The 1.70 Rust release fixed a bug that was allowing unstable compiler options to be used on stable toolchains. Unfortunately we were relying on this behavior to implement the test reporting added in RUST-1075. We'd need to switch over to the nightly toolchain to continue using the unstable options, but this would introduce too much noise to our builds. A few options here:

• Pin our Rust version to 1.69 in CI (probably not a good idea in the long term)
• Disable test reporting in evergreen until the unstable options we use are stabilized
• Figure out a different way to report test results that doesn't use unstable options