when do things get copied in R?

mem_copy.R

fn_list <- function(y){
    y$a <- y$a + 1
    return(y)
}

fn_env <- function(y){
    y$a <- y$a + 1
}

x <- list(a=matrix(1:9,3,3),b=matrix(1:36,6,6))
.Internal(inspect(x))
x <- fn_list(x)
.Internal(inspect(x))

y <- list2env(list(a=matrix(1:9,3,3),b=matrix(1:36,6,6)))
.Internal(inspect(y))
fn_env(y)
.Internal(inspect(y))

stopifnot(x$a == y$a)
stopifnot(x$b == y$b)