc (x: openArray[int])
+    NotView2 = ptr openArray[char]
+    NotView3 = ptr array[4, var int]
+
+
+A *mutable* view type is a type that is or contains a ``var T`` type.
+An *immutable* view type is a view type that is not a mutable view type.
+
+A *view* is a symbol (a let, var, const, etc.) that has a view type.
+
 Since version 1.4 Nim allows view types to be used as local variables.
 This feature needs to be enabled via ``{.experimental: "views".}``.
 
@@ -1860,10 +1891,159 @@ For example:
   main(@[11, 22, 33])
 
 
+A local variable of a view type can borrow from a location
+derived from a parameter, another local variable, a global ``const`` or ``let``
+symbol or a thread-local ``var`` or ``let``.
 
-If a view type is used as a return type, the location must borrow from the
+Let ``p`` the proc that is analysed for the correctness of the borrow operation.
-first parameter that is passed to the proc.
+
+Let ``source`` be one of:
+
+- A formal parameter of ``p``. Note that this does not cover parameters of
+  inner procs.
+- The ``result`` symbol of ``p``.
+- A local ``var`` or ``let`` or ``const`` of ``p``. Note that this does
+  not cover locals of inner procs.
+- A thread-local ``var`` or ``let``.
+- A global ``let`` or ``const``.
+- A constant arraq/seq/object/tuple constructor.
+
+
+Path expressions
+----------------
+
+A location derived from ``source`` is then defined as a path expression that
+has ``source`` as the owner. A path expression ``e`` is defined recursively:
+
+- ``source`` itself is a path expression.
+- Container access like ``e[i]`` is a path expression.
+- Tuple access ``e[0]`` is a path expression.
+- Object field access ``e.field`` is a path expression.
+- ``system.toOpenArray(e, ...)`` is a path expression.
+- Pointer dereference ``e[]`` is a path expression.
+- An address ``addr e``, ``unsafeAddr e`` is a path expression.
+- A type conversion ``T(e)`` is a path expression.
+- A cast expression ``cast[T](e)`` is a path expression.
+- ``f(e, ...)`` is a path expression if ``f``'s return type is a view type.
+  Because the view can only have been borrowed from ``e``, we then know
+  that owner of ``f(e, ...)`` is ``e``.
+
+
+If a view type is used as a return type, the location must borrow from a location
+that is derived from the first parameter that is passed to the proc.
 See https://nim-lang.org/docs/manual.html#procedures-var-return-type for
 details about how this is done for ``var T``.
 
-The algorithm behind this analysis is currently not documented.
+A mutable view can borrow from a mutable location, an immutable view can borrow
+from both a mutable or an immutable location.
+
+The *duration* of a borrow is the span of commands beginning from the assignment
+to the view and ending with the last usage of the view.
+
+For the duration of the borrow operation, no mutations to the borrowed locations
+may be performed except via the potentially mutable view that borrowed from the
+location. The borrowed location is said to be *sealed* during the borrow.
+
+.. code-block:: nim
+
+  {.experimental: "views".}
+
+  type
+    Obj = object
+      field: string
+
+  proc dangerous(s: var seq[Obj]) =
+    let v: lent Obj = s[0] # seal 's'
+    s.setLen 0  # prevented at compile-time because 's' is sealed.
+    echo v.field
+
+
+The scope of the view does not matter:
+
+.. code-block:: nim
+
+  proc valid(s: var seq[Obj]) =
+    let v: lent Obj = s[0]  # begin of borrow
+    echo v.field            # end of borrow
+    s.setLen 0  # valid because 'v' isn't used afterwards
+
+
+The analysis requires as much precision about mutations as is reasonably obtainable,
+so it is more effective with the experimental `strict funcs <#strict-funcs>`_
+feature. In other words ``--experimental:views`` works better
+with ``--experimental:strictFuncs``.
+
+The analysis is currently control flow insensitive:
+
+.. code-block:: nim
+
+  proc invalid(s: var seq[Obj]) =
+    let v: lent Obj = s[0]
+    if false:
+      s.setLen 0
+    echo v.field
+
+In this example, the compiler assumes that ``s.setLen 0`` invalidates the
+borrow operation of ``v`` even though a human being can easily see that it
+will never do that at runtime.
+
+
+Reborrows
+---------
+
+A view ``v`` can borrow from multiple different locations. However, the borrow
+is always the full span of ``v``'s lifetime and every location that is borrowed
+from is sealed during ``v``'s lifetime.
+
+
+Algorithm
+---------
+
+The following section is an outline of the algorithm that the current implementation
+uses. The algorithm performs two traversals over the AST of the procedure or global
+section of code that uses a view variable. No fixpoint iterations are performed, the
+complexity of the analysis is O(N) where N is the number of nodes of the AST.
+
+The first pass over the AST computes the lifetime of each local variable based on
+a notion of an "abstract time", in the implementation it's a simple integer that is
+incremented for every visited node.
+
+In the second pass information about the underlying object "graphs" is computed.
+Let ``v`` be a parameter or a local variable. Let ``G(v)`` be the graph
+that ``v`` belongs to. A graph is defined by the set of variables that belong
+to the graph. Initially for all ``v``: ``G(v) = {v}``. Every variable can only
+be part of a single graph.
+
+Assignments like ``a = b`` "connect" two variables, both variables end up in the
+same graph ``{a, b} = G(a) = G(b)``. Unfortunately, the pattern to look for is
+much more complex than that and can involve multiple assignment targets
+and sources::
+
+  f(x, y) = g(a, b)
+
+connects ``x`` and ``y`` to ``a`` and ``b``: ``G(x) = G(y) = G(a) = G(b) = {x, y, a, b}``.
+A type based alias analysis rules out some of these combinations, for example
+a ``string`` value cannot possibly be connected to a ``seq[int]``.
+
+A pattern like ``v[] = value`` or ``v.field = value`` marks ``G(v)`` as mutated.
+After the second pass a set of disjoint graphs was computed.
+
+For strict functions it is then enforced that there is no graph that is both mutated
+and has an element that is an immutable parameter (that is a parameter that is not
+of type ``var T``).
+
+For borrow checking a different set of checks is performed. Let ``v`` be the view
+and ``b`` the location that is borrowed from.
+
+- The lifetime of ``v`` must not exceed ``b``'s lifetime. Note: The lifetime of
+  a parameter is the complete proc body.
+- If ``v`` is a mutable view and ``v`` is used to actually mutate the
+  borrowed location, then ``b`` has to be a mutable location.
+  Note: If it is not actually used for mutation, borrowing a mutable view from an
+  immutable location is allowed! This allows for many important idioms and will be
+  justified in an upcoming RFC.
+- During ``v``'s lifetime, ``G(b)`` can only be modified by ``v`` (and only if
+  ``v`` is a mutable view).
+- If ``v`` is ``result`` then ``b`` has to be a location derived from the first
+  formal parameter or from a constant location.
+- A view cannot be used for a read or a write access before it was assigned to.

tests/views/tcannot_borrow.nim

@@ -1,17 +1,22 @@
 discard """
   errormsg: "cannot borrow"
-  nimout: '''tcannot_borrow.nim(16, 7) Error: cannot borrow meh; what it borrows from is potentially mutated
+  nimout: '''tcannot_borrow.nim(21, 7) Error: cannot borrow meh; what it borrows from is potentially mutated
-tcannot_borrow.nim(17, 3) the mutation is here
+tcannot_borrow.nim(22, 3) the mutation is here'''
-tcannot_borrow.nim(16, 7) is the statement that connected the mutation to the parameter'''
+  line: 21
-  line: 16
 """
 
+
 {.experimental: "views".}
 
 type
   Foo = object
     field: string
 
+proc valid(s: var seq[Foo]) =
+  let v: lent Foo = s[0]  # begin of borrow
+  echo v.field            # end of borrow
+  s.setLen 0  # valid because 'v' isn't used afterwards
+
 proc dangerous(s: var seq[Foo]) =
   let meh: lent Foo = s[0]
   s.setLen 0

tests/views/tmust_borrow_from_first_parameter.nim

@@ -0,0 +1,9 @@
+discard """
+  errormsg: "'result' must borrow from the first parameter"
+  line: 9
+"""
+
+{.experimental: "views".}
+
+proc p(a, b: openArray[char]): openArray[char] =
+  result = b

tests/views/tsplit_into_seq.nim

@@ -23,9 +23,8 @@ proc split*(s: string, seps: set[char] = Whitespace,
     while last < len(s) and s[last] notin seps:
       inc(last)
     if splits == 0: last = len(s)
-    {.noSideEffect.}:
+    result.add toOpenArray(s, first, last-1)
-      result.add toOpenArray(s, first, last-1)
+    result.add toOpenArray(s, first, last-1)
-      result.add toOpenArray(s, first, last-1)
     if splits == 0: break
     dec(splits)
     inc(last)

tests/views/tviews1.nim

@@ -4,6 +4,7 @@ discard """
 33
 3
 2
+3
 3'''
   targets: "c cpp"
 """
@@ -22,6 +23,6 @@ proc main(s: seq[int]) =
   take(x.toOpenArray(0, 1))
   let y = x
   take y
-
+  take x
 
 main(@[11, 22, 33])