saw-rustc --version panics (but crux-rustc --version doesn't)

[RyanGlScott]

While crux-rustc --version works as you would expect:

$ crux-rustc --version
normal build - ["rustc", "--version", "--target", "x86_64-unknown-linux-gnu", "--cfg", "crux"]
rustc 1.69.0-nightly (5e37043d6 2023-01-22)

saw-rustc --version, on the other hand, will panic:

$ RUST_BACKTRACE=1 saw-rustc --version
test build - extract output path - ["rustc", "--version", "--test", "--target", "x86_64-unknown-linux-gnu", "--cfg", "crux"]
rustc 1.69.0-nightly (5e37043d6 2023-01-22)
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/bin/mir-json-rustc-wrapper.rs:86:27
stack backtrace:
   0: rust_begin_unwind
             at /rustc/5e37043d63bfe2f3be8fa5a05b07d6c0dad5775d/library/std/src/panicking.rs:575:5
   1: core::panicking::panic_fmt
             at /rustc/5e37043d63bfe2f3be8fa5a05b07d6c0dad5775d/library/core/src/panicking.rs:64:14
   2: core::panicking::panic
             at /rustc/5e37043d63bfe2f3be8fa5a05b07d6c0dad5775d/library/core/src/panicking.rs:114:5
   3: mir_json_rustc_wrapper_real::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

The backtrace points to this line as the culprit:

mir-json/src/bin/mir-json-rustc-wrapper.rs

Line 86 in 3b53858

callbacks.output_path.unwrap()

I'm somewhat surprised by this, given how similar the two commands are. At first glance, one possible explanation is that saw-rustc calls rustc with --test, but crux-rustc does not.

[RyanGlScott]

See also #77. I suspect that we may need to reimplement some or all of the logic that powers the --version option in order to fix this.