go/ast, parser: remember short variable decls. w/ correspoding ident objects

The ast.Object's Decl field pointed back to the corresponding declaration for
all but short variable declarations. Now remember corresponding assignment
statement in the Decl field.

Also: simplified some code for parsing select statements.

R=golang-dev, r, bradfitz
CC=golang-dev
https://golang.org/cl/5492072

src/pkg/go/ast/scope.go

@@ -80,7 +80,7 @@ func (s *Scope) String() string {
 type Object struct {
 	Kind ObjKind
 	Name string      // declared name
-	Decl interface{} // corresponding Field, XxxSpec, FuncDecl, or LabeledStmt; or nil
+	Decl interface{} // corresponding Field, XxxSpec, FuncDecl, LabeledStmt, or AssignStmt; or nil
 	Data interface{} // object-specific data; or nil
 	Type interface{} // place holder for type information; may be nil
 }
@@ -125,6 +125,12 @@ func (obj *Object) Pos() token.Pos {
 		if d.Label.Name == name {
 			return d.Label.Pos()
 		}
+	case *AssignStmt:
+		for _, x := range d.Lhs {
+			if ident, isIdent := x.(*Ident); isIdent && ident.Name == name {
+				return ident.Pos()
+			}
+		}
 	}
 	return token.NoPos
 }

src/pkg/go/parser/parser.go

@@ -144,17 +144,17 @@ func (p *parser) declare(decl, data interface{}, scope *ast.Scope, kind ast.ObjK
 	}
 }
 
-func (p *parser) shortVarDecl(idents []*ast.Ident) {
+func (p *parser) shortVarDecl(decl *ast.AssignStmt, list []ast.Expr) {
 	// Go spec: A short variable declaration may redeclare variables
 	// provided they were originally declared in the same block with
 	// the same type, and at least one of the non-blank variables is new.
 	n := 0 // number of new variables
-	for _, ident := range idents {
+	for _, x := range list {
+		if ident, isIdent := x.(*ast.Ident); isIdent {
 			assert(ident.Obj == nil, "identifier already declared or resolved")
 			obj := ast.NewObj(ast.Var, ident.Name)
-		// short var declarations cannot have redeclaration errors
-		// and are not global => no need to remember the respective
-		// declaration
+			// remember corresponding assignment for other tools
+			obj.Decl = decl
 			ident.Obj = obj
 			if ident.Name != "_" {
 				if alt := p.topScope.Insert(obj); alt != nil {
@@ -163,9 +163,12 @@ func (p *parser) shortVarDecl(idents []*ast.Ident) {
 					n++ // new declaration
 				}
 			}
+		} else {
+			p.errorExpected(x.Pos(), "identifier")
+		}
 	}
 	if n == 0 && p.mode&DeclarationErrors != 0 {
-		p.error(idents[0].Pos(), "no new variables on left side of :=")
+		p.error(list[0].Pos(), "no new variables on left side of :=")
 	}
 }
 
@@ -522,7 +525,7 @@ func (p *parser) makeIdentList(list []ast.Expr) []*ast.Ident {
 	for i, x := range list {
 		ident, isIdent := x.(*ast.Ident)
 		if !isIdent {
-			pos := x.(ast.Expr).Pos()
+			pos := x.Pos()
 			p.errorExpected(pos, "identifier")
 			ident = &ast.Ident{pos, "_", nil}
 		}
@@ -1400,10 +1403,11 @@ func (p *parser) parseSimpleStmt(mode int) (ast.Stmt, bool) {
 		} else {
 			y = p.parseRhsList()
 		}
+		as := &ast.AssignStmt{x, pos, tok, y}
 		if tok == token.DEFINE {
-			p.shortVarDecl(p.makeIdentList(x))
+			p.shortVarDecl(as, x)
 		}
-		return &ast.AssignStmt{x, pos, tok, y}, isRange
+		return as, isRange
 	}
 
 	if len(x) > 1 {
@@ -1715,34 +1719,28 @@ func (p *parser) parseCommClause() *ast.CommClause {
 			comm = &ast.SendStmt{lhs[0], arrow, rhs}
 		} else {
 			// RecvStmt
-			pos := p.pos
-			tok := p.tok
-			var rhs ast.Expr
-			if tok == token.ASSIGN || tok == token.DEFINE {
+			if tok := p.tok; tok == token.ASSIGN || tok == token.DEFINE {
 				// RecvStmt with assignment
 				if len(lhs) > 2 {
 					p.errorExpected(lhs[0].Pos(), "1 or 2 expressions")
 					// continue with first two expressions
 					lhs = lhs[0:2]
 				}
+				pos := p.pos
 				p.next()
-				rhs = p.parseRhs()
-				if tok == token.DEFINE && lhs != nil {
-					p.shortVarDecl(p.makeIdentList(lhs))
+				rhs := p.parseRhs()
+				as := &ast.AssignStmt{lhs, pos, tok, []ast.Expr{rhs}}
+				if tok == token.DEFINE {
+					p.shortVarDecl(as, lhs)
 				}
+				comm = as
 			} else {
-				// rhs must be single receive operation
+				// lhs must be single receive operation
 				if len(lhs) > 1 {
 					p.errorExpected(lhs[0].Pos(), "1 expression")
 					// continue with first expression
 				}
-				rhs = lhs[0]
-				lhs = nil // there is no lhs
-			}
-			if lhs != nil {
-				comm = &ast.AssignStmt{lhs, pos, tok, []ast.Expr{rhs}}
-			} else {
-				comm = &ast.ExprStmt{rhs}
+				comm = &ast.ExprStmt{lhs[0]}
 			}
 		}
 	} else {