This is preparation for reverting 81238 for short-term resolution of issue 84297.

Support for force-warns

Implements https://github.com/rust-lang/rust/issues/85512.

This PR adds a new command line option `force-warns` which will force the provided lints to warn even if they are allowed by some other mechanism such as `#![allow(warnings)]`.

Some remaining issues:
* https://github.com/rust-lang/rust/issues/85512 mentions that `force-warns` should also be capable of taking lint groups instead of individual lints. This is not implemented.
* If a lint has a higher warning level than `warn`, this will cause that lint to warn instead. We probably want to allow the lint to error if it is set to a higher lint and is not allowed somewhere else.
* One test is currently ignored because it's not working - when a deny-by-default lint is allowed, it does not currently warn under `force-warns`. I'm not sure why, but I wanted to get this in before the weekend.

r? `@nikomatsakis`

Reland - Report coverage `0` of dead blocks

Fixes: #84018

With `-Z instrument-coverage`, coverage reporting of dead blocks
(for example, blocks dropped because a conditional branch is dropped,
based on const evaluation) is now supported.

Note, this PR relands an earlier, reverted PR that failed when compiling
generators. The prior issues with generators has been resolved and a new
test was added to prevent future regressions.

Check out the resulting changes to test coverage of dead blocks in the
test coverage reports in this PR.

r? `@tmandry`
fyi: `@wesleywiser`

fix testing Miri with --stage 0

Fixes https://github.com/rust-lang/rust/issues/78778 for Miri.

The issue remains open for error_index_generator, which has its own dylib logic:
https://github.com/rust-lang/rust/blob/903e369c831d52726a5292f847e384622189d9a0/src/bootstrap/tool.rs#L396-L400
`@jyn514` I could just copy the logic from `add_rustc_lib_path`, but that does not seem great. Any other suggestions?

Also I wonder if maybe `prepare_tool_cargo` should already call `add_rustc_lib_path`.

Rollup of 8 pull requests

Successful merges:

 - #85717 (Document `From` impls for cow.rs)
 - #85850 (Remove unused feature gates)
 - #85888 (Fix typo in internal documentation for `TrustedRandomAccess`)
 - #85889 (Restoring the `num_def_ids` function in the CStore API )
 - #85899 (jsondocck small cleanup)
 - #85937 (Fix bad suggestions for code from proc_macro)
 - #85963 (Show `::{{constructor}}` in std::any::type_name().)
 - #85977 (Fix linkcheck script from getting out of sync.)

Failed merges:

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

Fix linkcheck script from getting out of sync.

When there are changes to the linkcheck script, the CI jobs used in the books would download the latest version on master, but run it against nightly. During that 24 hour window, the CI can fail if the script has changes that are incompatible with the last nightly. This fixes it so that it downloads the linkchecker that matches the version of nightly.

This also includes a fix to build with release to make it run much faster (I forgot to add this in #85652).

Show `::{{constructor}}` in std::any::type_name().

Fix #84666

Before:
```
[src/main.rs:6] type_name::<T>() = "playground::Velocity"
[src/main.rs:6] type_name::<T>() = "playground::Velocity"
```

After:
```
[src/main.rs:6] type_name::<T>() = "scratchpad::Velocity::{{constructor}}"
[src/main.rs:6] type_name::<T>() = "scratchpad::Velocity"
```

cc ``@scottmcm``

Fix bad suggestions for code from proc_macro

Fixes #85932

This disables these suggestions for spans from external macros, while keeping them for macros defined locally:

Before:
```
3 | #[hello]
  | ^^^^^^^^
  | |
  | expected `&mut i32`, found integer
  | help: consider mutably borrowing here: `&mut #[hello]`
```

After:
```
3 | #[hello]
  | ^^^^^^^^ expected `&mut i32`, found integer
```

Unchanged:
```
26 | macro_rules! bla { () => { x(123); } }
   |                              ^^^
   |                              |
   |                              expected `&mut i32`, found integer
   |                              help: consider mutably borrowing here: `&mut 123`
...
29 |     bla!();
   |     ------- in this macro invocation
```

jsondocck small cleanup

updated `shlex` (there was some fix https://github.com/comex/rust-shlex/commit/6db4704fcad8393dd6947b1d31720327546a84a2)
replaced `lazy_static` with `once_cell`
removed `serde` direct dependency (`serde_json` will pull it)

Restoring the `num_def_ids` function in the CStore API

## The context

I am the maintainer of https://github.com/hacspec/hacspec, an embedded Rust DSL aimed at cryptographic specifications. As it is normal for an embedded DSL, Hacspec's compiler relies on being plugged to the internal API of the Rust compiler, which is unstable and subject to changes.

## The problem

The Hacspec compiler features its own typechecker, that performs an additional, more restrictive typechecking pass over the Rust code of a crate. To complete this typechecking, the Hacspec compiler needs to retrieve the signature of functions defined in non-local imported crates. Rather than retrieving these signatures on-demand, the Hacspec compiler pre-populates its typechecking context with all the Hacspec-compatible symbols defined in non-local crates first. This requires having a way to iterate over all the definitions in a non-local crate.

I used to do this with `CrateMetadata::all_def_path_hashes_and_def_ids`, but this function was deleted in 908bf5a3. Then, I fellback on `CStore::num_def_ids`, exploiting the fact that all the `DefIds` for a crate have the same `krate_num` and range from `0` to `num_def_ids(krate_num)`. But `num_def_ids` was deleted in b6120bfb.

I looked to the `Cstore::item_children_untracked` function to replicate the feature of traversing through all the `DefId` for a crate, using `CRATE_DEF_INDEX` as the root, but this does not work as recursive `Cstore::item_children_untracked` calls do not reach all the symbols I was able to reach using the two previous methods.

## Description of this PR

This PR simply restores in the public API of `CStore` the `num_def_ids` function, giving the size of the definition table for a given crate.

Fix typo in internal documentation for `TrustedRandomAccess`

`next_back` is a method of DoubleEndedIterator, not Iterator.

Remove unused feature gates

The first commit removes a usage of a feature gate, but I don't expect it to be controversial as the feature gate was only used to workaround a limitation of rust in the past. (closures never being `Clone`)

The second commit uses `#[allow_internal_unstable]` to avoid leaking the `trusted_step` feature gate usage from inside the index newtype macro. It didn't work for the `min_specialization` feature gate though.

The third commit removes (almost) all feature gates from the compiler that weren't used anyway.

Document `From` impls for cow.rs

Update Clippy

Bi-weekly Clippy update.

r? `@Manishearth`

Remove unnecessary SpecFromIter impls

Unless I’m missing something, these `SpecFromIter<&'a T, …> for Vec<T>` implementations were completely unused.

shrinking the deprecated method span

part of https://github.com/rust-lang/rust/issues/85403

r? `@estebank`

Improve debugging experience for enums on windows-msvc

This PR makes significant improvements over the status quo of debugging enums on the windows-msvc platform with either WinDbg or Visual Studio in three ways:

1. Improves the debugger experience for directly tagged enums.
2. Fixes a bug which caused the debugger to sometimes show the wrong debug info for niche layout enums. For example, `Option<&u32>` could sometimes use the debug info for `Option<&f64>` instead leading to nonsensical variable values in the debugger.
3. Significantly improves the debugger experience for niche-layout enums.

Let's look at a few examples:

```rust
pub enum CStyleEnum {
    Base = 2,
    Exponent = 16,
}

pub enum NicheLayoutEnum {
    Tag1,
    Data { my_data: CStyleEnum },
    Tag2,
    Tag3,
    Tag4,
}

pub enum OtherEnum<T> {
    Case1(T),
    Case2(T),
}

fn main() {
    let a = Some(CStyleEnum::Base);
    let b = Option::<CStyleEnum>::None;
    let c = NicheLayoutEnum::Tag1;
    let d = NicheLayoutEnum::Data { my_data: CStyleEnum::Exponent };
    let e = NicheLayoutEnum::Tag2;
    let f = Some(&1u32);
    let g = Option::<&'static u32>::None;
    let h = Some(&2u64);
    let i = Option::<&'static u64>::None;
    let j = Some(12u32);
    let k = Option::<u32>::None;
    let l = Some(12.34f64);
    let m = Option::<f64>::None;
    let n = CStyleEnum::Base;
    let o = CStyleEnum::Exponent;
    let p = Some("IAMA optional string!".to_string());
    let q = OtherEnum::Case1(42u32);
}
```

This is what WinDbg Preview shows using the latest rustc nightly:

![image](https://user-images.githubusercontent.com/831192/118285353-57c10780-b49f-11eb-97aa-db3abfc09508.png)

Most of the variables don't show a meaningful value expect for a few cases that we have targeted natvis definitions covering. Even worse, drilling into many of these variables shows information that can be difficult to interpret without an understanding of the layout of Rust types:

![image](https://user-images.githubusercontent.com/831192/118285609-a1a9ed80-b49f-11eb-9c29-b14576984647.png)

With the changes in this PR, we're able to write two natvis definitions that cover all enum cases generally. After building with these changes, WinDbg now shows this instead:

![image](https://user-images.githubusercontent.com/831192/118287730-be472500-b4a1-11eb-8cad-8f6a91c7516b.png)

Drilling into the same variables, we can see much more useful information:

![image](https://user-images.githubusercontent.com/831192/118287888-e20a6b00-b4a1-11eb-927f-32cf33a31c16.png)

Fixes #84670
Fixes #84671

Update Miri

Fixes #85946.

r? `@RalfJung`

Rollup of 13 pull requests

Successful merges:

 - #83362 (Stabilize `vecdeque_binary_search`)
 - #85706 (Turn off frame pointer elimination on all Apple platforms. )
 - #85724 (Fix issue 85435 by restricting Fake Read precision)
 - #85852 (Clarify meaning of MachineApplicable suggestions.)
 - #85877 (Intra doc link-ify a reference to a function)
 - #85880 (convert assertion on rvalue::threadlocalref to delay bug)
 - #85896 (Add test for forward declared const param defaults)
 - #85897 (Update I-unsound label for triagebot)
 - #85900 (Use pattern matching instead of checking lengths explicitly)
 - #85911 (Avoid a clone of output_filenames.)
 - #85926 (Update cargo)
 - #85934 (Add `Ty::is_union` predicate)
 - #85935 (Validate type of locals used as indices)

Failed merges:

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

Restore the num_def_ids_untracked public function giving the total number of exported symbols for each crate

Restored underlying num_def_ids_method

Update compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

Changed name to fit with naming convention
Co-authored-by: Nbjorn3 <bjorn3@users.noreply.github.com>

Update compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

Replace regular doc with Rustdoc comment
Co-authored-by: NJoshua Nelson <jyn514@gmail.com>

Clarifies third-party use of num_def_ids_untracked

Rustup

Empty rustup. No changes to Clippy in the Rust repo for the last 2 weeks 😮

changelog: none

Validate type of locals used as indices

Add `Ty::is_union` predicate

Update cargo

10 commits in e931e4796b61de593aa1097649445e535c9c7ee0..0cecbd67323ca14a7eb6505900d0d7307b00355b
2021-05-24 16:17:27 +0000 to 2021-06-01 20:09:13 +0000
- Configure hosts separately from targets when --target is specified. (rust-lang/cargo#9322)
- Add some validation to rustc-link-arg (rust-lang/cargo#9523)
- Implement suggestions for unknown features in workspace (rust-lang/cargo#9420)
- Extract common `make_dep_path` to cargo_util (rust-lang/cargo#9529)
- Add a note about rustflags compatibility. (rust-lang/cargo#9524)
- Consolidate doc collision detection. (rust-lang/cargo#9526)
- Add `--depth` option for `cargo-tree` (rust-lang/cargo#9499)
- `cargo tree -e no-proc-macro` to hide procedural macro dependencies (rust-lang/cargo#9488)
- Update to semver 1.0.0 (rust-lang/cargo#9508)
- Update tar dependency to 0.4.35 (rust-lang/cargo#9517)

Avoid a clone of output_filenames.

Part of #85153

Use pattern matching instead of checking lengths explicitly

This piece of code checks that there are exaclty two variants, one having
exactly one field, the other having exactly zero fields. If any of these
conditions is violated, it returns `None`. Otherwise it assigns that one
field's ty to `field_ty`.

Instead of fiddling with indices and length checks explicitly, use pattern
matching to simplify this.

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

Update I-unsound label for triagebot

Following the remaming of the `I-unsound` label (removing the space and the emoji) as discussed [on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/A.20way.20to.20self-label.20soundness.20issues.3F/near/240962907) (or [in the archive](https://zulip-archive.rust-lang.org/122651general/88362Awaytoselflabelsoundnessissues.html#240962907)), this change of the `triagebot.toml` is necessary to keep the automatic `I-prioritize` flagging.

Add test for forward declared const param defaults