FastR used to have prototypical integration with RStudio and we are planning to revive this at some point, however, there is no concrete time frame for this yet.
Update: FastR works well with this Visual Studio code plugin. You just need to set the path to FastR executable instead of GNU-R.
More technical details: RStudio (rserver more precisely) uses R in the embedded mode, which is supported by FastR 1[2], but has not been tested with RStudio since we did the prototype in 2016. Additionally, RStudio uses R API to directly access R's execution contexts, which is hard to emulate for FastR since it is just bunch of C global variables. FastR provides functions to access the execution contexts[3][4] and RStudio needs to be patched to use those functions instead.
1 https://github.com/oracle/fastr/blob/master/com.oracle.truffle.r.native/fficall/src/truffle_nfi/Rembedded.c
[2] tests: https://github.com/oracle/fastr/tree/master/com.oracle.truffle.r.test.native/embedded
[3] native side: https://github.com/oracle/fastr/blob/master/com.oracle.truffle.r.native/fficall/src/truffle_nfi/Rembedded.c#L262
[4] Java side: https://github.com/oracle/fastr/blob/master/com.oracle.truffle.r.ffi.impl/src/com/oracle/truffle/r/ffi/impl/upcalls/IDEUpCallsRFFI.java