Const tuple unpacking (#9964)

* tuple unpacking is now supported for consts

* Move nkConstTuple to the end of TNodeKind

* Add nnkConstTuple in macros.nim

* Fix Formatting

compiler/ast.nim

@@ -222,7 +222,8 @@ type
     nkState,              # give a label to a code section (for iterators)
     nkBreakState,         # special break statement for easier code generation
     nkFuncDef,            # a func
-    nkTupleConstr         # a tuple constructor
+    nkTupleConstr,        # a tuple constructor
+    nkConstTuple          # a ``const (a, b) = expr`` construct
 
   TNodeKinds* = set[TNodeKind]
 

compiler/parser.nim

@@ -1764,8 +1764,30 @@ proc parseSection(p: var TParser, kind: TNodeKind,
   else:
     parMessage(p, errIdentifierExpected, p.tok)
 
+proc parseConstTuple(p: var TParser): PNode =
+  result = newNodeP(nkConstTuple, p)
+  getTok(p)                   # skip '('
+  optInd(p, result)
+  while p.tok.tokType in {tkSymbol, tkAccent}:
+    addSon(result, identWithPragma(p))
+    if p.tok.tokType == tkColon:
+      getTok(p)
+      optInd(p, result)
+      addSon(result, parseTypeDesc(p))
+    if p.tok.tokType != tkComma: break
+    getTok(p)
+
+  addSon(result, p.emptyNode)
+  eat(p, tkParRi)
+  eat(p, tkEquals)
+  optInd(p, result)
+  addSon(result, parseExpr(p))
+  indAndComment(p, result)
+
 proc parseConstant(p: var TParser): PNode =
   #| constant = identWithPragma (colon typeDesc)? '=' optInd expr indAndComment
+  if p.tok.tokType == tkParLe: result = parseConstTuple(p)
+  else:
     result = newNodeP(nkConstDef, p)
     addSon(result, identWithPragma(p))
     if p.tok.tokType == tkColon:

compiler/semstmts.nim

@@ -542,17 +542,17 @@ proc semConst(c: PContext, n: PNode): PNode =
     var a = n.sons[i]
     if c.config.cmd == cmdIdeTools: suggestStmt(c, a)
     if a.kind == nkCommentStmt: continue
-    if a.kind != nkConstDef: illFormedAst(a, c.config)
+    if a.kind notin {nkConstDef, nkConstTuple}: illFormedAst(a, c.config)
-    checkSonsLen(a, 3, c.config)
+    checkMinSonsLen(a, 3, c.config)
-    var v = semIdentDef(c, a.sons[0], skConst)
+    var length = sonsLen(a)
-    styleCheckDef(c.config, v)
-    onDef(a[0].info, v)
-    var typ: PType = nil
-    if a.sons[1].kind != nkEmpty: typ = semTypeNode(c, a.sons[1], nil)
 
-    var def = semConstExpr(c, a.sons[2])
+    var typ: PType = nil
+    if a.sons[length-2].kind != nkEmpty:
+      typ = semTypeNode(c, a.sons[length-2], nil)
+
+    var def = semConstExpr(c, a.sons[length-1])
     if def == nil:
-      localError(c.config, a.sons[2].info, errConstExprExpected)
+      localError(c.config, a.sons[length-1].info, errConstExprExpected)
       continue
     # check type compatibility between def.typ and typ:
     if typ != nil:
@@ -560,20 +560,42 @@ proc semConst(c: PContext, n: PNode): PNode =
     else:
       typ = def.typ
     if typ == nil:
-      localError(c.config, a.sons[2].info, errConstExprExpected)
+      localError(c.config, a.sons[length-1].info, errConstExprExpected)
       continue
     if typeAllowed(typ, skConst) != nil and def.kind != nkNilLit:
       localError(c.config, a.info, "invalid type for const: " & typeToString(typ))
       continue
-    setVarType(c, v, typ)
+
-    v.ast = def               # no need to copy
+    var b: PNode
+    if a.kind == nkConstTuple:
+      if typ.kind != tyTuple:
+        localError(c.config, a.info, errXExpected, "tuple")
+      elif int(length/2) != sonsLen(typ):
+        localError(c.config, a.info, errWrongNumberOfVariables)
+      b = newNodeI(nkConstTuple, a.info)
+      newSons(b, length)
+      b.sons[length-2] = a.sons[length-2]
+      b.sons[length-1] = def
+  
+    for j in countup(0, length-3):
+      var v = semIdentDef(c, a.sons[j], skConst)
       if sfGenSym notin v.flags: addInterfaceDecl(c, v)
       elif v.owner == nil: v.owner = getCurrOwner(c)
-    var b = newNodeI(nkConstDef, a.info)
+      styleCheckDef(c.config, v)
+      onDef(a[j].info, v)
+
+      if a.kind != nkConstTuple:
+          setVarType(c, v, typ)
+          v.ast = def               # no need to copy
+          b = newNodeI(nkConstDef, a.info)
           if importantComments(c.config): b.comment = a.comment
           addSon(b, newSymNode(v))
           addSon(b, a.sons[1])
           addSon(b, copyTree(def))
+      else:
+        setVarType(c, v, typ.sons[j])
+        v.ast = def[j]
+        b.sons[j] = newSymNode(v)
     addSon(result,b)
 
 include semfields

lib/core/macros.nim

@@ -81,7 +81,8 @@ type
     nnkState,
     nnkBreakState,
     nnkFuncDef,
-    nnkTupleConstr
+    nnkTupleConstr,
+    nnkConstTuple
 
   NimNodeKinds* = set[NimNodeKind]
   NimTypeKind* = enum  # some types are no longer used, see ast.nim