Sorry, It worked when I test it home.
Try to remove `recursive = FALSE` (set it to `TRUE`)
I didn't see any change in my setup but you can also do
```{r}
rcppset <- dependsOnPkgs("Rcpp", recursive = FALSE, installed = installed.packages())
```

Here is the results of my test
```{r}
library(tools)
IP <- installed.packages()
rcppset <- dependsOnPkgs("Rcpp", recursive = FALSE)
rcppset2 <- unique(sort(unname(IP[unique(c(grep("Rcpp", as.character(IP[,"Depends"])),
                                          grep("Rcpp", as.character(IP[,"LinkingTo"])),
                                          grep("Rcpp", as.character(IP[,"Imports"])))),"Package"])))
all.equal(rcppset, rcppset2)
[1] TRUE
```