To me, that seems overly complicated. I think 90% of the time people just want variables to go in the front, instead of in the back.

If we're only interested in supporting adding to the front, maybe we could use transmute()?

@lionel-: What would it take to make the following syntax work?

library(tidyverse)
tibble(a = 2, b = 3) %>% transmute(x = 1, everything())
#> Error in mutate_impl(.data, dots): Evaluation error: No tidyselect variables were registered.

Created on 2018-03-30 by the reprex package (v0.2.0).

In MySQL we have two verbs, first and after , to add a column in a table with alter. Maybe dplyr can copy that behavior? It could also solve #4598.

I'm once again leaning towards a .where argument, which would use tidyselect along the lines of r-lib/tidyselect#151.

Discussion of dedicated move verb at #4598

You can now use transmute() + across() to add new cols at the start:

df %>%
  transmute(x = y ^ 2, across(everything())

@hadley, this is a great commit, but perhaps it should support multiple columns relocation (the thread title is indeed Allow mutate() to choose the position of new columns):

iris %>%
  as_tibble() %>%
  mutate(Petal.Length.mm = Petal.Length * 10,
         Sepal.Length.mm = Sepal.Length * 10,
         .after = c(Petal.Length, Sepal.Length))

produces

# A tibble: 150 x 6
   Petal.Length Sepal.Length Petal.Length.mm Sepal.Length.mm Petal.Width Species

and not

# A tibble: 150 x 6
   Petal.Length Petal.Length.mm Sepal.Length Sepal.Length.mm Petal.Width Species

@courtiol Your suggestion would not work with plural selections like starts_with() or is.numeric.

I agree @lionel-, but it would make sense in mutate() calls without suffix (as shown above), wouldn't it?
Perhaps for mutate_if() and mutate_at() a possibility would be to consider a .before = .x (or .after = .x) to imply that each newly created column goes before (or after) its source.