Bump bootstrap compiler to 1.50 beta

r? `@pietroalbini`

Rollup of 7 pull requests

Successful merges:

 - #80185 (Fix ICE when pointing at multi bytes character)
 - #80260 (slightly more typed interface to panic implementation)
 - #80311 (Improvements to NatVis support)
 - #80337 (Use `desc` as a doc-comment for queries if there are no doc comments)
 - #80381 (Revert "Cleanup markdown span handling")
 - #80492 (remove empty wraps, don't return Results from from infallible functions)
 - #80509 (where possible, pass slices instead of &Vec or &String (clippy::ptr_arg))

Failed merges:

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

where possible, pass slices instead of &Vec or &String (clippy::ptr_arg)

remove empty wraps, don't return Results from from infallible functions

This makes code easier to understand because it is more obvious when a function actually can't fail (return Err or None)

Make functions that only ever return Some(x), return x directly
Remove return type from functions that return Option<(), Err> but would only ever return Ok(()).

Found with `clippy::unnecessary_wraps`

Revert "Cleanup markdown span handling"

Reverts rust-lang/rust#80244. This caused a diagnostic regression, originally it was:

```
warning: unresolved link to `std::process::Comman`
 --> link.rs:3:10
  |
3 | //! [a]: std::process::Comman
  |          ^^^^^^^^^^^^^^^^^^^^ no item named `Comman` in module `process`
  |
  = note: `#[warn(broken_intra_doc_links)]` on by default
```
but after that PR rustdoc now displays
```
warning: unresolved link to `std::process::Comman`
 --> link.rs:1:14
  |
1 | //! Links to [a] [link][a]
  |              ^^^ no item named `Comman` in module `process`
  |
  = note: `#[warn(broken_intra_doc_links)]` on by default
```
which IMO is much less clear.

cc `@bugadani,` thanks for catching this in https://github.com/rust-lang/rust/pull/77859.
r? `@GuillaumeGomez`

Use `desc` as a doc-comment for queries if there are no doc comments

This at least gives *some* idea of what the query does even if it's not very readable. Some examples:

![image](https://user-images.githubusercontent.com/23638587/103021399-13e15c00-4518-11eb-8121-940774ae2fd1.png)
![image](https://user-images.githubusercontent.com/23638587/103021448-222f7800-4518-11eb-8ee6-cc10795fdc22.png)
![image](https://user-images.githubusercontent.com/23638587/103021434-1d6ac400-4518-11eb-885b-59d00c57bc70.png)

I want to turn `{}` into either `_` or the stringified expr, but [I'm not sure how to do that](https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/Evaluate.20format.20string.20in.20proc-macro). In the meantime, this is better than having no docs at all.

Improvements to NatVis support

NatVis files describe how to display types in some Windows debuggers,
such as Visual Studio, WinDbg, and VS Code.

This commit makes several improvements:

* Adds visualizers for Rc<T>, Weak<T>, and Arc<T>.

* Changes [size] to [len], for consistency with the Rust API.
  Visualizers often use [size] to mirror the size() method on C++ STL
  collections.

* Several visualizers used the PVOID and ULONG typedefs. These are part
  of the Windows API; they are not guaranteed to always be defined in a
  pure Rust DLL/EXE. I converted PVOID to `void*` and `ULONG` to
  `unsigned long`.

* Cosmetic change: Removed {} braces around the visualized display
  for `Option` types. They now display simply as `Some(value)` or
  `None`, which reflects what is written in source code.

* The visualizer for `alloc::string::String` makes assumptions about
  the layout of `String` (it casts `String*` to another type), rather
  than using symbolic expressions. This commit changes the visualizer
  so that it simply uses symbolic expressions to access the string
  data and string length.

* The visualizers for `str` and `String` now place the character data
  array under a synthetic `[chars]` node. When expanding a `String`
  node, users rarely want to see an array of characters. This just places
  them behind one expansion node / level.

slightly more typed interface to panic implementation

The panic payload is currently being passed around as a `usize`. However, it actually is a pointer, and the involved types are available on all ends of this API, so I propose we use the proper pointer type to avoid some casts. Avoiding int-to-ptr casts also makes this code work with `miri -Zmiri-track-raw-pointers`.

Fix ICE when pointing at multi bytes character

Fixes #80134

Seems this ICE was introduced by #73953, checking width of span to avoid ICE.

Make copy[_nonoverlapping] const

Constifies
* `intrinsics::copy` and `intrinsics::copy_nonoverlapping`
* `ptr::read` and `ptr::read_unaligned`
  * `*const T::read` and `*const T::read_unaligned`
  * `*mut T::read` and `*mut T::read_unaligned`
* `MaybeUninit::assume_init_read`

remove unused return type of dropck::check_drop_obligations()
don't wrap return type in Option in get_macro_by_def_id() since we would always return Some(..)
remove redundant return type of back::write::optimize()
don't Option-wrap return type of compute_type_parameters() since we always return Some(..)
don't return empty Result in assemble_generator_candidates()
don't return empty Result in assemble_closure_candidates()
don't return empty result in assemble_fn_pointer_candidates()
don't return empty result in assemble_candidates_from_impls()
don't return empty result in assemble_candidates_from_auto_impls()
don't return emtpy result in assemble_candidates_for_trait_alias()
don't return empty result in assemble_builtin_bound_candidates()
don't return empty results in assemble_extension_candidates_for_traits_in_scope() and assemble_extension_candidates_for_trait()
remove redundant wrapping of return type of StripItem::strip() since it always returns Some(..)
remove unused return type of assemble_extension_candidates_for_all_traits()

Rollup of 13 pull requests

Successful merges:

 - #79812 (Lint on redundant trailing semicolon after item)
 - #80348 (remove redundant clones (clippy::redundant_clone))
 - #80358 (Edit rustc_span documentation)
 - #80457 (Add missing commas to `rustc_ast_pretty::pp` docs)
 - #80461 (Add llvm-libunwind change to bootstrap CHANGELOG)
 - #80464 (Use Option::map_or instead of open coding it)
 - #80465 (Fix typo in ffi-pure.md)
 - #80467 (More uses of the matches! macro)
 - #80469 (Fix small typo in time comment)
 - #80472 (Use sans-serif font for the "all items" page links)
 - #80477 (Make forget intrinsic safe)
 - #80482 (don't clone copy types)
 - #80487 (don't redundantly repeat field names)

Failed merges:

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

don't redundantly repeat field names

r? `@Dylan-DPC`

don't clone copy types

r? `@Dylan-DPC`

Make forget intrinsic safe

Use sans-serif font for the "all items" page links

The "all items" pages' links aren't using a sans-serif font unlike the rest of equivalent items in the other module pages. ``@Nemo157`` reported me this issue so here is the fix!

r? ``@Nemo157``

Fix small typo in time comment

I was reading through [this PR](https://github.com/rust-lang/rust/pull/77727/files) and noticed a [small typo](https://github.com/rust-lang/rust/pull/77727/files#diff-3183738c82f747fd92bb573712073aa384b16e14d154bf2df18f41077edc31cbR240) smuggled though. This just fixes that.

More uses of the matches! macro

`@rustbot` modify labels +C-cleanup +T-compiler

Fix typo in ffi-pure.md

accesing -> accessing

Use Option::map_or instead of open coding it

``@rustbot`` modify labels +C-cleanup +T-compiler

Add llvm-libunwind change to bootstrap CHANGELOG

From #77703.

This doesn't need a `changelog-seen` version bump because the old values aren't accepted anymore, meaning anyone who was using this had to change it already.

r? ``@Mark-Simulacrum``

Add missing commas to `rustc_ast_pretty::pp` docs

Edit rustc_span documentation

Various changes to the `rustc_span` docs, including the following:

- Additions to top-level docs
- Edits to the source_map module docs
- Edits to documentation for `Span` and `SpanData`
- Added intra-docs links
- Documentation for Levenshtein distances
- Fixed missing punctuation

remove redundant clones (clippy::redundant_clone)

Lint on redundant trailing semicolon after item

We now lint on code like this:

```rust
fn main() {
    fn foo() {};
    struct Bar {};
}
```

Previously, this caused warnings in Cargo, so it was disabled.

Replace pretty-print/compare/retokenize hack with targeted workarounds

Based on https://github.com/rust-lang/rust/pull/78296
cc https://github.com/rust-lang/rust/issues/43081

The 'pretty-print/compare/retokenize' hack is used to try to avoid passing an outdated `TokenStream` to a proc-macro when the underlying AST is modified in some way (e.g. cfg-stripping before derives). Unfortunately, retokenizing throws away spans (including hygiene information), which causes issues of its own. Every improvement to the accuracy of the pretty-print/retokenize comparison has resulted in non-trivial ecosystem breakage due to hygiene changes. In extreme cases, users deliberately wrote unhygienic `macro_rules!` macros (likely because they did not realize that the compiler's behavior was a bug).

Additionaly, the comparison between the original and pretty-printed/retoknized token streams comes at a non-trivial runtime cost, as shown by https://github.com/rust-lang/rust/pull/79338

This PR removes the pretty-print/compare/retokenize logic from `nt_to_tokenstream`. We only discard the original `TokenStream` under two circumstances:
* Inner attributes are used (detected by examining the AST)
* `cfg`/`cfg_attr` processing modifies the AST. This is detected by making the visitor update a flag when it performs a modification, instead of trying to detect the modification after-the-fact. Note that a 'matching' `cfg` (e.g. `#[cfg(not(FALSE)]`) does not actually get removed from the AST, allowing us to preserve the original `TokenStream`.

In all other cases, we preserve the original `TokenStream`.

This could use a bit of refactoring/renaming - opening for a Crater run.

r? `@ghost`

- Replace {} with the stringified expr

  Giant thank you to `@danielhenrymantilla` for figuring out how to make
  this work ❤

- Note that this is just an approximation and it would be better to add
  a doc-comment

Remove `compile-fail` test suite

By moving all of its tests to `ui` test suite.
Now we have directives like `// dont-check-compiler-stderr` that allow to disable `.stderr` comparison for platform-dependent tests without introducing a whole new test suite.

We now lint on code like this:

```rust
fn main() {
    fn foo() {};
    struct Bar {};
}
```

Previously, this caused warnings in Cargo, so it was disabled.