Fix for #62691: use the largest niche across all fields
fixes#62691
(The second commit is a small optimization but it makes the code less pretty and i don't know if it is worth it.)
permit negative impls for non-auto traits
This is a prototype impl that extends `impl !Trait` beyond auto traits. It is not integrated with coherence or anything else, and hence only serves to prevent downstream impls (but not to allow downstream crates to rely on the absence of such impls for coherence purposes).
Fixes https://github.com/rust-lang/rust/issues/66544
TODO:
- [x] need a test that you can't rely on negative impls for coherence purposes
- [x] test that negative impls cannot specialize positive ones
- [x] test that positive impls cannot specialize negative ones
- [x] extend negative impl to `Clone` in order to fully fix#66544
- [x] and maybe make `CoerceUnsized` unsafe? -- that problem is now split out into https://github.com/rust-lang/rust/issues/68015
- [x] introduce feature flag and prepare a write-up
- [x] improve diagnostics?
They used to be covered by `optin_builtin_traits` but negative impls
are now applicable to all traits, not just auto traits.
This also adds docs in the unstable book for the current state of auto traits.
Tweak chained operators diagnostic
Use more selective spans
Improve suggestion output
Be more selective when displaying suggestions
Silence some knock-down type errors
r? @Centril
Implement Fuse with Option
The former `done` flag was roughly similar to an `Option` tag, but left
the possibity of misuse. By using a real `Option`, we can set `None`
when the iterator is exhausted, removing any way to call it again. We
also allow niche layout this way, so the `Fuse` may be smaller.
The `FusedIterator` specialization does want to ignore the possibility
of exhaustion though, so it uses `unsafe { intrinsics::unreachable() }`
to optimize that branch away. The entire `Fuse` implementation is now
isolated in its own module to contain that unsafety.
r? @scottmcm
use checked casts and arithmetic in Miri engine
This is unfortunately pretty annoying because we have to cast back and forth between `u64` and `usize` more often that should be necessary, and that cast is considered fallible.
For example, should [this](https://doc.rust-lang.org/nightly/nightly-rustc/rustc/mir/interpret/value/enum.ConstValue.html) really be `usize`?
Also, `LayoutDetails` uses `usize` for field indices, but in Miri we use `u64` to be able to also handle array indexing. Maybe methods like `mplace_field` should be suitably generalized to accept both `u64` and `usize`?
r? @oli-obk Cc @eddyb
