env: remove unwrap in examples in favor of try op

Fix rustdoc panic with `impl Trait` in type parameters

Fixes #50702.

I'm not sure `impl Trait`s neither in arguments nor in return types are supposed to work, though.

read2: Use inner function instead of closure

Very minor thing, but there doesn't appear to be a reason to use a closure here.

Generated code is identical in my tests, but I believe it's clearer that nothing from the environment is being used.

rustdoc: deprecate `#![doc(passes, plugins, no_default_passes)]`

Closes https://github.com/rust-lang/rust/issues/48164

Blocked on https://github.com/rust-lang/rust/pull/50541 - this includes those changes, which were necessary to create the UI test

cc https://github.com/rust-lang/rust/issues/44136

Turns out, there were special attributes to mess with rustdoc passes and plugins! Who knew! Since we deprecated the CLI flags for this functionality, it makes sense that we do the same for the attributes.

This PR also introduces a `#![doc(document_private_items)]` attribute, to match the `--document-private-items` flag introduced in https://github.com/rust-lang/rust/pull/44138 when the passes/plugins flags were deprecated.

I haven't done a search to see whether these attributes are being used at all, but if the flags were any indication, i don't expect to see any users of these.

Fix `fn main() -> impl Trait` for non-`Termination` trait

Fixes #50595.

This bug currently affects stable. Why I think we can go for hard error:
- It will in stable for at most one cycle and there is no legitimate reason to abuse it, nor any known uses in the wild.
- It only affects `bin` crates (which have a `main`), so there is little practical difference between a hard error or a deny lint, both are a one line fix.

The fix was to just unshadow a variable. Thanks @nikomatsakis for the mentoring!

r? @nikomatsakis

Don't unconditionally set CLOEXEC twice on every fd we open on Linux

Previously, every `open64` was accompanied by a `ioctl(…, FIOCLEX)`,
because some old Linux version would ignore the `O_CLOEXEC` flag we pass
to the `open64` function.

Now, we check whether the `CLOEXEC` flag is set on the first file we
open – if it is, we won't do extra syscalls for every opened file. If it
is not set, we fall back to the old behavior of unconditionally calling
`ioctl(…, FIOCLEX)` on newly opened files.

On old Linuxes, this amounts to one extra syscall per process, namely
the `fcntl(…, F_GETFD)` call to check the `CLOEXEC` flag.

On new Linuxes, this reduces the number of syscalls per opened file by
one, except for the first file, where it does the same number of
syscalls as before (`fcntl(…, F_GETFD)` to check the flag instead of
`ioctl(…, FIOCLEX)` to set it).

Review proc macro API 1.2

cc https://github.com/rust-lang/rust/issues/38356

Summary of applied changes:
- Documentation for proc macro API 1.2 is expanded.
- Renamed APIs: `Term` -> `Ident`, `TokenTree::Term` -> `TokenTree::Ident`, `Op` -> `Punct`, `TokenTree::Op` -> `TokenTree::Punct`, `Op::op` -> `Punct::as_char`.
- Removed APIs: `Ident::as_str`, use `Display` impl for `Ident` instead.
- New APIs (not stabilized in 1.2): `Ident::new_raw` for creating a raw identifier (I'm not sure `new_x` it's a very idiomatic name though).
- Runtime changes:
    - `Punct::new` now ensures that the input `char` is a valid punctuation character in Rust.
    - `Ident::new` ensures that the input `str` is a valid identifier in Rust.
    - Lifetimes in proc macros are now represented as two joint tokens - `Punct('\'', Spacing::Joint)` and `Ident("lifetime_name_without_quote")` similarly to multi-character operators.
- Stabilized APIs: None yet.

A bit of motivation for renaming (although it was already stated in the review comments):
- With my compiler frontend glasses on `Ident` is the single most appropriate name for this thing, *especially* if we are doing input validation on construction. `TokenTree::Ident` effectively wraps `token::Ident` or `ast::Ident + is_raw`, its meaning is "identifier" and it's already named `ident` in declarative macros.
- Regarding `Punct`, the motivation is that `Op` is actively misleading. The thing doesn't mean an operator, it's neither a subset of operators (there is non-operator punctuation in the language), nor superset (operators can be multicharacter while this thing is always a single character). So I named it `Punct` (first proposed in [the original RFC](https://github.com/rust-lang/rfcs/pull/1566), then [by @SimonSapin](https://github.com/rust-lang/rust/issues/38356#issuecomment-276676526)) , together with input validation it's now a subset of ASCII punctuation character category (`u8::is_ascii_punctuation`).

Implement RFC 2056 trivial constraints in where clauses

This is an implementation of the new behaviour for #48214. Tests are mostly updated to show the effects of this. Feature gate hasn't been added yet.

Some things that are worth noting and are maybe not want we want

* `&mut T: Copy` doesn't allow as much as someone might expect because there is often an implicit reborrow.
* ~There isn't a check that a where clause is well-formed any more, so `where Vec<str>: Debug` is now allowed (without a `str: Sized` bound).~

r? @nikomatsakis

Update RLS and Rustfmt

Fixes RLS tests

r? @alexcrichton

 Remove ScopeTarget and LoopIdResult

* Remove ScopeTarget in preparation of label-break-value (PR #50045)
* Replace LoopIdResult by Result which is possible now thanks to commit 8ac65af8 " Implement Encodable and Decodable for Result."

r? @EddyB

rustdoc: replace most (e)println! statements with structured warnings/errors

Turns out, the rustc diagnostic handler doesn't need a whole lot of setup that we weren't already doing. For errors that occur outside a "dealing with source code" context, we can just use the format/color config we were already parsing and make up a `Handler` that we can emit structured warnings/errors from. So i did that. This will make it way easier to test things with `rustdoc-ui` tests, since those require the JSON error output. (In fact, this PR is a yak shave for a different one where i was trying to do just that. `>_>`)

The Great Generics Generalisation: Ty Edition

Part of the generic parameter refactoring effort, split off from https://github.com/rust-lang/rust/pull/48149. Contains the `ty`-relative refactoring.

r? @EddyB

Remove unnecessary proc-macro-related `feature`s

Don't inject clippy into the rls anymore

r? @kennytm

sorry about breaking nightlies.

The issue is that the `[patch.crates-io]` doesn't work if the versions differ. So every time we update clippy, we can only update it to the verison that rls is depending on.

I'm disabling the injection of clippy into rls for now. I'm not sure how to do this properly. We could

* add a version check, so rls only builds clippy if its dependency clippy is the same as the submodule clippy
* do something crazy like auto-patching the Cargo.toml of the rls tool repo to just use a path dependency on clippy
* build crates-io clippy instead of submodule clippy and gate clippy injection on that
    * that's somewhat automatic, and is essentially what is necessary right now, but done manually
* make clippy 0.1.* instead of 0.0.* and update patch versions for nightly updates and minor version updates for `clippy_lints` api changes.
    * not sure how semver-great this is

It's redundant as Result already implements Encodable
as well as Decodable.

When we want to implement label-break-value,
we can't really decide whether to emit ScopeTarget::Loop or
ScopeTarget::Block in the code that is supposed to create it.
So we get rid of it and reconstruct the information when
needed.