How is dot (.) alias for list constructor implemented in data.table package?

I wonder how is is the dot alias (.) for the list constructor function implemented in the data.table package. Just to be clear I am thinking about this functionality:

library(data.table)
D = data.table(iris)
x1 = D[, .(Sepal.Length, Sepal.Width)]    # dot alias
x2 = D[, list(Sepal.Length, Sepal.Width)] # standard name
identical(x1, x2)    # TRUE

I tried to find it in the source code on github, but it is too dense for me to understand it in any reasonable amount of time.

EDIT. I know this can be easily done by defining an alias like: . <- list or . <- function(...) list(...). However, this is not exactly what I am looking for. I want to define such an alias, so it only works in the context of a given function/method.

Example.

L <- .(1)   # This throws error
L <- func(.(1)) # This works

Actually I can get what I want using the rlang tools for tidy evaluation. Below is a simple example.

library(rlang)
func <- function(...) {
    . <- list
    eval_tidy(enexpr(x))
}
x1 <- func(.(1))
x2 <- list(1)
identical(x1, x2)   # TRUE

So I wonder how this kind of functionality is implemented in the data.table specifically, since it was developed a way earlier than rlang?