Create a profile option for autodiff to replace RUSTFLAGS requirement.

I'm not sure if you could convince the cargo team to get dedicated profiles. I was referring to a profile option autodiff = true similar to ones like lto = true or opt-level = 3 that you can specify within the [profile.release] and [profile.debug] sections.

[profile.debug]
autodiff = true

[profile.release]
autodiff = true

Originally posted by @bjorn3 in #147487

Right now, we need two options to make autodiff work.
You need to set lto=fat in your cargo.toml, and add RUSTFLAGS=-Zautodiff=Enable.
It would be good to bundle both in the same place, preferably Cargo.toml.
As a result, each compiler invocation would also become shorter, which makes autodiff much nicer to use.

Creating this issue since the main issue from the thread linked above has ben solved, so I'm closing that one. This would still be good to solve. I'm not experienced with how we parse Cargo.toml, but I assume it's a sufficiently nice beginner issue, if someone is interested.

Start with the Cargo.toml [profile.debug] and [profile.release] settings, then trace how existing lto and opt-level options are parsed and how RUSTFLAGS=-Zautodiff=Enable currently reaches compiler invocations. Done means autodiff can be enabled through the profile option in both profiles without requiring the separate RUSTFLAGS setting.