Streamlined the explanation of covariance for `NonNull<T>`,
focusing on practical usage and reducing scary explanation.
Added a concise example for cases where invariance is required,
showing how to use `PhantomData<Cell<T>>

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

Introduce the `MetaSized` and `PointeeSized` traits as supertraits of
`Sized` and initially implement it on everything that currently
implements `Sized` to isolate any changes that simply adding the
traits introduces.

more information to Display implementation for BorrowError/BorrowMutError

- The BorrowError/BorrowMutError Debug implementations do not print
anything differently from what the derived implementation does, so we
don't need it.

- This change also adds the location field of
BorrowError/BorrowMutError to the the Display output when it is
present, rewords the error message, and uses the Display trait for
outputting the error message instead of Debug.

It's actually used as a counter so update the name to reflect that.

As core uses an extern type (`ptr::VTable`), the default `?Sized` to
`MetaSized` migration isn't sufficient, and some code that previously
accepted `VTable` needs relaxed to continue to accept extern types.

Similarly, the compiler uses many extern types in `rustc_codegen_llvm`
and in the `rustc_middle::ty::List` implementation (`OpaqueListContents`)
some bounds must be relaxed to continue to accept these types.

Unfortunately, due to the current inability to relax `Deref::Target`,
some of the bounds in the standard library are forced to be stricter than
they ideally would be.

Adding a sizedness supertrait shouldn't require multiple vtables so
shouldn't be linted against.

Make performance description of String::{insert,insert_str,remove} more precise

std: refactor explanation of `NonNull`

Fixes rust-lang#141933

I cut out the excessive explanation and used an example to explain how to maintain invariance, but I think what is quoted in the *rust reference* in the document needs to be added with a more layman's explanation and example.

(I'm not sure if I deleted too much)

r? `@workingjubilee`

Miscellaneous RefCell cleanups

- Clarify `RefCell` error messages when borrow rules are broken
- Remove `Debug` impl for `BorrowError`/`BorrowMutError` since `#derive(Debug)` provides identical functionality
- Rename `BorrowFlag` to `BorrowCounter`

Sized Hierarchy: Part I

This patch implements the non-const parts of rust-lang/rfcs#3729. It introduces two new traits to the standard library, `MetaSized` and `PointeeSized`. See the RFC for the rationale behind these traits and to discuss whether this change makes sense in the abstract.

These traits are unstable (as is their constness), so users cannot refer to them without opting-in to `feature(sized_hierarchy)`. These traits are not behind `cfg`s as this would make implementation unfeasible, there would simply be too many `cfg`s required to add the necessary bounds everywhere. So, like `Sized`, these traits are automatically implemented by the compiler.

RFC 3729 describes changes which are necessary to preserve backwards compatibility given the introduction of these traits, which are implemented and as follows:

- `?Sized` is rewritten as `MetaSized`
- `MetaSized` is added as a default supertrait for all traits w/out an explicit sizedness supertrait already.

There are no edition migrations implemented in this,  as these are primarily required for the constness parts of the RFC and prior to stabilisation of this (and so will come in follow-up PRs alongside the const parts). All diagnostic output should remain the same (showing `?Sized` even if the compiler sees `MetaSized`) unless the `sized_hierarchy` feature is enabled.

Due to the use of unstable extern types in the standard library and rustc, some bounds in both projects have had to be relaxed already - this is unfortunate but unavoidable so that these extern types can continue to be used where they were before. Performing these relaxations in the standard library and rustc are desirable longer-term anyway, but some bounds are not as relaxed as they ideally would be due to the inability to relax `Deref::Target` (this will be investigated separately).

It is hoped that this is implemented such that it could be merged and these traits could exist "under the hood" without that being observable to the user (other than in any performance impact this has on the compiler, etc). Some details might leak through due to the standard library relaxations, but this has not been observed in test output.

**Notes:**

- Any commits starting with "upstream:" can be ignored, as these correspond to other upstream PRs that this is based on which have yet to be merged.
- This best reviewed commit-by-commit. I've attempted to make the implementation easy to follow and keep similar changes and test output updates together.
  - Each commit has a short description describing its purpose.
  - This patch is large but it's primarily in the test suite.
- I've worked on the performance of this patch and a few optimisations are implemented so that the performance impact is neutral-to-minor.
- `PointeeSized` is a different name from the RFC just to make it more obvious that it is different from `std::ptr::Pointee` but all the names are yet to be bikeshed anyway.
- `@nikomatsakis` has confirmed [that this can proceed as an experiment from the t-lang side](https://rust-lang.zulipchat.com/#narrow/channel/435869-project-goals/topic/SVE.20and.20SME.20on.20AArch64.20.28goals.23270.29/near/506196491)
- FCP in rust-lang#137944 (comment)

Fixes rust-lang#79409.

r? `@ghost` (I'll discuss this with relevant teams to find a reviewer)

…kingjubilee

Rollup of 13 pull requests

Successful merges:

 - rust-lang#138538 (Make performance description of String::{insert,insert_str,remove} more precise)
 - rust-lang#141946 (std: refactor explanation of `NonNull`)
 - rust-lang#142216 (Miscellaneous RefCell cleanups)
 - rust-lang#142542 (Manually invalidate caches in SimplifyCfg.)
 - rust-lang#142563 (Refine run-make test ignores due to unpredictable `i686-pc-windows-gnu` unwind mechanism)
 - rust-lang#142570 (Reject union default field values)
 - rust-lang#142584 (Handle same-crate macro for borrowck semicolon suggestion)
 - rust-lang#142585 (Update books)
 - rust-lang#142586 (Fold unnecessary `visit_struct_field_def` in AstValidator)
 - rust-lang#142587 (Make sure to propagate result from `visit_expr_fields`)
 - rust-lang#142595 (Revert overeager warning for misuse of `--print native-static-libs`)
 - rust-lang#142598 (Set elf e_flags on ppc64 targets according to abi)
 - rust-lang#142601 (Add a comment to `FORMAT_VERSION`.)

r? `@ghost`
`@rustbot` modify labels: rollup