rule/expr.go
func makePlatformStringsExpr(ps platformStringsExprs) bzl.Expr {
	makeSelect := func(dict *bzl.DictExpr) bzl.Expr {
		return &bzl.CallExpr{
			X:    &bzl.Ident{Name: "select"},
			List: []bzl.Expr{dict},
		}
	}
	forceMultiline := func(e bzl.Expr) {
		switch e := e.(type) {
		case *bzl.ListExpr:
			e.ForceMultiLine = true
		case *bzl.CallExpr:
			e.List[0].(*bzl.DictExpr).ForceMultiLine = true
		}
	}

	var parts []bzl.Expr
	if ps.generic != nil {
		parts = append(parts, ps.generic)
	}
	if ps.os != nil {
		parts = append(parts, makeSelect(ps.os))
	}
	if ps.arch != nil {
		parts = append(parts, makeSelect(ps.arch))
	}
	if ps.platform != nil {
		parts = append(parts, makeSelect(ps.platform))
	}

	if len(parts) == 0 {
		return nil
	}
	if len(parts) == 1 {
		return parts[0]
	}
	expr := parts[0]
	forceMultiline(expr)
	for _, part := range parts[1:] {
		forceMultiline(part)
		expr = &bzl.BinaryExpr{
			Op: "+",
			X:  expr,
			Y:  part,
		}
	}
	return expr
}
func makePlatformStringsExpr(ps platformStringsExprs) bzl.Expr {
	makeSelect := func(dict *bzl.DictExpr) bzl.Expr {
		return &bzl.CallExpr{
			X:    &bzl.Ident{Name: "select"},
			List: []bzl.Expr{dict},
		}
	}
	forceMultiline := func(e bzl.Expr) {
		switch e := e.(type) {
		case *bzl.ListExpr:
			e.ForceMultiLine = true
		case *bzl.CallExpr:
			e.List[0].(*bzl.DictExpr).ForceMultiLine = true
		}
	}

	var parts []bzl.Expr
	if ps.generic != nil {
		parts = append(parts, ps.generic)
	}
	if ps.os != nil {
		parts = append(parts, makeSelect(ps.os))
	}
	if ps.arch != nil {
		parts = append(parts, makeSelect(ps.arch))
	}
	if ps.platform != nil {
		parts = append(parts, makeSelect(ps.platform))
	}

	if len(parts) == 0 {
		return nil
	}
	if len(parts) == 1 {
		return parts[0]
	}
	expr := parts[0]
	forceMultiline(expr)
	for _, part := range parts[1:] {
		forceMultiline(part)
		expr = &bzl.BinaryExpr{
			Op: "+",
			X:  expr,
			Y:  part,
		}
	}
	return expr
}
// makePlatformStringsExpr constructs a single expression from the
// sub-expressions in ps.