Simplify visit_{foreign,trait}_item

Using an `if` seems like a better semantic fit and saves a few lines.

Noticed while looking at https://github.com/rust-lang/rust/pull/79752, but that's already merged.
r? `@lcnr,` cc `@cjgillot`

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

Strip prefix instead of replacing it with empty string

r? `@lcnr,` since you reviewed my other PR in the area.
`@rustbot` modify labels +C-cleanup +T-compiler

Fixes to Rust coverage

Fixes: #79725

Some macros can create a situation where `fn_sig_span` and `body_span`
map to different files.

New documentation on coverage tests incorrectly assumed multiple test
binaries could just be listed at the end of the `llvm-cov` command,
but it turns out each binary needs a `--object` prefix.

This PR fixes the bug and updates the documentation to correct that
issue. It also fixes a few other minor issues in internal implementation
comments, and adds documentation on getting coverage results for doc
tests.

Account for gaps in def path table during decoding

When encoding a proc-macro crate, there may be gaps in the table (since
we only encode the crate root and proc-macro items). Account for this by
checking if the entry is present, rather than using `unwrap()`

Update xsv to prevent random CI failures

This fixes occasional proptest failures due to a bug in xsv, which
aren't related to bugs in the rust compiler.

See https://github.com/rust-lang/rust/pull/79751#issuecomment-740027046 for context.

Privatize some of libcore unicode_internals

My understanding is that these API are perma unstable, so it doesn't
make sense to pollute docs & IDE completion[1] with them.

[1]: https://github.com/rust-analyzer/rust-analyzer/issues/6738

Remove `first_merge` from liveness debug logs

Fix trimming of lint docs

Fixes #79748.

It was removing all the indentation before.

r? `@Mark-Simulacrum`

minor stylistic clippy cleanups

simplify if let Some(_) = x  to  if x.is_some()  (clippy::redundant_pattern_matching)
don't create owned values for comparison (clippy::cmp_owned)
use .contains() or .any() instead of find(x).is_some() (clippy::search_is_some)
don't wrap code block in Ok()  (clipppy::unit_arg)

ext/ucred: Support PID in peer creds on macOS

This is a follow-up to https://github.com/rust-lang/rust/pull/75148 (RFC: https://github.com/rust-lang/rust/issues/42839).

The original PR used `getpeereid` on macOS and the BSDs, since they don't (generally) support the `SO_PEERCRED` mechanism that Linux supplies.

This PR splits the macOS/iOS implementation of `peer_cred()` from that of the BSDs, since macOS supplies the `LOCAL_PEERPID` sockopt as a source of the missing PID. It also adds a `cfg`-gated tests that ensures that platforms with support for PIDs in `UCred` have the expected data.

Properly re-use def path hash in incremental mode

Fixes #79661

In incremental compilation mode, we update a `DefPathHash -> DefId`
mapping every time we create a `DepNode` for a foreign `DefId`.
This mapping is written out to the on-disk incremental cache, and is
read by the next compilation session to allow us to lazily decode
`DefId`s.

When we decode a `DepNode` from the current incremental cache, we need
to ensure that any previously-recorded `DefPathHash -> DefId` mapping
gets recorded in the new mapping that we write out. However, PR #74967
didn't do this in all cases, leading to us being unable to decode a
`DefPathHash` in certain circumstances.

This PR refactors some of the code around `DepNode` deserialization to
prevent this kind of mistake from happening again.

Also generate `StorageDead` in constants

r? `@EddyB`

None of this special casing is actually necessary since we started promoting within constants and statics.

We may want to keep some of it around out of perf reasons, but it's not required for user visible behaviour

somewhat related: #68622

remove this weird special case from promotion

Promotion has a special case to ignore interior mutability under some specific circumstances. The purpose of this PR is to figure out what changes if we remove that. Since `Cell::new` and friends only get promoted inside `const`/`static` initializers these days, it actually is not easy to exploit this case: you need something like
```rust
const TEST_INTERIOR_MUT: () = {
    // The "0." case is already ruled out by not permitting any interior mutability in `const`.
    let _val: &'static _ = &(Cell::new(1), 2).1;
};
```

I assume something like `&Some(&(Cell::new(1), 2).1)` would hit the nested case inside `validate_rvalue`... though I am not sure why that would not just trigger nested promotion, first promoting the inner reference and then the outer one?

Fixes https://github.com/rust-lang/rust/issues/67534 (by simply rejecting that code^^)

r? `@oli-obk` (but for now this is not meant to be merged!)
Cc `@rust-lang/wg-const-eval`

Don't ICE on malformed `rustc_args_required_const` attribute

Don't time `emit_ignored_resolution_errors`

This printed several hundred lines each time rustdoc was run, almost all
of which rounded to 0.000. Since this isn't useful info, don't print it
everywhere, so other perf info is easier to read.

r? `@Mark-Simulacrum`

Use is_write_vectored to optimize the write_vectored implementation for BufWriter

In case when the underlying writer does not have an efficient implementation `write_vectored`, the present implementation of
`write_vectored` for `BufWriter` may still forward vectored writes directly to the writer depending on the total length of the data. This misses the advantage of buffering, as the actually written slice may be small.

Provide an alternative code path for the non-vectored case, where the slices passed to `BufWriter` are coalesced in the buffer before being flushed to the underlying writer with plain `write` calls. The buffer is only bypassed if an individual slice's length is at least as large as the buffer.

Remove a FIXME comment referring to #72919 as the issue has been closed with an explanation provided.

Compress RWU from at least 32 bits to 4 bits

The liveness uses a mixed representation of RWUs based on the
observation that most of them have invalid reader and invalid
writer. The packed variant uses 32 bits and unpacked 96 bits.
Unpacked data contains reader live node and writer live node.

Since live nodes are used only to determine their validity,
RWUs can always be stored in a packed form with four bits for
each: reader bit, writer bit, used bit, and one extra padding
bit to simplify packing and unpacking operations.

When encoding a proc-macro crate, there may be gaps in the table (since
we only encode the crate root and proc-macro items). Account for this by
checking if the entry is present, rather than using `unwrap()`

Using an `if` seems like a better semantic fit and saves a few lines.

Replace simple `if let` constructs with Option::map

Replaces a few constructs of the form

```
if let Some(x) = var {
    Some(...)
} else {
    None
}
```

with calls to `Option::map`.

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

Visit ForeignItems when marking dead code

Follow-up to #79318

r? `@lcnr`

Fixup: `filter().is_none()` -> `!any()`

Add a regression test for issue-66286

Fixes #66422

Update cargo

10 commits in 63d0fe43449adcb316d34d98a982b597faca4178..d274fcf862b89264fa2c6b917b15230705257317
2020-12-02 01:44:30 +0000 to 2020-12-07 23:08:44 +0000
- Clarify cargo manifest edition field docs (rust-lang/cargo#8953)
- Run rustdoc doctests relative to the workspace (rust-lang/cargo#8954)
- Workaround fs issue in `cargo publish`. (rust-lang/cargo#8950)
- Fix panic with -Zbuild-std and no roots. (rust-lang/cargo#8942)
- Slightly optimize `cargo vendor` (rust-lang/cargo#8937)
- Fixes rust-lang/cargo#8783 , cargo new fails without a author name or email (rust-lang/cargo#8912)
- Fix test escaping __CARGO_TEST_ROOT (rust-lang/cargo#8929)
- Add period to allowed feature name characters. (rust-lang/cargo#8932)
- faq: small fixes (rust-lang/cargo#8931)
- Fix semver documentation tests. (rust-lang/cargo#8930)

Fixes: #79725

Some macros can create a situation where `fn_sig_span` and `body_span`
map to different files.

New documentation on coverage tests incorrectly assumed multiple test
binaries could just be listed at the end of the `llvm-cov` command,
but it turns out each binary needs a `--object` prefix.

This PR fixes the bug and updates the documentation to correct that
issue. It also fixes a few other minor issues in internal implementation
comments, and adds documentation on getting coverage results for doc
tests.

Replaces a few constructs of the form

if let Some(x) = var {
    Some(...)
} else {
    None
}

with calls to Option::map.

Use `summary_opts()` in another spot

I added `summary_opts()` before I cut the branch for #77686 (2 months
ago!), so this "slipped through the cracks".

Validate naked functions definitions

Validate that naked functions are defined in terms of a single inline assembly
block that uses only `const` and `sym` operands and has `noreturn` option.

Implemented as future incompatibility lint with intention to migrate it into
hard error. When it becomes a hard error it will ensure that naked functions are
either unsafe or contain an unsafe block around the inline assembly. It will
guarantee that naked functions do not reference functions parameters (obsoleting
part of existing checks from #79411). It will limit the definitions of naked
functions to what can be reliably supported. It will also reject naked functions
implemented using legacy LLVM style assembly since it cannot satisfy those
conditions.

https://github.com/rust-lang/rfcs/pull/2774
https://github.com/rust-lang/rfcs/pull/2972