Rust fails to build on Big Sur (Public Beta).

[5nefarious (Arvin Ignaci)]

I'm trying to create a package and it looks like rust is one of the sub-dependencies (chromaprint > ffmpeg > librsvg > rust). However, it fails to build on the darwin_20 platform, which is what I'm currently running. Looking at the log output, it looks like it's failing to link an FFI library:

:info:build    Compiling regex v1.3.9
:info:build    Compiling globset v0.4.5
:info:build    Compiling ignore v0.4.16
:info:build    Compiling toml v0.5.6
:info:build    Compiling bootstrap v0.0.0 (/opt/local/var/macports/build/_opt_mports_macports-ports_lang_rust/rust/work/rustc-1.47.0-src/src/bootstrap)
:info:build     Finished dev [unoptimized + debuginfo] target(s) in 1m 02s
:info:build running: /opt/local/var/macports/build/_opt_mports_macports-ports_lang_rust/rust/work/rustc-1.47.0-src/build/bootstrap/debug/bootstrap build --stage 2 -v -j8
:info:build dyld: Library not loaded: /opt/local/lib/libffi.6.dylib
:info:build   Referenced from: /opt/local/libexec/llvm-9.0/lib/libLLVM.dylib
:info:build   Reason: image not found
:info:build thread 'main' panicked at 'command did not execute successfully: "/opt/local/libexec/llvm-9.0/bin/llvm-config" "--bindir"
:info:build expected success, got: signal: 6', src/build_helper/lib.rs:139:9
:info:build note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
:info:build Traceback (most recent call last):
:info:build   File "/opt/local/var/macports/build/_opt_mports_macports-ports_lang_rust/rust/work/rustc-1.47.0-src/src/bootstrap/bootstrap.py", line 1026, in <module>
:info:build     main()
:info:build   File "/opt/local/var/macports/build/_opt_mports_macports-ports_lang_rust/rust/work/rustc-1.47.0-src/src/bootstrap/bootstrap.py", line 1009, in main
:info:build     bootstrap(help_triggered)
:info:build   File "/opt/local/var/macports/build/_opt_mports_macports-ports_lang_rust/rust/work/rustc-1.47.0-src/src/bootstrap/bootstrap.py", line 995, in bootstrap
:info:build     run(args, env=env, verbose=build.verbose)
:info:build   File "/opt/local/var/macports/build/_opt_mports_macports-ports_lang_rust/rust/work/rustc-1.47.0-src/src/bootstrap/bootstrap.py", line 143, in run
:info:build     raise RuntimeError(err)
:info:build RuntimeError: failed to run: /opt/local/var/macports/build/_opt_mports_macports-ports_lang_rust/rust/work/rustc-1.47.0-src/build/bootstrap/debug/bootstrap build --stage 2 -v -j8
:info:build make: *** [all] Error 1
:info:build make: Leaving directory `/opt/local/var/macports/build/_opt_mports_macports-ports_lang_rust/rust/work/rustc-1.47.0-src'
:info:build Command failed:  cd "/opt/local/var/macports/build/_opt_mports_macports-ports_lang_rust/rust/work/rustc-1.47.0-src" && /usr/bin/make -j8 -w all VERBOSE=1 BOOTSTRAP_ARGS="-v -j8" 
:info:build Exit code: 2
:error:build Failed to build rust: command execution failed
:debug:build Error code: CHILDSTATUS 81677 2
:debug:build Backtrace: command execution failed
:debug:build     while executing
:debug:build "system {*}$notty {*}$callback {*}$nice $fullcmdstring"
:debug:build     invoked from within
:debug:build "command_exec -callback portprogress::target_progress_callback build"
:debug:build     (procedure "portbuild::build_main" line 8)
:debug:build     invoked from within
:debug:build "$procedure $targetname"
:error:build See /opt/local/var/macports/logs/_opt_mports_macports-ports_lang_rust/rust/main.log for details.

I checked and found that there is no /opt/local/lib/libffi.6.dylib, only /opt/local/lib/libffi.7.dylib. Is this something that needs to be fixed in MacPorts or an upstream problem?