Spill error when `-Cforce-frame-pointers` is passed

[wisp3rwind]

I've just encountered this error:

$ cargo +esp run
   Compiling compiler_builtins v0.1.160 (/home/benedikt/.rustup/toolchains/esp/lib/rustlib/src/rust/library/compiler-builtins/compiler-builtins)
   Compiling alloc v0.0.0 (/home/benedikt/.rustup/toolchains/esp/lib/rustlib/src/rust/library/alloc)
rustc-LLVM ERROR: Error while trying to spill A8 from class AR: Cannot scavenge register without an emergency spill slot!
error: could not compile `compiler_builtins` (lib)

This seems to happen only when adding

[build]
rustflags = [
  "-C", "force-frame-pointers",
]

to .cargo/config.toml. I got here because I merged in some files from a risc-v project (where frame pointers are required for esp-backtrace). Is this error expected, or rather a bug?

[MabezDev]

It's not expected, but I do know about it... I just haven't had time to file it upstream yet, with a min repro.