Implements #2154.

When unpacking tuples in var/let declarations a part of the tuple can now
be discarded using a single underscore.

compiler/ast.nim

@@ -221,6 +221,7 @@ type
     nkGotoState,          # used for the state machine (for iterators)
     nkState,              # give a label to a code section (for iterators)
     nkBreakState,         # special break statement for easier code generation
+    nkUnderscore,         # underscore inside a tuple unpack ``(_, x) = foo()``
   TNodeKinds* = set[TNodeKind]
 
 type

compiler/docgen.nim

@@ -384,7 +384,8 @@ proc genItem(d: PDoc, n, nameNode: PNode, k: TSymKind) =
        tkBracketDotLe, tkBracketDotRi, tkCurlyDotLe, tkCurlyDotRi, tkParDotLe,
        tkParDotRi, tkComma, tkSemiColon, tkColon, tkEquals, tkDot, tkDotDot,
        tkAccent, tkColonColon,
-       tkGStrLit, tkGTripleStrLit, tkInfixOpr, tkPrefixOpr, tkPostfixOpr:
+       tkGStrLit, tkGTripleStrLit, tkInfixOpr, tkPrefixOpr, tkPostfixOpr,
+       tkUnderscore:
       dispA(result, "<span class=\"Other\">$1</span>", "\\spanOther{$1}",
             [toRope(esc(d.target, literal))])
   inc(d.id)

compiler/lexer.nim

@@ -62,6 +62,7 @@ type
     tkColon, tkColonColon, tkEquals, tkDot, tkDotDot,
     tkOpr, tkComment, tkAccent,
     tkSpaces, tkInfixOpr, tkPrefixOpr, tkPostfixOpr,
+    tkUnderscore
 
   TTokTypes* = set[TTokType]
 
@@ -96,7 +97,7 @@ const
     ":", "::", "=", ".", "..",
     "tkOpr", "tkComment", "`",
     "tkSpaces", "tkInfixOpr",
-    "tkPrefixOpr", "tkPostfixOpr"]
+    "tkPrefixOpr", "tkPostfixOpr", "_"]
 
 type
   TNumericalBase* = enum
@@ -874,6 +875,9 @@ proc rawGetTok(L: var TLexer, tok: var TToken) =
     of '`':
       tok.tokType = tkAccent
       inc(L.bufpos)
+    of '_':
+      tok.tokType = tkUnderscore
+      inc(L.bufpos)
     of '\"':
       # check for extended raw string literal:
       var rawMode = L.bufpos > 0 and L.buf[L.bufpos-1] in SymChars

compiler/lookups.nim

@@ -20,6 +20,7 @@ proc considerQuotedIdent*(n: PNode): PIdent =
   case n.kind
   of nkIdent: result = n.ident
   of nkSym: result = n.sym.name
+  of nkUnderscore: result = getIdent"_"
   of nkAccQuoted:
     case n.len
     of 0:

compiler/parser.nim

@@ -296,12 +296,15 @@ proc colcom(p: var TParser, n: PNode) =
   skipComment(p, n)
 
 proc parseSymbol(p: var TParser, allowNil = false): PNode =
-  #| symbol = '`' (KEYW|IDENT|literal|(operator|'('|')'|'['|']'|'{'|'}'|'=')+)+ '`'
+  #| symbol = '`' (KEYW|IDENT|literal|(operator|'('|')'|'['|']'|'{'|'}'|'='|'_')+)+ '`'
   #|        | IDENT | 'addr' | 'type'
   case p.tok.tokType
   of tkSymbol, tkAddr, tkType:
     result = newIdentNodeP(p.tok.ident, p)
     getTok(p)
+  of tkUnderscore:
+    result = newNodeP(nkUnderscore, p)
+    getTok(p)
   of tkAccent:
     result = newNodeP(nkAccQuoted, p)
     getTok(p)
@@ -643,6 +646,9 @@ proc identOrLiteral(p: var TParser, mode: TPrimaryMode): PNode =
   of tkNil:
     result = newNodeP(nkNilLit, p)
     getTok(p)
+  of tkUnderscore:
+    result = newNodeP(nkUnderscore, p)
+    getTok(p)
   of tkParLe:
     # () constructor
     if mode in {pmTypeDesc, pmTypeDef}:
@@ -1814,7 +1820,7 @@ proc parseVarTuple(p: var TParser): PNode =
   result = newNodeP(nkVarTuple, p)
   getTok(p)                   # skip '('
   optInd(p, result)
-  while p.tok.tokType in {tkSymbol, tkAccent}:
+  while p.tok.tokType in {tkSymbol, tkAccent, tkUnderscore}:
     var a = identWithPragma(p)
     addSon(result, a)
     if p.tok.tokType != tkComma: break

compiler/renderer.nim

@@ -349,6 +349,7 @@ proc atom(n: PNode): string =
     else:
       result = litAux(n, (cast[PInt64](addr(n.floatVal)))[], 8) & "\'f64"
   of nkNilLit: result = "nil"
+  of nkUnderscore: result = "_"
   of nkType:
     if (n.typ != nil) and (n.typ.sym != nil): result = n.typ.sym.name.s
     else: result = "[type node]"
@@ -805,7 +806,7 @@ proc gsub(g: var TSrcGen, n: PNode, c: TContext) =
   of nkTripleStrLit: putRawStr(g, tkTripleStrLit, n.strVal)
   of nkEmpty: discard
   of nkType: put(g, tkInvalid, atom(n))
-  of nkSym, nkIdent: gident(g, n)
+  of nkSym, nkIdent, nkUnderscore: gident(g, n)
   of nkIntLit: put(g, tkIntLit, atom(n))
   of nkInt8Lit: put(g, tkInt8Lit, atom(n))
   of nkInt16Lit: put(g, tkInt16Lit, atom(n))

compiler/semstmts.nim

@@ -403,7 +403,8 @@ proc semVarOrLet(c: PContext, n: PNode, symkind: TSymKind): PNode =
 
     for j in countup(0, length-3):
       var v = semIdentDef(c, a.sons[j], symkind)
-      if sfGenSym notin v.flags: addInterfaceDecl(c, v)
+      if sfGenSym notin v.flags and
+         a.sons[j].kind != nkUnderscore: addInterfaceDecl(c, v)
       when oKeepVariableNames:
         if c.inUnrolledContext > 0: v.flags.incl(sfShadowed)
         else:

tests/parser/ttupleunpack.nim

@@ -0,0 +1,18 @@
+discard """
+  file: "ttupleunpack.nim"
+  output: ""
+  exitcode: 0
+"""
+proc foo(): tuple[x, y, z: int] =
+  return (4, 2, 3)
+
+var (x, _, y) = foo()
+doAssert x == 4
+doAssert y == 3
+
+iterator bar(): tuple[x, y, z: int] =
+  yield (1,2,3)
+
+for x, y, _ in bar():
+  doAssert x == 1
+  doAssert y == 2