Switch to Rust 2021.

[eddyb]

This PR is built on the following PRs: (EDIT: now all of them merged)

• linker/duplicates: handle all decorations instead of special-casing zombies. #949
• Use Spans to detect "system crate"s, for error deferral (zombie) purposes. #952
  • itself required linker/zombies: stop compilation after zombie errors. #950
• Replace bimap dependency with a more efficient pair of maps, and arena-allocate slices. #951

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Sadly, this is not ready. The big issue here is that Rust 2021 makes panic!("...") go through the same format_args!("...") machinery as everything else (i.e. its special-casing was removed - this means that e.g. panic!("foo: {}") no long bypasses format string checks, and actually errors).

While we should have the ability to ignore such things, there's enough going on that our panic entry-point special-casing (plus the fn param weakening, maybe?) doesn't actually bypass the "zombie" systems enough.

My best guess so far is that, unlike a string literal constant, format_args! actually needs local variables, and the "zombie" system's concept of "dead code" might not include them.

So far my efforts have focused on improving the granularity of the "zombie" system (hence the other PRs), to better debug such failure modes (when I started, it kept pointing at ptr::metadata, which is pretty useless).

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EDIT: the new Rust 2021 panic! (which always uses format_args!) is now supported (by removing simple invocations of format_args! feeding directly into a panic entry-point)

[oisyn]

Oh good point, I also forgot the changelog with the nightly update.