Using glue-like constructs on RHS in R/Tidyeval

I've spent hours trying to make glue on the RHS of a formula work and out of clues. Here is a simple reprex.

meta <- function(x, var, suffix){
  x<- x %>% mutate("{{var}}_{suffix}":= 5)
  
  x<- x %>% mutate("{{var}}_{suffix}_new":= {{var}} - "{{var}}_{suffix}")
  
  
}

x<- meta(mtcars, mpg, suf)

#Should be equivalent to

x<- mtcars %>% mutate(mpg_suf:= 5)

x<- x%>% mutate(mpg_suf_new:= mpg - mpg_suf)

#N: Tried https://stackoverflow.com/questions/70427403/how-to-correctly-glue-together-prefix-suffix-in-a-function-call-rhs but none of the methods in it worked, unfortunately

Meta function gives me "Error in local_error_context(dots = dots, .index = i, mask = mask) : promise already under evaluation: recursive default argument reference or earlier problems? "

Went over all hits for the searchwords for it on SO but nothing worked at the moment.

Would really appreciate any insights. Thank you!

Solution

Here is a working version:

meta <- function(x, var, suffix){
  new_name <- rlang::englue("{{ var }}_{{ suffix }}")
  x %>%
    mutate("{new_name}" := 5) %>%
    mutate("{new_name}_new" := {{ var }} - .data[[new_name]])
}

names(meta(mtcars, mpg, suf))
#>  [1] "mpg"         "cyl"         "disp"        "hp"
#>  [5] "drat"        "wt"          "qsec"        "vs"
#>  [9] "am"          "gear"        "carb"        "mpg_suf"
#> [13] "mpg_suf_new"

To understand what is going on:

Learn about the difference between "{{ var }}" and "{var}" in tidyeval glue strings: https://rlang.r-lib.org/reference/glue-operators.html
Learn about englue() to create glue strings outside of the LHS of :=: https://rlang.r-lib.org/reference/englue.html. This part is not necessary but I thought it was nicer to create and reuse a variable.
Tricky part, you create a new column with a constructed name and then want to use the new column that this name refers to. You'll have to subset it with .data, see: https://rlang.r-lib.org/reference/dot-data.html