dplyr, rlang, etc.

code.R

library(dplyr)

df <- nycflights13::flights

.a <- quote(sched_dep_time + dep_delay)
.b <- function() quote(sched_dep_time + dep_delay)
.c <- function(x) bquote(.(substitute(x)) + dep_delay)
.d <- function(x) {
  x <- enquo(x)
  expr(!!x + dep_delay)
}
.e <- function(x) {
  expr(!!x) # + dep_delay
}
.f <- function(x) x

df %>%
  mutate(
    a = !!.a,
    b = !!.b(),
    c = !!.c(sched_dep_time),
    d = !!.d(sched_dep_time),
    e = .e(sched_dep_time),
    f = .f(sched_dep_time)
  ) %>%
  select(a, b, c, d, e, f)