Cstring `from_raw` and `into_raw` safety precisions

Fixes #48525.
Fixes #68456.

This issue had two points:

- The one about `from_raw` has been addressed (I hope).
- The other one, about `into_raw`, has only been partially fixed.

About `into_raw`: the idea was to:

> steer users away from using the pattern of CString::{into_raw,from_raw} when interfacing with C APIs that may change the effective length of the string by writing interior NULs or erasing the final NUL

I tried making a `Vec<c_char>` like suggested but my current solution feels very unsafe and *hacky* to me (most notably the type cast), I included it here to make it available for discussion:

```rust
fn main() {
    use std::os::raw::c_char;

    let v = String::from("abc")
        .bytes()
        // From u8 to i8,
        // I feel like it will be a problem for values of u8 > 255
        .map(|c| c as c_char)
        .collect::<Vec<_>>();

    dbg!(v);
}
```

Liballoc impl

Mainly code rearrangements

Add `-Z span-debug` to allow for easier debugging of proc macros

Currently, the `Debug` impl for `proc_macro::Span` just prints out
the byte range. This can make debugging proc macros (either as a crate
author or as a compiler developer) very frustrating, since neither the
actual filename nor the `SyntaxContext` is displayed.

This commit adds a perma-unstable flag `-Z span-debug`. When enabled,
the `Debug` impl for `proc_macro::Span` simply forwards directly to
`rustc_span::Span`. Once #72618 is merged, this will start displaying
actual line numbers.

While `Debug` impls are not subject to Rust's normal stability
guarnatees, we probably shouldn't expose any additional information on
stable until `#![feature(proc_macro_span)]` is stabilized. Otherwise,
we would be providing a 'backdoor' way to access information that's
supposed be behind unstable APIs.

Added the documentation for the 'use' keyword

This is a partial fix of #34601.

I heavily inspired myself from the Reference on the `use` keyword.

I checked the links when compiling the documentation, they should be ok.

I also added an example for the wildcard `*` in the case of types, because it's behaviour is not *import everything* like one might think at first.

Fix documentation example for gcov profiling

closes #72546
Improves the documentation for the unstable Rustflag `-Zprofile` by:
- stating that Incremental compilation must be turned off.
- Adding the other `RUSTFLAGS` that should/need to be turned on (taken from [grcov documentation](https://github.com/mozilla/grcov#example-how-to-generate-gcda-files-for-a-rust-project))
- Mentioning `RUSTC_WRAPPER` to prevent everything getting instrumented.

r? @steveklabnik

impl AsRef<[T]> for vec::IntoIter<T>

Adds `impl<T> AsRef<[T]> for vec::IntoIter<T>`. This mirrors the same trait impl for [`slice::Iter`](https://doc.rust-lang.org/nightly/std/slice/struct.Iter.html). Both types already offer `fn as_slice(&self) -> &[T]`, this just adds the trait impl for `vec::IntoIter`.

If/when `fn as_slice(&self) -> &[T]` stabilizes for `vec::Drain` and `slice::IterMut`, they should get `AsRef<[T]>` impls as well. As thus, tangentially related to #58957.

My ultimate goal here: being able to use `for<T, I: Iterator<Item=T> + AsRef<[T]>> I` to refer to `vec::IntoIter`, `vec::Drain`, and eventually `array::IntoIter`, as an approximation of the set of by-value iterators that can be "previewed" as by-ref iterators. (Actually expressing that as a trait requires GAT.)

Update annotate-snippets-rs to 0.8.0

#59346
I made major changes to this library. In the previous version we worked with owned while in the current one with borrowed.

I have adapted it without changing the behavior.
I have modified the coverage since the previous one did not return correctly the index of the character in the line.

save_analysis: fix some ICEs

Fixes #73020
Fixes #73022
Fixes #73041

Order the Rust and C ABIs first to reduce test churn

Rollup of 2 pull requests

Successful merges:

 - #72952 (run-make regression test for issue #70924.)
 - #72977 (Fix codegen tests for RISC-V)

Failed merges:

r? @ghost

Fix codegen tests for RISC-V

Some codegen tests didn't seem relevant (e.g. unsupported annotations).

The RISC-V abi tests were broken by LLVM 10, c872dcf9 fixes that (cc: @msizanoen1)

I'm not sure about skipping catch-unwind.rs and included that change here mostly as a request for comment - I can't tell if that's a bug.

run-make regression test for issue #70924.

Sometime after my PR #72767 (to fix issue #70924) landed, I realized that I *could* make a local regression test, thanks to `rustc --print sysroot`: I can make a fresh "copy" (really mostly symlinks) of the sysroot, and then modify it to recreate the terms of this bug.

Rollup of 5 pull requests

Successful merges:

 - #72764 (Be more careful around ty::Error in generators)
 - #72908 (rename FalseEdges -> FalseEdge)
 - #72970 (Properly handle feature-gated lints)
 - #72998 (Mention that some atomic operations may not be available on some platforms)
 - #73063 (Elide type on liballoc vec)

Failed merges:

r? @ghost

Elide type on liballoc vec

Mention that some atomic operations may not be available on some platforms

fixes #54250

This simply adds a line saying the type/function/method may not be available on some platforms, depending on said platform capabilities.

I *think* I got them all.

Properly handle feature-gated lints

Closes #72694

rename FalseEdges -> FalseEdge

There's just a single false edge in this terminator, not multiple of them.

r? @matthewjasper @jonas-schievink

Be more careful around ty::Error in generators

cc https://github.com/rust-lang/rust/issues/72685

(doesn't close it because it's missing a reproduction to use as a test case)

r? @estebank

Update LLVM submodule to include lld NOLOAD fix

> Rust nightly 2020-05-22 and later ships lld with a regression related to linker scripts: NOLOAD sections incorrectly generate sections filled with 0s. This causes gdb and other elf loaders to write to reserved or otherwise invalid addresses (gdb also seems confused by the resulting ELF files and spits out a warning about the sections). This is particularly a problem for embedded rust projects that use lld by default and have affected linker scripts (cortex-m-rt based projects for instance).

https://github.com/rust-lang/llvm-project/pull/64

Note that this also pulls in llvm changes from #72937

Rollup of 5 pull requests

Successful merges:

 - #72810 (validate basic sanity for TerminatorKind)
 - #72989 (Revert pr 71840)
 - #72993 (Count the beta prerelease number just from master)
 - #73057 (Clean up E0644 explanation)
 - #73059 (remove outdated comment)

Failed merges:

r? @ghost

remove outdated comment

r? @matthewjasper

Clean up E0644 explanation

r? @Dylan-DPC

Count the beta prerelease number just from master

We were computing a merge-base between the remote beta and master
branches, but this was giving incorrect answers for the first beta if
the remote hadn't been pushed yet. For instance, `1.45.0-beta.3359`
corresponds to the number of merges since the 1.44 beta, but we really
want just `.1` for the sole 1.45 beta promotion merge.

We don't really need to query the remote beta at all -- `master..HEAD`
suffices if we assume that we're on the intended beta branch already.

Revert pr 71840

Revert7 PR #71840 to fix issue #72470

This will need a backport to beta if we do not want #72470 to hit stable.

validate basic sanity for TerminatorKind

r? @jonas-schievink

This mainly checks that all `BasicBlock` actually exist. On top of that, it checks that `Call` actually calls something of `FnPtr`/`FnDef` type, and `Assert` has to work on a `bool`. Also `SwitchInt` cannot have an empty target list.

Co-authored-by: NAmanieu d'Antras <amanieu@gmail.com>

Rollup of 3 pull requests

Successful merges:

 - #71796 (de-promote Duration::from_secs)
 - #72508 (Make `PolyTraitRef::self_ty` return `Binder<Ty>`)
 - #72708 (linker: Add a linker rerun hack for gcc versions not supporting -static-pie)

Failed merges:

r? @ghost

linker: Add a linker rerun hack for gcc versions not supporting -static-pie

Which mirrors the existing `-no-pie` linker rerun hack, but the logic is a bit more elaborated in this case.

If the linker (gcc or clang) errors on `-static-pie` we rerun in with `-static` instead.
We must also replace CRT objects corresponding to `-static-pie` with ones corresponding to `-static` in this case.

(One sanity check for CRT objects in target specs is also added as a drive-by fix.)

To do in the future: refactor all linker rerun hacks into separate functions and share more code with `add_(pre,post)_link_objects`.

This PR accompanies https://github.com/rust-lang/rust/pull/71804 and unblocks https://github.com/rust-lang/rust/pull/70740.

Make `PolyTraitRef::self_ty` return `Binder<Ty>`

This came up during review of #71618. The current implementation is the same as a call to `skip_binder` but harder to audit. Make it preserve binding levels and add a call to `skip_binder` at all use sites so they can be audited as part of #72507.

de-promote Duration::from_secs

In https://github.com/rust-lang/rust/pull/67531, we removed the `rustc_promotable` attribute from a bunch of `Duration` methods, but not from `Duration::from_secs`. This makes the current list of promotable functions the following (courtesy of @ecstatic-morse):

* `INT::min_value`, `INT::max_value`
* `std::mem::size_of`, `std::mem::align_of`
* `RangeInclusive::new` (backing `x..=y`)
* `std::ptr::null`, `std::ptr::null_mut`
* `RawWaker::new`, `RawWakerVTable::new` ???
* `Duration::from_secs`

I feel like the last one stands out a bit here -- the rest are all very core language primitives, and `RawWaker` has a strong motivation for getting a `'static` vtable. But a `&'static Duration`? That seems unlikely. So I propose we no longer promote calls to `Duration::from_secs`, which is what this PR does.

https://github.com/rust-lang/rust/pull/67531 saw zero regressions and I am not aware of anyone complaining that this broke their (non-cratered) code, so I consider it likely the same will be true here, but of course we'd do a crater run.

See [this document](https://github.com/rust-lang/const-eval/blob/master/promotion.md) for some more background on promotion and https://github.com/rust-lang/const-eval/issues/19 for some of the concerns around promoting function calls.

Rename all remaining compiler crates to use the `rustc_foo` pattern

libarena -> librustc_arena
libfmt_macros -> librustc_parse_format
libgraphviz -> librustc_graphviz
libserialize -> librustc_serialize

Closes https://github.com/rust-lang/rust/issues/71177 in particular.

- lint: add `feature_gate` field, allow specifying it in `declare_lint!`

- generalize the lint feature gate check

- librustdoc: generalize whitelisting of feature-gated lints

resolve: Sort E0408 errors by Symbol str

This is a request for comments implementing my suggested solution to https://github.com/rust-lang/rust/issues/72913

Previously errors were sorted by Symbol index instead of the string. The indexes are not the same between architectures because Symbols for architecture extensions (e.g. x86 AVX or RISC-V d) are interned before the source file is parsed. RISC-V's naming of extensions after single letters led to it having errors sorted differently for test cases using single letter variable names. Instead sort the errors by the Symbol string so that it is stable across architectures.

While I was at it, there's also 8edb05c2  skipping some ui tests which I think are irrelevant for risc-v.