Rollup of 9 pull requests

Successful merges:

 - #78934 (refactor: removing library/alloc/src/vec/mod.rs ignore-tidy-filelength)
 - #79479 (Add `Iterator::intersperse`)
 - #80128 (Edit rustc_ast::ast::FieldPat docs)
 - #80424 (Don't give an error when creating a file for the first time)
 - #80458 (Some Promotion Refactoring)
 - #80488 (Do not create dangling &T in Weak<T>::drop)
 - #80491 (Miri: make size/align_of_val work for dangling raw ptrs)
 - #80495 (Rename kw::Invalid -> kw::Empty)
 - #80513 (Add regression test for #80062)

Failed merges:

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

Add regression test for #80062

Closes #80062.
r? `@lcnr`

Rename kw::Invalid -> kw::Empty

See https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Is.20there.20a.20symbol.20for.20the.20empty.20string.3F/near/220054471
for context.

r? `@petrochenkov`

Miri: make size/align_of_val work for dangling raw ptrs

This is needed for https://github.com/rust-lang/rust/issues/80365#issuecomment-752128105.

r? `@oli-obk`

Do not create dangling &T in Weak<T>::drop

Since at this point all strong pointers have been dropped, the wrapped `T` has also been dropped. As such, creating a `&T` to the dropped place is negligent at best (language UB at worst). Since we have `Layout::for_value_raw` now, use that instead of `Layout::for_value` to avoid creating the `&T`.

This does have implications for custom (potentially thin) DSTs, though much less severe than those discussed in #80407. Specifically, one of two things has to be true:

- It has to be possible to use a `*const T` to a dropped (potentially custom, potentially thin) unsized tailed object to determine the layout (size/align) of the object. This is what is currently implemented (though with `&T` instead of `&T`). The validity of reading some location after it has been dropped is an open question IIUC (https://github.com/rust-lang/unsafe-code-guidelines/issues/188) (except when the whole type is `Copy`, per `drop_in_place`'s docs).
  In this design, custom DSTs would get a `*mut T` and use that to return layout, and must be able to do so while in the "zombie" (post-drop, pre-free) state.
- `RcBox`/`ArcInner` compute and store layout eagerly, so that they don't have to ask the type for its layout after dropping it.

Importantly, this is already true today, as you can construct `Rc<DST>`, create a `Weak<DST>`, and drop the `Rc` before the `Weak`. This PR is a strict improvement over the status quo, and the above question about potentially thin DSTs will need to be resolved by any custom DST proposal.

Some Promotion Refactoring

Clean up promotion a bit:
* factor out some common code
* more exhaustive matches

This *should* not break anything... the only potentially-breaking change is that `BorrowKind::Shallow | BorrowKind::Unique` are now rejected for internal references.

r? ``@oli-obk``

Don't give an error when creating a file for the first time

Previously, `os.remove` would always give a FileNotFound error the first
time you called it, causing bootstrap to make unnecessary copies. This
now only calls `remove()` if the file exists, avoiding the unnecessary
error.

This is a pretty small cleanup but I think it's useful. Taken from https://github.com/rust-lang/rust/pull/79540.

Edit rustc_ast::ast::FieldPat docs

Punctuation fixes.

Add `Iterator::intersperse`

This is a rebase of #75784. I'm hoping to push this past the finish line!

cc `@jonas-schievink`

refactor: removing library/alloc/src/vec/mod.rs ignore-tidy-filelength

This PR removes the need for ignore-tidy-filelength for library/alloc/src/vec/mod.rs which is part of the issue #60302

It is probably easiest to review this PR by looking at it commit by commit rather than looking at the overall diff.

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

See https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Is.20there.20a.20symbol.20for.20the.20empty.20string.3F/near/220054471
for context.

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