Also, turn off the lint when the unused dependencies json flag
is specified so that cargo doesn't have to supress the lint

Recent FreeBSD gdb packages have a different format for the version string.

Instead of producing `<span class=macro>assert_eq</span><span class=macro>!</span>`,
just produce `<span class=macro>assert_eq!</span>`.

Co-authored-by: Ralf Jung <post@ralfj.de>

…crum

Add an unstable --json=unused-externs flag to print unused externs

This adds an unstable flag to print a list of the extern names not used by cargo.

This PR will enable cargo to collect unused dependencies from all units and provide warnings.
The companion PR to cargo is: rust-lang/cargo#8437

The goal is eventual stabilization of this flag in rustc as well as in cargo.

Discussion of this feature is mostly contained inside these threads: rust-lang#57274 rust-lang#72342 rust-lang#72603

The feature builds upon the internal datastructures added by rust-lang#72342

Externs are uniquely identified by name and the information is sufficient for cargo.
If the mode is enabled, rustc will print json messages like:

```
{"unused_extern_names":["byteorder","openssl","webpki"]}
```

For a crate that got passed byteorder, openssl and webpki dependencies but needed none of them.

### Q: Why not pass -Wunused-crate-dependencies?
A: See [ehuss's comment here](rust-lang#57274 (comment))
   TLDR: it's cleaner. Rust's warning system wasn't built to be filtered or edited by cargo.
   Even a basic implementation of the feature would have to change the "n warnings emitted" line that rustc prints at the end.
   Cargo ideally wants to synthesize its own warnings anyways. For example, it would be hard for rustc to emit warnings like
   "dependency foo is only used by dev targets", suggesting to make it a dev-dependency instead.

### Q: Make rustc emit used or unused externs?
A: Emitting used externs has the advantage that it simplifies cargo's collection job.
   However, emitting unused externs creates less data to be communicated between rustc and cargo.
   Often you want to paste a cargo command obtained from `cargo build -vv` for doing something
   completely unrelated. The message is emitted always, even if no warning or error is emitted.
   At that point, even this tiny difference in "noise" matters. That's why I went with emitting unused externs.

### Q: One json msg per extern or a collective json msg?
A: Same as above, the data format should be concise. Having 30 lines for the 30 crates a crate uses would be disturbing to readers.
   Also it helps the cargo implementation to know that there aren't more unused deps coming.

### Q: Why use names of externs instead of e.g. paths?
A: Names are both sufficient as well as neccessary to uniquely identify a passed `--extern` arg.
   Names are sufficient because you *must* pass a name when passing an `--extern` arg.
   Passing a path is optional on the other hand so rustc might also figure out a crate's location from the file system.
   You can also put multiple paths for the same extern name, via e.g. `--extern hello=/usr/lib/hello.rmeta --extern hello=/usr/local/lib/hello.rmeta`,
   but rustc will only ever use one of those paths.
   Also, paths don't identify a dependency uniquely as it is possible to have multiple different extern names point to the same path.
   So paths are ill-suited for identification.

### Q: What about 2015 edition crates?
A: They are fully supported.
   Even on the 2015 edition, an explicit `--extern` flag is is required to enable `extern crate foo;` to work (outside of sysroot crates, which this flag doesn't warn about anyways).
   So the lint would still fire on 2015 edition crates if you haven't included a dependency specified in Cargo.toml using `extern crate foo;` or similar.
   The lint won't fire if your sole use in the crate is through a `extern crate foo;`   statement, but that's not its job.
   For detecting unused `extern crate foo` statements, there is the `unused_extern_crates` lint
   which can be enabled by `#![warn(unused_extern_crates)]` or similar.

cc ```@jsgf``` ```@ehuss``` ```@petrochenkov``` ```@estebank```

…e8472

Implement `SourceIterator` and `InPlaceIterable` for `ResultShunt`

…-Simulacrum

BTree: move blocks around in node.rs

Without changing any names or implementation, reorder some members:
- Move down the ones defined long ago on the demised `struct Root`, to below the definition of their current host `struct NodeRef`.
- Move up some defined on `struct NodeRef` that are interspersed with those defined on `struct Handle`.
- Move up the `correct_…` methods squeezed between the two flavours of `push`.
- Move the unchecked static downcasts (`cast_to_…`) after the upcasts (`forget_`) and the (weirdly named) dynamic downcasts (`force`).
r? ````@Mark-Simulacrum````

…=nikomatsakis

2229: Fix diagnostic issue when using FakeReads in closures

This PR fixes a diagnostic issue caused by rust-lang#82536. A temporary work around was used in this merged PR which involved feature gating the addition of FakeReads introduced as a result of pattern matching in closures.

The fix involves adding an optional closure DefId to ForLet and ForMatchedPlace FakeReadCauses. This DefId will only be added if a closure pattern matches a Place starting with an Upvar.

r? ```@nikomatsakis```

Fix compiletest on FreeBSD

Recent FreeBSD gdb packages have a different format for the version string.

…ht, r=GuillaumeGomez

rustdoc: highlight macros more efficiently

Instead of producing `<span class=macro>assert_eq</span><span class=macro>!</span>`,
just produce `<span class=macro>assert_eq!</span>`.

…r=camelid

Remove unneeded INITIAL_IDS const

Some IDs inside this map didn't exist anymore, some others were duplicates of what we have inside `IdMap`. So instead of keeping the two around and since `INITIAL_IDS` was only used by `IdMap`, no need to keep both of them.

…r=RalfJung

cleanup leak after test to make miri happy

Contains changes that were requested in rust-lang#83629 but didn't make it into the rollup.

r? `````@RalfJung`````