Error if an autodiff user does not set lto=fat

[ZuseZ4]

Based on your feedback, I started to provide a nice error message for a lack of lto=fat, instead of us forcing it.

In a next step, we should replace RUSTFLAGS="-Zautodiff=Enable" with another Cargo.toml setting, as discussed here: #147487 (comment)

As another improvement, we should also figure out why rlib builds do not properly obey the fat=lto setting.

@bjorn3

[rustbot]

r? @madsmtm

rustbot has assigned @madsmtm.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[ZuseZ4]

r? @bjorn3

[rustbot]

Failed to set assignee to bjorn: invalid assignee

Note: Only org members with at least the repository "read" role, users with write permissions, or people who have commented on the PR may be assigned.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.