This is a sequel to November 2017's rust-lang#46103 / 1a9dc2e. It had been
reported (more than once—at least rust-lang#19140, rust-lang#44083, and rust-lang#44565) that the
"never used" language was confusing for enum variants that were "used"
as match patterns, so the wording was changed to say never
"constructed" specifically for enum variants. More recently, the same
issue was raised for structs (rust-lang#52325). It seems consistent to say
"constructed" here, too, for the same reasons.

We considered using more specific word "called" for unused functions
and methods (while we declined to do this in rust-lang#46103, the rationale
given in the commit message doesn't actually make sense), but it turns
out that Cargo's test suite expects the "never used" message, and
maybe we don't care enough even to make a Cargo PR over such a petty
and subjective wording change.

This resolves rust-lang#52325.

Refer instead to thanks page.

…ler and port libstd to it.

As a start, the port uses the simplest possible configuration (no jemalloc, abort on panic)
and makes use of existing Unix-specific code wherever possible.
It adds targets for x86_64 (current main HermitCore platform) and aarch64 (HermitCore platform
under development).

Together with the patches to "liblibc" and "llvm", this enables HermitCore applications to be
written in Rust.

Recent changes to lints disallowed lints from being emitted against code located
in foreign macros, except for future-incompatible lints. For a future
incompatible lint, however, the automatic suggestions may not be applicable!

This commit updates this code path to force all applicability suggestions made
to foreign macros to never be `MachineApplicable`. This should avoid rustfix
actually attempting fixing these suggestions, causing non-compiling code to be
produced.

Closes rust-lang/cargo#5799

Its former contents are now in libcore.

`const LE_I128` needs parentheses to negate the value *before* calling
`to_le()`, otherwise it doesn't match the operations performed in the
black-boxed part of the test.  This only makes a tangible difference on
big-endian targets.

…ource order

(and thus, hopefully, more closely match user expectation).

…LL diagnostic change.

…ccmmodate diagnostic change.

RfC -> RFC

… rewrite.

 * Arguably this change is sometimes injecting noise into the output
   (namely in the cases where the suggested rewrite is inline with the
   suggestion and we end up highlighting the original source code).
   I would not be opposed to something more aggressive/dynamic, like
   revising the suggestion code to automatically print the original
   source when necessary (e.g. when the error does not have a span
   that includes the span of the suggestion).

 * Also, as another note on this change: The doc comment for `Diagnostic::span_suggestion`
   says:
    /// The message
    ///
    /// * should not end in any punctuation (a `:` is added automatically)
    /// * should not be a question
    /// * should not contain any parts like "the following", "as shown"

   but the `:` is *not* added when the emitted line appears
   out-of-line relative to the suggestion. I find that to be an
   unfortunate UI experience.

----

As a drive-by fix, also changed code to combine multiple suggestions
for a pattern into a single multipart suggestion (which vastly
improves user experience IMO).

----

Includes the updates to expected NLL diagnostics.

Parsing the code block's LangString (```foo) previously checked itself
to see if we were on nightly; that isn't the right place to do so. Move
that check slightly outwards to better abstract LangString.

(This is also an optimization as we avoid the costly environment
variable load of RUSTC_BOOTSTRAP).

Previously we would only warn on Rust code but we can also do so when
testing markdown (the diag::Handler is available).

This is far more sound than passing many different arguments of the same
type.

Removes dependency on UnstableFeatures from markdown rendering

This avoids a redundant length check in some cases when calling
`Vec::truncate` or `Vec::clear`.

Fixes rust-lang#51802

It is not required for LLVM to have SPARC target support, so it is
necessary to only run this test when LLVM does support SPARC. Sadly, it
isn’t possible to specify exactly this constraint. Instead, we specify
that this test should run on SPARC host only (it surely is sane
assumption to make that compiler running on a SPARC can generate
SPARC, right?)

Since you cannot specify multiple `only-*` to have it run on both 32-bit
and 64-bit SPARC we pick 64-bit SPARC, because it is exactly what is
being tested by this test.

Fixes rust-lang#52881

…sort_by`!).

Previously these functions relied on TLS but we can instead thread the
relevant state through explicitly.

Removes some unused code and de-publicizes structs

…y_2_electric_boogaloo, r=pnkfelix

dead-code lint: say "constructed" for structs

Respectively.

This is a sequel to November 2017's rust-lang#46103 / 1a9dc2e. It had been
reported (more than once—at least rust-lang#19140, rust-lang#44083, and rust-lang#44565) that the
"never used" language was confusing for enum variants that were "used"
as match patterns, so the wording was changed to say never "constructed"
specifically for enum variants. More recently, the same issue was raised
for structs (rust-lang#52325). It seems consistent to say "constructed" here,
too, for the same reasons.

~~While we're here, we can also use more specific word "called" for unused
functions and methods. (We declined to do this in rust-lang#46103, but the
rationale given in the commit message doesn't actually make sense.)~~

This resolves rust-lang#52325.

…r=steveklabnik

Document From trait implementations for OsStr, OsString, CString, and CStr

As part of issue rust-lang#51430 (cc @skade).

The allocation and copy claims should be double-checked.

r? @steveklabnik

…r=QuietMisdreavus

Cleanup some rustdoc code

Commits are mostly individual though some do depend on others.

Remove unstable and deprecated APIs

Update clippy to latest master

r? @oli-obk

There is a regression in the version in current nightly that falsely lints `println!` and `writeln!` that use named arguments, thinking all rhs values for the argument expressions are literals even when they are not. This update includes the fix for that.

rustc: Disallow machine applicability in foreign macros

Recent changes to lints disallowed lints from being emitted against code located
in foreign macros, except for future-incompatible lints. For a future
incompatible lint, however, the automatic suggestions may not be applicable!

This commit updates this code path to force all applicability suggestions made
to foreign macros to never be `MachineApplicable`. This should avoid rustfix
actually attempting fixing these suggestions, causing non-compiling code to be
produced.

Closes rust-lang/cargo#5799

Clarify thread::park semantics

It took me quite some time to realize that the example is not actually racy, so let's clarify it? :-)

…nkfelix

[NLL] Don't make "fake" match variables mutable

These variables can't be mutated by the user, but since they have names the unused-mut lint thinks that it should check them.

…aelwoerister

pretty print for std::collections::vecdeque

I want pretty print function for VecDeque like Vec.

```rust
use std::collections::VecDeque;

fn main() {
    let mut d: VecDeque<i32> = VecDeque::new();
    d.push_back(4);
    d.push_back(4);
    d.push_back(6);
    let mut v: Vec<i32> = Vec::new();
    v.push(4);
    v.push(4);
    v.push(6);
}
```

```
(gdb) p v
$1 = Vec<i32>(len: 3, cap: 4) = {4, 4, 6}
(gdb) p d
$2 = VecDeque<i32>(len: 3, cap: 8) = {4, 4, 6}
```

Thanks.

…, r=cramertj

Fix From<LocalWaker>

This is a follow-up to rust-lang#52640

Fixes `From<LocalWaker>` which is affected by the same accidental drop bug (unless I'm totally mistaken)

r? @cramertj

…r=rkruppe

Fix -Wpessimizing-move warnings in rustllvm/PassWrapper

These are producing warnings when building rustc (`warning: moving a local object in a return statement prevents copy elision [-Wpessimizing-move]`).

…lexcrichton

remove references to AUTHORS.txt file

Rust does not ship an AUTHORS.txt file anymore.

…=eddyb

Fix Alias intra doc ICE

Fixes rust-lang#52611.

cc @QuietMisdreavus

r? @varkor

update comment

r? @joshtriplett

Addressing comment: rust-lang#52761 (review)

…ark-Simulacrum

Add timeout to use of `curl` in bootstrap.py.

Recently we've seen a lot of "30 minutes no output" spurious errors while downloading the bootstrap compiler. This added several timeout options so if the "30 minutes no output" errors were caused by connection or transfer issue, we could fail quicker for curl to retry.

Make the tool_lints actually usable

cc rust-lang#44690

Necessary for rust-lang/rust-clippy#2955 and rust-lang/rust-clippy#2977

This PR makes it possible for lint tools (at the moment only for Clippy) to implement the `tool_lints`, like it was documented in rust-lang#52018.

Because the `declare_lint` macro is pretty cluttered right now, there was not really a good way to add the `tool_name` as an additional argument of the macro. That's why I chose to introduce the new `declare_tool_lint` macro.

The switch from `&str` to `String` in the `lint_groups` `FxHashMap` is because I got weird error messages in the `check_lint_name` method. And the `by_name` field of the `LintStore` also uses `String`.

### What comes with this PR:

If this PR lands and Clippy switches to the `tool_lints`, the currently used methods
```rust
#[cfg_attr(feature = "cargo-clippy", allow(clippy_lint))]
#[allow(unknown_lints, clippy_lint)]
```
to `allow`/`warn`/`deny`/`forbid` Clippy lints, won't have any effects anymore, but also won't produce a warning. That is because the name of `clippy_lint` will then be `clippy::clippy_lint`. (Maybe we can add a clippy lint to search for `cfg_attr` appearances with the `cargo-clippy` feature?)

r? @oli-obk

…hton

Improve bootstrap help on stages

Cc @eddyb

…-Simulacrum

Use Vec::extend in SmallVec::extend when applicable

As calculated in rust-lang#52738, `Vec::extend` is much faster than `push`ing to it in a loop. We can take advantage of this method in `SmallVec` too - at least in cases when its underlying object is an `AccumulateVec::Heap`.

~~This approach also accidentally improves the `push` loop of the `AccumulateVec::Array` variant, because it doesn't utilize `SmallVec::push` which performs `self.reserve(1)` with every iteration; this is unnecessary, because we're already reserving the whole space we will be needing by performing `self.reserve(iter.size_hint().0)` at the beginning.~~

Add targets for HermitCore (https://hermitcore.org) to the Rust compiler and port libstd to it.

As a start, the port uses the simplest possible configuration (no jemalloc, abort on panic) and makes use of existing Unix-specific code wherever possible.
It adds targets for x86_64 (current main HermitCore platform) and aarch64 (HermitCore platform under development).

Together with the patches to "liblibc" (rust-lang/libc#1048) and llvm (rust-lang/llvm#122), this enables HermitCore applications to be written in Rust.

…Simulacrum

releases.md: fix 2 typos

Implement Unpin for FutureObj and LocalFutureObj

cc @MajorBreakfast

r? @aturon

run-pass/const-endianness: negate before to_le()

`const LE_I128` needs parentheses to negate the value *before* calling
`to_le()`, otherwise it doesn't match the operations performed in the
black-boxed part of the test.  This only makes a tangible difference on
big-endian targets.

Fix wrong issue number in the test name

I made a mistake in previous PR rust-lang#52620, second issue number was wrong, changing from rust-lang#52133 to rust-lang#52113

r? @kennytm

Include lifetime in mutability suggestion in NLL messages

Fix rust-lang#52880.

…r=petrochenkov

NLL: sort diagnostics by span

Sorting the output diagnostics by span is a long planned revision to the NLL diagnostics that we hope will yield a less surprising user experience in some case.

Once we got them buffered, it was trivial to implement. (The hard part is skimming the resulting changes to the diagnostics to make sure nothing broke... Note that I largely rubber-stamped the `#[rustc_regions]` output change.)

Fix rust-lang#51167

Fix a typo in unsize.rs

RfC -> RFC

…e-should-precede-suggestions, r=petrochenkov

NLL: On "cannot move out of type" error, print original before rewrite

NLL: On "cannot move out of type" error, print original source before rewrite.

 * Arguably this change is sometimes injecting noise into the output  (namely in the cases where the suggested rewrite is inline with the   suggestion and we end up highlighting the original source code).   I would not be opposed to something more aggressive/dynamic, like   revising the suggestion code to automatically print the original  source when necessary (e.g. when the error does not have a span   that includes the span of the suggestion).

 * Also, as another note on this change: The doc comment for `Diagnostic::span_suggestion`  says:
```rust
    /// The message
    ///
    /// * should not end in any punctuation (a `:` is added automatically)
    /// * should not be a question
    /// * should not contain any parts like "the following", "as shown"
```
  *  but the `:` is *not* added when the emitted line appears  out-of-line relative to the suggestion. I find that to be an  unfortunate UI experience.

----

As a drive-by fix, also changed code to combine multiple suggestions for a pattern into a single multipart suggestion (which vastly improves user experience IMO).

----

Includes the updates to expected NLL diagnostics.

Fix rust-lang#52877

…chton

Use SetLenOnDrop in Vec::truncate()

This avoids a redundant length check in some cases when calling
`Vec::truncate` or `Vec::clear`.

Fixes rust-lang#51802

Note that the generated code still seems suboptimal. I tested with the following functions:

```rust
#[no_mangle]
pub extern fn foo(x: &mut Vec<u8>) {
    x.clear();
}

#[no_mangle]
pub extern fn bar(x: &mut Vec<u8>) {
    x.truncate(5);
}

#[no_mangle]
pub extern fn baz(x: &mut Vec<u8>, n: usize) {
    x.truncate(n);
}

#[no_mangle]
pub extern fn foo_string(x: &mut Vec<String>) {
    x.clear();
}

#[no_mangle]
pub extern fn bar_string(x: &mut Vec<String>) {
    x.truncate(5);
}

#[no_mangle]
pub extern fn baz_string(x: &mut Vec<String>, n: usize) {
    x.truncate(n);
}
```

<details>
  <summary>Old output</summary>

```asm
00000000000460a0 <foo>:
   460a0:       48 83 7f 10 00          cmpq   $0x0,0x10(%rdi)
   460a5:       74 08                   je     460af <foo+0xf>
   460a7:       48 c7 47 10 00 00 00    movq   $0x0,0x10(%rdi)
   460ae:       00
   460af:       c3                      retq

00000000000460b0 <bar>:
   460b0:       48 83 7f 10 06          cmpq   $0x6,0x10(%rdi)
   460b5:       72 08                   jb     460bf <bar+0xf>
   460b7:       48 c7 47 10 05 00 00    movq   $0x5,0x10(%rdi)
   460be:       00
   460bf:       c3                      retq

00000000000460c0 <baz>:
   460c0:       48 39 77 10             cmp    %rsi,0x10(%rdi)
   460c4:       76 04                   jbe    460ca <baz+0xa>
   460c6:       48 89 77 10             mov    %rsi,0x10(%rdi)
   460ca:       c3                      retq
   460cb:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)

00000000000460d0 <foo_string>:
   460d0:       41 57                   push   %r15
   460d2:       41 56                   push   %r14
   460d4:       53                      push   %rbx
   460d5:       48 8b 47 10             mov    0x10(%rdi),%rax
   460d9:       48 85 c0                test   %rax,%rax
   460dc:       74 4a                   je     46128 <foo_string+0x58>
   460de:       49 89 fe                mov    %rdi,%r14
   460e1:       48 8b 0f                mov    (%rdi),%rcx
   460e4:       48 8d 14 40             lea    (%rax,%rax,2),%rdx
   460e8:       48 8d 58 ff             lea    -0x1(%rax),%rbx
   460ec:       4c 8d 3c d1             lea    (%rcx,%rdx,8),%r15
   460f0:       49 83 c7 f0             add    $0xfffffffffffffff0,%r15
   460f4:       66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
   460fb:       00 00 00
   460fe:       66 90                   xchg   %ax,%ax
   46100:       49 89 5e 10             mov    %rbx,0x10(%r14)
   46104:       49 8b 37                mov    (%r15),%rsi
   46107:       48 85 f6                test   %rsi,%rsi
   4610a:       74 0e                   je     4611a <foo_string+0x4a>
   4610c:       49 8b 7f f8             mov    -0x8(%r15),%rdi
   46110:       ba 01 00 00 00          mov    $0x1,%edx
   46115:       e8 a6 e9 ff ff          callq  44ac0 <__rust_dealloc@plt>
   4611a:       48 83 c3 ff             add    $0xffffffffffffffff,%rbx
   4611e:       49 83 c7 e8             add    $0xffffffffffffffe8,%r15
   46122:       48 83 fb ff             cmp    $0xffffffffffffffff,%rbx
   46126:       75 d8                   jne    46100 <foo_string+0x30>
   46128:       5b                      pop    %rbx
   46129:       41 5e                   pop    %r14
   4612b:       41 5f                   pop    %r15
   4612d:       c3                      retq
   4612e:       66 90                   xchg   %ax,%ax

0000000000046130 <bar_string>:
   46130:       41 57                   push   %r15
   46132:       41 56                   push   %r14
   46134:       53                      push   %rbx
   46135:       4c 8b 7f 10             mov    0x10(%rdi),%r15
   46139:       49 83 ff 06             cmp    $0x6,%r15
   4613d:       72 49                   jb     46188 <bar_string+0x58>
   4613f:       49 89 fe                mov    %rdi,%r14
   46142:       48 8b 07                mov    (%rdi),%rax
   46145:       4b 8d 0c 7f             lea    (%r15,%r15,2),%rcx
   46149:       48 8d 1c c8             lea    (%rax,%rcx,8),%rbx
   4614d:       48 83 c3 f0             add    $0xfffffffffffffff0,%rbx
   46151:       66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
   46158:       00 00 00
   4615b:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
   46160:       49 83 c7 ff             add    $0xffffffffffffffff,%r15
   46164:       4d 89 7e 10             mov    %r15,0x10(%r14)
   46168:       48 8b 33                mov    (%rbx),%rsi
   4616b:       48 85 f6                test   %rsi,%rsi
   4616e:       74 0e                   je     4617e <bar_string+0x4e>
   46170:       48 8b 7b f8             mov    -0x8(%rbx),%rdi
   46174:       ba 01 00 00 00          mov    $0x1,%edx
   46179:       e8 42 e9 ff ff          callq  44ac0 <__rust_dealloc@plt>
   4617e:       48 83 c3 e8             add    $0xffffffffffffffe8,%rbx
   46182:       49 83 ff 05             cmp    $0x5,%r15
   46186:       77 d8                   ja     46160 <bar_string+0x30>
   46188:       5b                      pop    %rbx
   46189:       41 5e                   pop    %r14
   4618b:       41 5f                   pop    %r15
   4618d:       c3                      retq
   4618e:       66 90                   xchg   %ax,%ax

0000000000046190 <baz_string>:
   46190:       41 57                   push   %r15
   46192:       41 56                   push   %r14
   46194:       41 54                   push   %r12
   46196:       53                      push   %rbx
   46197:       50                      push   %rax
   46198:       4c 8b 67 10             mov    0x10(%rdi),%r12
   4619c:       49 39 f4                cmp    %rsi,%r12
   4619f:       76 46                   jbe    461e7 <baz_string+0x57>
   461a1:       49 89 f6                mov    %rsi,%r14
   461a4:       49 89 ff                mov    %rdi,%r15
   461a7:       48 8b 07                mov    (%rdi),%rax
   461aa:       4b 8d 0c 64             lea    (%r12,%r12,2),%rcx
   461ae:       48 8d 1c c8             lea    (%rax,%rcx,8),%rbx
   461b2:       48 83 c3 f0             add    $0xfffffffffffffff0,%rbx
   461b6:       66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
   461bd:       00 00 00
   461c0:       49 83 c4 ff             add    $0xffffffffffffffff,%r12
   461c4:       4d 89 67 10             mov    %r12,0x10(%r15)
   461c8:       48 8b 33                mov    (%rbx),%rsi
   461cb:       48 85 f6                test   %rsi,%rsi
   461ce:       74 0e                   je     461de <baz_string+0x4e>
   461d0:       48 8b 7b f8             mov    -0x8(%rbx),%rdi
   461d4:       ba 01 00 00 00          mov    $0x1,%edx
   461d9:       e8 e2 e8 ff ff          callq  44ac0 <__rust_dealloc@plt>
   461de:       48 83 c3 e8             add    $0xffffffffffffffe8,%rbx
   461e2:       4d 39 f4                cmp    %r14,%r12
   461e5:       77 d9                   ja     461c0 <baz_string+0x30>
   461e7:       48 83 c4 08             add    $0x8,%rsp
   461eb:       5b                      pop    %rbx
   461ec:       41 5c                   pop    %r12
   461ee:       41 5e                   pop    %r14
   461f0:       41 5f                   pop    %r15
   461f2:       c3                      retq
   461f3:       90                      nop
   461f4:       66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
   461fb:       00 00 00
   461fe:       66 90                   xchg   %ax,%ax
```
</details>

<details>
  <summary>New output</summary>

```asm
0000000000084d10 <foo>:
   84d10:       48 c7 47 10 00 00 00    movq   $0x0,0x10(%rdi)
   84d17:       00
   84d18:       c3                      retq
   84d19:       0f 1f 80 00 00 00 00    nopl   0x0(%rax)

0000000000084d20 <bar>:
   84d20:       48 8b 47 10             mov    0x10(%rdi),%rax
   84d24:       48 83 f8 05             cmp    $0x5,%rax
   84d28:       b9 05 00 00 00          mov    $0x5,%ecx
   84d2d:       48 0f 42 c8             cmovb  %rax,%rcx
   84d31:       48 89 4f 10             mov    %rcx,0x10(%rdi)
   84d35:       c3                      retq
   84d36:       66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
   84d3d:       00 00 00

0000000000084d40 <baz>:
   84d40:       48 8b 47 10             mov    0x10(%rdi),%rax
   84d44:       48 39 f0                cmp    %rsi,%rax
   84d47:       48 0f 47 c6             cmova  %rsi,%rax
   84d4b:       48 89 47 10             mov    %rax,0x10(%rdi)
   84d4f:       c3                      retq

0000000000084d50 <foo_string>:
   84d50:       41 57                   push   %r15
   84d52:       41 56                   push   %r14
   84d54:       53                      push   %rbx
   84d55:       49 89 fe                mov    %rdi,%r14
   84d58:       4c 8b 7f 10             mov    0x10(%rdi),%r15
   84d5c:       4d 85 ff                test   %r15,%r15
   84d5f:       74 2f                   je     84d90 <foo_string+0x40>
   84d61:       49 8b 06                mov    (%r14),%rax
   84d64:       4b 8d 0c 7f             lea    (%r15,%r15,2),%rcx
   84d68:       48 8d 1c c8             lea    (%rax,%rcx,8),%rbx
   84d6c:       48 83 c3 f0             add    $0xfffffffffffffff0,%rbx
   84d70:       48 8b 33                mov    (%rbx),%rsi
   84d73:       48 85 f6                test   %rsi,%rsi
   84d76:       74 0e                   je     84d86 <foo_string+0x36>
   84d78:       48 8b 7b f8             mov    -0x8(%rbx),%rdi
   84d7c:       ba 01 00 00 00          mov    $0x1,%edx
   84d81:       e8 1a b1 ff ff          callq  7fea0 <__rust_dealloc@plt>
   84d86:       48 83 c3 e8             add    $0xffffffffffffffe8,%rbx
   84d8a:       49 83 c7 ff             add    $0xffffffffffffffff,%r15
   84d8e:       75 e0                   jne    84d70 <foo_string+0x20>
   84d90:       49 c7 46 10 00 00 00    movq   $0x0,0x10(%r14)
   84d97:       00
   84d98:       5b                      pop    %rbx
   84d99:       41 5e                   pop    %r14
   84d9b:       41 5f                   pop    %r15
   84d9d:       c3                      retq
   84d9e:       66 90                   xchg   %ax,%ax

0000000000084da0 <bar_string>:
   84da0:       41 57                   push   %r15
   84da2:       41 56                   push   %r14
   84da4:       53                      push   %rbx
   84da5:       49 89 fe                mov    %rdi,%r14
   84da8:       4c 8b 7f 10             mov    0x10(%rdi),%r15
   84dac:       49 83 ff 06             cmp    $0x6,%r15
   84db0:       72 44                   jb     84df6 <bar_string+0x56>
   84db2:       49 8b 06                mov    (%r14),%rax
   84db5:       4b 8d 0c 7f             lea    (%r15,%r15,2),%rcx
   84db9:       48 8d 1c c8             lea    (%rax,%rcx,8),%rbx
   84dbd:       48 83 c3 f0             add    $0xfffffffffffffff0,%rbx
   84dc1:       49 83 c7 fb             add    $0xfffffffffffffffb,%r15
   84dc5:       66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
   84dcc:       00 00 00
   84dcf:       90                      nop
   84dd0:       48 8b 33                mov    (%rbx),%rsi
   84dd3:       48 85 f6                test   %rsi,%rsi
   84dd6:       74 0e                   je     84de6 <bar_string+0x46>
   84dd8:       48 8b 7b f8             mov    -0x8(%rbx),%rdi
   84ddc:       ba 01 00 00 00          mov    $0x1,%edx
   84de1:       e8 ba b0 ff ff          callq  7fea0 <__rust_dealloc@plt>
   84de6:       48 83 c3 e8             add    $0xffffffffffffffe8,%rbx
   84dea:       49 83 c7 ff             add    $0xffffffffffffffff,%r15
   84dee:       75 e0                   jne    84dd0 <bar_string+0x30>
   84df0:       41 bf 05 00 00 00       mov    $0x5,%r15d
   84df6:       4d 89 7e 10             mov    %r15,0x10(%r14)
   84dfa:       5b                      pop    %rbx
   84dfb:       41 5e                   pop    %r14
   84dfd:       41 5f                   pop    %r15
   84dff:       c3                      retq

0000000000084e00 <baz_string>:
   84e00:       41 57                   push   %r15
   84e02:       41 56                   push   %r14
   84e04:       41 54                   push   %r12
   84e06:       53                      push   %rbx
   84e07:       50                      push   %rax
   84e08:       49 89 ff                mov    %rdi,%r15
   84e0b:       48 8b 47 10             mov    0x10(%rdi),%rax
   84e0f:       49 89 c4                mov    %rax,%r12
   84e12:       49 29 f4                sub    %rsi,%r12
   84e15:       76 3c                   jbe    84e53 <baz_string+0x53>
   84e17:       49 89 f6                mov    %rsi,%r14
   84e1a:       49 8b 0f                mov    (%r15),%rcx
   84e1d:       48 8d 04 40             lea    (%rax,%rax,2),%rax
   84e21:       48 8d 1c c1             lea    (%rcx,%rax,8),%rbx
   84e25:       48 83 c3 f0             add    $0xfffffffffffffff0,%rbx
   84e29:       0f 1f 80 00 00 00 00    nopl   0x0(%rax)
   84e30:       48 8b 33                mov    (%rbx),%rsi
   84e33:       48 85 f6                test   %rsi,%rsi
   84e36:       74 0e                   je     84e46 <baz_string+0x46>
   84e38:       48 8b 7b f8             mov    -0x8(%rbx),%rdi
   84e3c:       ba 01 00 00 00          mov    $0x1,%edx
   84e41:       e8 5a b0 ff ff          callq  7fea0 <__rust_dealloc@plt>
   84e46:       48 83 c3 e8             add    $0xffffffffffffffe8,%rbx
   84e4a:       49 83 c4 ff             add    $0xffffffffffffffff,%r12
   84e4e:       75 e0                   jne    84e30 <baz_string+0x30>
   84e50:       4c 89 f0                mov    %r14,%rax
   84e53:       49 89 47 10             mov    %rax,0x10(%r15)
   84e57:       48 83 c4 08             add    $0x8,%rsp
   84e5b:       5b                      pop    %rbx
   84e5c:       41 5c                   pop    %r12
   84e5e:       41 5e                   pop    %r14
   84e60:       41 5f                   pop    %r15
   84e62:       c3                      retq
   84e63:       90                      nop
   84e64:       66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
   84e6b:       00 00 00
   84e6e:       66 90                   xchg   %ax,%ax
```
</details>

For calling `truncate` with non-zero lengths on non-`Drop` types, it seems that a redundant load and comparison gets replaced with an awkward sequence with a conditional move. In the unknown length case, the new code is no longer awkward.

Maybe someone moderately proficient at assembly could tell if this looks like a win or not.

---

This came up when discussing replacing `unsafe { vec.set_len(0) }` with `vec.clear()` in a project where the author was worried about potential performance degradation. It might be worth replacing some unsafe code, even it it's trivial to see that it's actually safe.

…s, r=alexcrichton

Only run the sparc-abi test on sparc

It is not required for LLVM to have SPARC target support, so it is
necessary to only run this test when LLVM does support SPARC. Sadly, it
isn’t possible to specify exactly this constraint. Instead, we specify
that this test should run on SPARC host only (it surely is sane
assumption to make that compiler running on a SPARC can generate
SPARC, right?)

Since you cannot specify multiple `only-*` to have it run on both 32-bit
and 64-bit SPARC we pick 64-bit SPARC, because it is exactly what is
being tested by this test.

Fixes rust-lang#52881

…imulacrum

Backport 1.27.2 release notes

r? @Mark-Simulacrum

Update compatibility note for 1.28.0 to be correct

You can still put implementations on `dyn Trait + Send + Send`, but it'd be the same as putting them on `dyn Trait + Send`. This is why the error is that there are duplicate definitions in the example.