Hygiene 2.0: Avoid comparing fields by name
There are two separate commits here (not counting tests):
- The first one unifies named (`obj.name`) and numeric (`obj.0`) field access expressions in AST and HIR. Before field references in these expressions are resolved it doesn't matter whether the field is named or numeric (it's just a symbol) and 99% of code is common. After field references are resolved we work with
them by index for all fields (see the second commit), so it's again not important whether the field was named or numeric (this includes MIR where all fields were already by index).
(This refactoring actually fixed some bugs in HIR-based borrow checker where borrows through names (`S {
0: ref x }`) and indices (`&s.0`) weren't considered overlapping.)
- The second commit removes all by-name field comparison and instead resolves field references to their indices once, and then uses those resolutions. (There are still a few name comparisons in save-analysis, because save-analysis is weird, but they are made correctly hygienic).
Thus we are fixing a bunch of "secondary" field hygiene bugs (in borrow checker, lints).
Fixes https://github.com/rust-lang/rust/issues/46314
Ak 44493 infer predicate
**WIP** Implements #44493
Things to do:
- [x] add feature gate and appropriate tests (see [forge](https://forge.rust-lang.org/feature-guide.html) for some details)
- [x] add a unit testing system similar to `#[rustc_variance]`
- [x] to see how, maybe `rg rustc_variance` and take some notes
- [ ] add more tests:
- [x] we need to decide how to handle `struct Foo<'a, T> { x: &'a T::Item }`
- [x] handle explicit predicates on types
- [ ] handle explicit predicates on `dyn Trait` (this could be put off to a follow-up PR)
- [ ] handle explicit predicates on projections (this could be put off to a follow-up PR)
Deprecate offset_to; switch core&alloc to using offset_from instead
Bonus: might make code than uses `.len()` on slice iterators faster
cc https://github.com/rust-lang/rust/issues/41079
Merge the std_unicode crate into the core crate
[The standard library facade](https://github.com/rust-lang/rust/issues/27783) has historically contained a number of crates with different roles, but that number has decreased over time. `rand` and `libc` have moved to crates.io, and [`collections` was merged into `alloc`](https://github.com/rust-lang/rust/pull/42648). Today we have `core` that applies everywhere, `std` that expects a full operating system, and `alloc` in-between that only requires a memory allocator (which can be provided by users)… and `std_unicode`, which doesn’t really have a reason to be separate anymore. It contains functionality based on Unicode data tables that can be large, but as long as relevant functions are not called the tables should be removed from binaries by linkers.
This deprecates the unstable `std_unicode` crate and moves all of its contents into `core`, replacing them with `pub use` reexports. The crate can be removed later. This also removes the `CharExt` trait (replaced with inherent methods in libcore) and `UnicodeStr` trait (merged into `StrExt`). There traits were both unstable and not intended to be used or named directly.
A number of new items are newly-available in libcore and instantly stable there, but only if they were already stable in libstd.
Fixes#49319.
