Why do `rlang::sym` and `rlang::quo_name` in LHS of expression behave similarly?

Question

Call the function below using foo(c("b")). The outputs are shown inline.

I'm confused as to why both (1) df %>% mutate(!!x_ := 100 + !!x)) and (2) df %>% mutate(!!x := 100 + !!x)) work identically; based on dplyr programming recipes only (1) should work.

foo <- function(variables) {

  x <- rlang::sym(variables[[1]])

  x_ <- quo_name(x)

  print(x)
  #> b

  print(typeof(x))
  #> [1] "symbol"

  print(x_)
  #> [1] "b"

  print(typeof(x_))
  #> [1] "character"

  df <- data_frame(a = 1, b = 2)

  print(df %>% mutate(!!x_ := 100 + !!x))

  #> # A tibble: 1 x 2
  #>         a     b
  #>       <dbl> <dbl>
  #>   1     1   102  

  print(df %>% mutate(!!x := 100 + !!x))

  #> # A tibble: 1 x 2
  #>         a     b
  #>       <dbl> <dbl>
  #>   1     1   102  

}

Answer 1

Moving comment to answer.

As per mentioned in the documentation you are referring to:

The rules on the LHS are slightly different: the unquoted operand should evaluate to a string or a symbol.

Here, it works because x_ is, in fact, a character.