curly brackets for {{hightlight}} conflicts with recent changes in rlang

[filipwastberg]

I'm writing slides in xaringan to show how you can use {{}} for tidy evaluation with rlang 0.4.0.

However, this is also how we can highlight code in xaringan

---
title: "Presentation Ninja"
subtitle: "⚔<br/>with xaringan"
author: "Yihui Xie"
institute: "RStudio, Inc."
date: "2016/12/12 (updated: `r Sys.Date()`)"
output:
  xaringan::moon_reader:
    lib_dir: libs
    nature:
      highlightStyle: github
      highlightLines: true
      countIncrementalSlides: false
---

## How you can use tidy evaluation

```{r message=FALSE}
library(dplyr)
sum_function <- function(df, group) {
  {{df}} %>% 
    group_by({{group}}) %>% 
    summarise(mean_hp = mean(hp, na.rm = T))
}

sum_function(mtcars, cyl)

Which yields:

![Screenshot 2019-07-03 at 11 23 31](https://user-images.githubusercontent.com/20815525/60580233-031db300-9d85-11e9-9b87-7fc0273a2758.png)

Not sure how to properly handle this.

xfun::session_info('xaringan')

> R version 3.6.0 (2019-04-26)

> Platform: x86_64-apple-darwin15.6.0 (64-bit)

> Running under: macOS Mojave 10.14.5

>

> Locale: en_US.UTF-8 / en_US.UTF-8 / en_US.UTF-8 / C / en_US.UTF-8 / en_US.UTF-8

>

> Package version:

> base64enc_0.1.3 BH_1.69.0.1 digest_0.6.19 evaluate_0.13

> glue_1.3.1.9000 graphics_3.6.0 grDevices_3.6.0 highr_0.8

> htmltools_0.3.6 httpuv_1.5.1 jsonlite_1.6 knitr_1.23

> later_0.8.0 magrittr_1.5 markdown_0.9 methods_3.6.0

> mime_0.6 promises_1.0.1 R6_2.4.0 Rcpp_1.0.1

> rlang_0.4.0 rmarkdown_1.12 servr_0.13 stats_3.6.0

> stringi_1.4.3 stringr_1.4.0 tinytex_0.13 tools_3.6.0

> utils_3.6.0 xaringan_0.10.1 xfun_0.7 yaml_2.2.0

<sup>Created on 2019-07-03 by the [reprex package](https://reprex.tidyverse.org) (v0.2.1)</sup>
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[yihui]

Should be fixed now. Thanks for the report!

BTW, please read the issue guide more carefully next time to correctly format your example (which requires four backticks). I have edited it.