cleanup: `Span::new` -> `Span::with_lo`

Extracted from https://github.com/rust-lang/rust/pull/84373 as suggested in https://github.com/rust-lang/rust/pull/84373#issuecomment-857773867.
It turned out less useful then I expected, but anyway.
r? `@cjgillot`
`@bors` rollup

PassWrapper: adapt for LLVM 14 changes

These API changes appear to have all taken place in
https://reviews.llvm.org/D105007, which moved HWAddressSanitizerPass and
AddressSanitizerPass to only accept their options type as a ctor
argument instead of the sequence of bools etc. This required a couple of
parameter additions, which I made match the default prior to the
mentioned upstream LLVM change.

This patch restores rustc to building (though not quite passing all
tests, I've mailed other patches for those issues) against LLVM HEAD.

Generate an iOS LLVM target with a specific version

This commit adds the `LC_VERSION_MIN_IPHONEOS` load command to the Mach-O header generated for `aarch64-apple-ios` binaries. The operating system will look for this load command to determine the minimum supported operating system version and will not allow the binary to run if it's absent. This logic already exists for the simulator toolchain.

I've been using `otool` from a [cctools](https://github.com/tpoechtrager/cctools-port) toolchain to parse the header and validate that this change adds the required load command.

This change appears to be enough to build Rust binaries that can run on a jailbroken iPhone.

Show discriminant before overflow in diagnostic for duplicate values.

This PR adds the value before overflow for explicit discriminant values in the error for duplicate discriminant values.
I found it rather confusing to see only the overflowed value.

It only does this for literals, since overflows in const evaluated arithmetic are already a hard error.

This is my first PR to the compiler, so please let me know if the implementation can be improved :)

Before:
![image](https://user-images.githubusercontent.com/786213/125850097-bf5fb7e0-d800-4386-a738-c30f41822964.png)

After:
![image](https://user-images.githubusercontent.com/786213/125850120-e2bb765d-ad86-4888-a6cb-dec34fba3fea.png)

Improve wording of the `drop_bounds` lint

This PR addresses #86653. The issue is sort of a false positive of the `drop_bounds` lint, but I would argue that the best solution for #86653 is simply a rewording of the warning message and lint description, because even if the lint is _technically_ wrong, it still forces the programmer to think about what they are doing, and they can always use `#[allow(drop_bounds)]` if they think that they really need the `Drop` bound.

There are two issues with the current warning message and lint description:
- First, it says that `Drop` bounds are "useless", which is technically incorrect because they actually do have the effect of allowing you e.g. to call methods that also have a `Drop` bound on their generic arguments for some reason. I have changed the wording to emphasize not that the bound is "useless", but that it is most likely not what was intended.
- Second, it claims that `std::mem::needs_drop` detects whether a type has a destructor. But I think this is also technically wrong: The `Drop` bound says whether the type has a destructor or not, whereas `std::mem::needs_drop` also takes nested types with destructors into account, even if the top-level type does not itself have one (although I'm not 100% sure about the exact terminology here, i.e. whether the "drop glue" of the top-level type counts as a destructor or not).

cc `@jonhoo,` does this solve the issue for you?

r? `@GuillaumeGomez`

Fix clippy::collapsible_match with let expressions

This fixes rust-lang/rust-clippy#7575 which is a regression from #80357. I am fixing the bug here instead of in the clippy repo (if that's okay) because a) the regression has not been synced yet and b) I would like to land the fix on nightly asap.

The fix is basically to re-generalize `match` and `if let` for the lint implementation (they were split because `if let` no longer desugars to `match` in the HIR).

Also fixes rust-lang/rust-clippy#7586 and fixes rust-lang/rust-clippy#7591
cc `@rust-lang/clippy`
`@xFrednet` do you want to review this?

marker_traits: require `EvaluatedToOk` during winnowing

closes #84955, while it doesn't really fix it in a way that makes me happy it should prevent the issue for now and this
test can't be reproduced anyways, so it doesn't make much sense to keep it open.

fixes #84917 as only one of the impls depends on regions, so we now drop the ambiguous one instead of the correct one.

cc https://rust-lang.zulipchat.com/#narrow/stream/144729-wg-traits/topic/winnowing.20soundly/near/247899832

r? `@nikomatsakis`

Fix example in `Extend<(A, B)>` impl

After looking over the examples in my last PR (#85835) on doc.rust-lang.org/nightly I realized that the example didn't actually show what I wanted it to show 😅
So here's the better example

add file_prefix method to std::path

This is an initial implementation of `std::path::Path::file_prefix`. It is effectively a "left" variant of the existing [`file_stem`](https://doc.rust-lang.org/std/path/struct.Path.html#method.file_stem) method. An illustration of the difference is

```rust
use std::path::Path;

let path = Path::new("foo.tar.gz");
assert_eq!(path.file_stem(), Some("foo.tar"));
assert_eq!(path.file_prefix(), Some("foo"));
```

In my own development, I generally find I almost always want the prefix, rather than the stem, so I thought it might be best to suggest it's addition to libstd.

Of course, as this is my first contribution, I expect there is probably more work that needs to be done. Additionally, if the libstd team feel this isn't appropriate then so be it.

There has been some [discussion about this on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/file_lstem/near/238076313) and a user there suggested I open a PR to see whether someone in the libstd team thinks it is worth pursuing.

Rollup of 13 pull requests

Successful merges:

 - #87604 (CI: Verify commits in beta & stable are in upstream branches.)
 - #88057 (Update RELEASES to clarify attribute macro values.)
 - #88072 (Allow the iOS toolchain to be built on Linux)
 - #88170 (Update release note for 1.55.0.)
 - #88172 (Test that type alias impl trait happens in a submodule)
 - #88179 (Mailmap entry for myself)
 - #88182 (We meant to use a trait instead of lifetime here)
 - #88183 (test TAIT in different positions)
 - #88189 (Add TAIT struct test)
 - #88192 (Use of impl trait in an impl as the value for an associated type in a dyn)
 - #88194 (Test use of impl Trait in an impl as the value for an associated type in an impl trait)
 - #88197 (Test tait use in a fn type)
 - #88201 (Test that incomplete inference for TAITs fail)

Failed merges:

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

Test that incomplete inference for TAITs fail

r? `@oli-obk`

Related to #86727

Test tait use in a fn type

r? `@oli-obk`

I thought this was going to work but doesn't, quickly checked with Niko and he told me that we ruled this out for now. I'm not exactly sure why and how but here we have a test with a FIXME :)

Related to #86727

Test use of impl Trait in an impl as the value for an associated type in an impl trait

r? `@oli-obk`

Related to #86727

Use of impl trait in an impl as the value for an associated type in a dyn

r? `@oli-obk`

Related to #86727

Add TAIT struct test

r? `@oli-obk`

Related to #86727

test TAIT in different positions

r? `@oli-obk`

Related to #86727

We meant to use a trait instead of lifetime here

r? `@oli-obk`

Related to #86727

Mailmap entry for myself

Test that type alias impl trait happens in a submodule

r? `@oli-obk`

Related to #86727

Update release note for 1.55.0.

Added a line about new formatting option, `{lib}`, for `cargo
tree` (https://github.com/rust-lang/cargo/pull/9663).

Allow the iOS toolchain to be built on Linux

The iOS toolchain can be built on Linux with minor changes. The compilation will invoke `xcrun` to find the path to the iPhone SDK but a fake `xcrun` executable can be used.

```
#!/bin/sh
echo "/path/to/sdk"
```

The iOS toolchain can then be built and linked with rustup.

```
$ ./x.py build --stage 2 --host x86_64-unknown-linux-gnu \
  	 --target aarch64-apple-ios
$ rustup toolchain link stage1 build/x86_64-unknown-linux-gnu/stage1
```

It's possible to take this toolchain and compile an iOS executable with it. This requires the ld64 linker and an iOS SDK. The ld64 linker can be taken from [cctools](https://github.com/tpoechtrager/cctools-port). A project's .cargo/config can then be edited to use the linker for this target.

```
[target.aarch64-apple-ios]
linker = "/path/to/cctools/bin/arm-apple-darwin-ld"
rustflags = [
    "-C",
    """
link-args=
    -F/path/to/sdk/System/Library/Frameworks
    -L/path/to/sdk/usr/lib
    -L/path/to/sdk/usr/lib/system/
    -adhoc_codesign
    """,
]
```

Update RELEASES to clarify attribute macro values.

As noted in #87681, macros do not work with the `#[path]` attribute.  Since the places where macros *can* be used is very limited, I have changed this to just focus on `#[doc]` which is the only attribute where this is really useful.

CI: Verify commits in beta & stable are in upstream branches.

Closes #74721

I think this does the trick. https://github.com/rust-lang/rust/pull/87597 is an example of it failing as it should.

Optimize unnecessary check in VecDeque::retain

This pr is highly inspired by https://github.com/rust-lang/rust/pull/88060 which shared the same idea: we can split the `for` loop into stages so that we can remove unnecessary checks like `del > 0`.

## Benchmarks

Before

```rust
test collections::vec_deque::tests::bench_retain_half_10000  ... bench:     290,125 ns/iter (+/- 8,717)
test collections::vec_deque::tests::bench_retain_odd_10000   ... bench:     291,588 ns/iter (+/- 9,621)
test collections::vec_deque::tests::bench_retain_whole_10000 ... bench:     287,426 ns/iter (+/- 9,009)
```

After

```rust
test collections::vec_deque::tests::bench_retain_half_10000  ... bench:     243,940 ns/iter (+/- 8,563)
test collections::vec_deque::tests::bench_retain_odd_10000   ... bench:     242,768 ns/iter (+/- 3,903)
test collections::vec_deque::tests::bench_retain_whole_10000 ... bench:     202,926 ns/iter (+/- 6,332)
```

Based on the current benchmark, this PR will improve the perf of `VecDeque::retain` by around 16%. For special cases, the improvement will be up to 30%.
Signed-off-by: NXuanwo <github@xuanwo.io>

Trait upcasting coercion (part 3)

By using separate candidates for each possible choice, this fixes type-checking issues in previous commits.

r? `@nikomatsakis`

Give precedence to `html_root_url` over `--extern-html-root-url` by default, but add a way to opt-in to the previous behavior

## What is an HTML root url?

It tells rustdoc where it should link when documentation for a crate is
not available locally; for example, when a crate is a dependency of a
crate documented with `cargo doc --no-deps`.

 ## What is the difference between `html_root_url` and `--extern-html-root-url`?

Both of these tell rustdoc what the HTML root should be set to.
`doc(html_root_url)` is set by the crate author, while
`--extern-html-root-url` is set by the person documenting the crate.
These are often different. For example, docs.rs uses
`--extern-html-root-url https://docs.rs/crate-name/version` to ensure
all crates have documentation, even if `html_root_url` is not set.
Conversely, crates such as Rocket set `doc(html_root_url =
"https://api.rocket.rs")`, because they prefer users to view the
documentation on their own site.

Crates also set `html_root_url` to ensure they have
documentation when building locally when offline. This is unfortunate to
require, because it's more work from the library author. It also makes
it impossible to distinguish between crates that want to be viewed on a
different site (e.g. Rocket) and crates that just want documentation to
be visible offline at all (e.g. Tokio). I have authored a separate
change to the API guidelines to no longer recommend doing this:
rust-lang/api-guidelines#230.

 ## Why change the default?

In the past, docs.rs has been the main user of `--extern-html-root-url`.
However, it's useful for other projects as well. In particular, Cargo
wants to pass it by default when running `--no-deps`
(rust-lang/cargo#8296).

Unfortunately, for these other use cases, the priority order is
inverted. They want to give *precedence* to the URL the crate picks, and
only fall back to the `--extern-html-root` if no `html_root_url` is
present. That allows passing `--extern-html-root` unconditionally,
without having to parse the source code to see what attributes are
present.

For docs.rs, however, we still want to keep the old behavior, so that
all links on docs.rs stay on the site.

Force warn improvements

As part of stablization of the `--force-warn` option (#86516) I've made the following changes:
* Error when the `warnings` lint group is based to the `--force-warn` option
* Tests have been updated to make it easier to understand the semantics of `--force-warn`

r? `@nikomatsakis`

Add needs-asm-support to more tests

These were found as test failures on s390x for RHEL and Fedora.

Upgrade to LLVM 13

Work in progress update to LLVM 13. Main changes:

 * InlineAsm diagnostics reported using SrcMgr diagnostic kind are now handled. Previously these used a separate diag handler.
 * Codegen tests are updated for additional attributes.
 * Some data layouts have changed.
 * Switch `#[used]` attribute from `llvm.used` to `llvm.compiler.used` to avoid SHF_GNU_RETAIN flag introduced in https://reviews.llvm.org/D97448, which appears to trigger a bug in older versions of gold.
 * Set `LLVM_INCLUDE_TESTS=OFF` to avoid Python 3.6 requirement.

Upstream issues:

 * ~~https://bugs.llvm.org/show_bug.cgi?id=51210 (InlineAsm diagnostic reporting for module asm)~~ Fixed by https://github.com/llvm/llvm-project/commit/1558bb80c01b695ce12642527cbfccf16cf54ece.
 * ~~https://bugs.llvm.org/show_bug.cgi?id=51476 (Miscompile on AArch64 due to incorrect comparison elimination)~~ Fixed by https://github.com/llvm/llvm-project/commit/81b106584f2baf33e09be2362c35c1bf2f6bfe94.
 * https://bugs.llvm.org/show_bug.cgi?id=51207 (Can't set custom section flags anymore). Problematic change reverted in our fork, https://reviews.llvm.org/D107216 posted for upstream revert.
 * https://bugs.llvm.org/show_bug.cgi?id=51211 (Regression in codegen for #83623). This is an optimization regression that we may likely have to eat for this release. The fix for #83623 was based on an incorrect premise, and this needs to be properly addressed in the MergeICmps pass.

The [compile-time impact](https://perf.rust-lang.org/compare.html?start=ef9549b6c0efb7525c9b012148689c8d070f9bc0&end=0983094463497eec22d550dad25576a894687002) is mixed, but quite positive as LLVM upgrades go.

The LLVM 13 final release is scheduled for Sep 21st. The current nightly is scheduled for stable release on Oct 21st.

r? `@ghost`

This follows what clang does in CoverageMappingGen. Using just
llvm.compiler.used is insufficient at least for MSVC targets.

⬆ rust-analyzer

Refactor fallback code to prepare for never type

This PR contains cherry-picks of some of `@nikomatsakis's` work from #79366, and shouldn't (AFAICT) represent any change in behavior. However, the refactoring is good regardless of the never type work being landed, and will reduce the size of those eventual PR(s) (and rebase pain).

I am not personally an expert on this code, and the commits are essentially 100% `@nikomatsakis's,` but they do seem reasonable to me by my understanding. Happy to edit with review, of course. Commits are best reviewed in sequence rather than all together.

r? `@jackh726` perhaps?

Check that a box expression's type is Sized

This resolves [issue 87935](https://github.com/rust-lang/rust/issues/87935).

This makes E0161 (move from an unsized rvalue) much less common.  I've replaced the test to use [this case](https://github.com/rust-lang/rust/blob/master/src/test/ui/object-safety/object-safety-by-value-self-use.rs), when a boxed `dyn` trait is passed by value, but that isn't an error when `unsized_locals` is enabled.  I think it may be possible to get rid of E0161 entirely by checking that case earlier, but I'm not sure if that's desirable?

Along the way, simplify and document the logic more clearly.

Motivation: in upcoming commits, we are going to create a graph of the
coercion relationships between variables. We want to
distinguish *coercion* specifically from other sorts of subtyping, as
it indicates values flowing from one place to another via assignment.

I didn't like the sub-unify code executing when a predicate was
ENQUEUED, that felt fragile. I would have preferred to move the
sub-unify code so that it only occurred during generalization, but
that impacted diagnostics, so having it also occur when we process
subtype predicates felt pretty reasonable. (I guess we only need one
or the other, but I kind of prefer both, since the generalizer
ultimately feels like the *right* place to guarantee the properties we
want.)