[beta] Add back Manifest::targets_mut

[Mark-Simulacrum]

It is needed by cargo-c, it was removed in df5cb70

This is coupled with stable backport approved yesterday, #8527.

[rust-highfive]

r? @ehuss

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[rust-highfive]

⚠️ Warning ⚠️

• Pull requests are usually filed against the master branch for this repo, but this one is against rust-1.46.0. Please double check that you specified the right target!

[alexcrichton]

Mind cherry-picking #8528 here to get green CI?

[ehuss]

I think it would be fine to manually merge this. The beta branch would need a fair bit of work to get CI passing (at least #8528 and #8467, I'm not sure what the cfg_if failure is about).

[Mark-Simulacrum]

Happy with whatever, just let me know. Do we expect these test failures to lead to problems when integrating into upstream (rust-lang/rust)?

[alexcrichton]

Ah that's a good point, I think it's fine to just hit merge. I'll work on getting a green CI later for more backports.

[alexcrichton]

Oh and these are all nightly-only tests so shouldn't cause issues in rust-lang/rust.

[ehuss]

Oh, haha, sorry for giving conflicting advice! These won't fail in rust-lang/rust because they don't run there (all of our nightly-only tests are disabled on rust-lang/rust).