Add Read/Write::can_read/write_vectored

When working with an arbitrary reader or writer, code that uses vectored
operations may end up being slower than code that copies into a single
buffer when the underlying reader or writer doesn't actually support
vectored operations. These new methods allow you to ask the reader or
witer up front if vectored operations are efficiently supported.

Currently, you have to use some heuristics to guess by e.g. checking if
the read or write only accessed the first buffer. Hyper is one concrete
example of a library that has to do this dynamically:
https://github.com/hyperium/hyper/blob/0eaf304644a396895a4ce1f0146e596640bb666a/src/proto/h1/io.rs#L582-L594

Remove some `Vec` allocations to improve performance

This claws back most of the performance lost in https://github.com/rust-lang/rust/pull/69745.
r? @EddyB

Don't run various MIR optimizations at mir-opt-level=0

Add missing checks for mir-opt-level to non-essential MIR passes.

I verified that this can still bootstrap even with these passes disabled.

r? @oli-obk cc @RalfJung

Rollup of 6 pull requests

Successful merges:

 - #68716 (Stabilize `Span::mixed_site`)
 - #71263 (Remove unused abs_path method from rustc_span::source_map::FileLoader)
 - #71409 (Point at the return type on `.into()` failure caused by `?`)
 - #71419 (add message for resolution failure because wrong namespace)
 - #71438 (Tweak some suggestions in `rustc_resolve`)
 - #71589 (remove Unique::from for shared pointer types)

Failed merges:

r? @ghost

remove Unique::from for shared pointer types

r? @SimonSapin

Tweak some suggestions in `rustc_resolve`

add message for resolution failure because wrong namespace

closes https://github.com/rust-lang/rust/issues/71406

Point at the return type on `.into()` failure caused by `?`

Fix #35946.

Remove unused abs_path method from rustc_span::source_map::FileLoader

Stabilize `Span::mixed_site`

Closes https://github.com/rust-lang/rust/issues/65049.
cc https://github.com/rust-lang/rust/issues/54727#issuecomment-580647446

Pre-requisite for https://github.com/rust-lang/rust/pull/68717 ("Stabilize fn-like proc macros in expression, pattern and statement positions").

Stabilization report: https://github.com/rust-lang/rust/pull/68716#issuecomment-581076337.

submodules: update clippy from 891e1a85 to d01a4981

Changes:
````
`predecessors_for` will be removed soon
Rustup "Remove `BodyAndCache`"
span_lint_and_note now takes an Option<Span> for the note_span instead of just a span
Make lint also capture blocks and closures, adjust language to mention other mutex types
don't test the code in the lint docs
Switch to matching against full paths instead of just the last element of the path
Lint for holding locks across await points
fix crash on issue-69020-assoc-const-arith-overflow.rs
update stderr file
util/fetch_prs_between.sh: Add Markdown formatted Link
factor ifs into function, add differing mutex test
Update the changelog update documentation
Apply suggestions from PR review
update span_lint_and_help call to six args
test for mutex eq, add another test case
use if chain
cargo dev fmt
fix map import to rustc_middle
dev update_lints
fix internal clippy warnings
change visitor name to OppVisitor
use Visitor api to find Mutex::lock calls
add note about update-all-refs script, revert redundant pat to master
move closures to seperate fns, remove known problems
use span_lint_and_help, cargo dev fmt
creating suggestion
progress work on suggestion for auto fix
Implement unsafe_derive_deserialize lint
Update empty_enum.stderr
Formatting and naming
Formatting and naming
Cleanup: `node_id` -> `hir_id`
Fix issue #2907.
Don't trigger toplevel_ref_arg for `for` loops
Cleanup: future_not_send: use `return_ty` method
Remove badge FIXME from Cargo.toml
Change note_span argument for span_lint_and_note.
Add an Option<Span> argument to span_lint_and_help.
Fixes internal lint warning in code base.
Implement collapsible_span_lint_calls lint.
````

Fixes #71453

r? @Dylan-DPC

Rollup of 5 pull requests

Successful merges:

 - #71421 (Add a function to turn Box<T> into Box<[T]>)
 - #71537 (Remove support for self-opening)
 - #71551 (Minor refactoring around IndexVec usage in generator transformation)
 - #71569 ([miri] Throw UB if target size and data size don't match)
 - #71576 (check that `AsRef` and `AsMut` are inlined)

Failed merges:

 - #71558 (Cleanup and document `-Z tls-model` )

r? @ghost

check that `AsRef` and `AsMut` are inlined

Adds a regression test for #58867

r? @Dylan-DPC

[miri] Throw UB if target size and data size don't match

Issue: https://github.com/rust-lang/miri/issues/1355

If an extern C function is defined as

```
extern "C" {
    fn malloc(size: u32) -> *mut std::ffi::c_void;
}
```

on a 64-bit machine(ie. pointer sizes don't match), return undefined behaviour from Miri when [converting the argument into machine_usize](https://github.com/rust-lang/miri/blob/master/src/shims/foreign_items.rs#L200)

Minor refactoring around IndexVec usage in generator transformation

Replace hash map with IndexVec for liveness data.
Utilize IndexVec::push return value to avoid redundant object creation.

r? @EddyB

Remove support for self-opening

This was only used for linkage test cases, which is already covered by
the [run-make-fulldeps/symbol-visibility test](https://github.com/rust-lang/rust/blob/master/src/test/run-make-fulldeps/symbol-visibility/Makefile) -- which fairly extensively makes
sure we're correctly exporting the right symbols at the right visibility (for
various Rust crate types).

This fixes #10379 and resolves #10356 by removing the test case (and underlying support in the compiler). AFAICT, the better way to test visibility is via nm, like the symbol visibility test. It seems like that's sufficient; I suspect that given that we don't use this we should just drop it (android is tier 2 anyway). But happy to hear otherwise.

Add a function to turn Box<T> into Box<[T]>

Hi,
I think this is very useful, as currently it's not possible in safe rust to do this without re-allocating.
an alternative implementation of the same function can be:
```rust
pub fn into_boxed_slice<T>(boxed: Box<T>) -> Box<[T]> {
    unsafe {
        let slice = slice::from_raw_parts_mut(Box::into_raw(boxed), 1);
        Box::from_raw(slice)
    }
}
```

The only thing that makes me a little uncomfortable is this line :
> The alignment of array types is greater or equal to the alignment of its element type

from https://rust-lang.github.io/unsafe-code-guidelines/layout/arrays-and-slices.html

But then I see:
> The alignment of &T, &mut T, *const T and *mut T are the same, and are at least the word size.
> The alignment of &[T] is the word size.

from https://rust-lang.github.io/unsafe-code-guidelines/layout/pointers.html#representation

So I do believe this is valid(FWIW it also passes in miri https://play.rust-lang.org/?gist=c002b99364ee6b29862aeb3565a91c19)

Fix #35946.

Rollup of 5 pull requests

Successful merges:

 - #71490 (Cleanup and document `-C relocation-model`)
 - #71562 (fix more clippy warnings)
 - #71571 (Fix since attribute for nonzero_bitor impl's)
 - #71574 (proc_macro: Fix since attributes for new Span methods)
 - #71575 (Fix stable(since) attribute for BTreeMap::remove_entry)

Failed merges:

r? @ghost

Changes:
````
rustup to https://github.com/rust-lang/rust/pull/70043
map_clone: avoid suggesting `copied()` for &mut
fix redundant_pattern_matching lint
Add tests for #1654
Don't trigger while_let_on_iterator when the iterator is recreated every iteration
Update issue_2356.stderr reference file
Update while_let_on_iterator tests
Fix while_let_on_iterator suggestion and make it MachineApplicable
Add lifetime test case for `new_ret_no_self`
rustup https://github.com/rust-lang/rust/pull/71215/
Downgrade match_bool to pedantic
Run fetch before testing if master contains beta
The beta branch update should not require a force push
Add a note to the beta sections of release.md
Remove apt-get upgrade again
Always use the deploy script and templates of the master branch
README: fix lit count line
clippy_dev: make it fatal when the regex for updating lint count does not match
`predecessors_for` will be removed soon
Rustup "Remove `BodyAndCache`"
Only run (late) internal lints, when they are warn/deny/forbid
Only run cargo lints, when they are warn/deny/forbid
span_lint_and_note now takes an Option<Span> for the note_span instead of just a span
Make lint also capture blocks and closures, adjust language to mention other mutex types
don't test the code in the lint docs
Switch to matching against full paths instead of just the last element of the path
Lint for holding locks across await points
Also mention `--fix` for nightly users
fix crash on issue-69020-assoc-const-arith-overflow.rs
Address review comments
remark fixes
Update CHANGELOG.md for Rust 1.43 and 1.44
update stderr file
util/fetch_prs_between.sh: Add Markdown formatted Link
factor ifs into function, add differing mutex test
Update the changelog update documentation
Apply suggestions from PR review
update span_lint_and_help call to six args
test for mutex eq, add another test case
use if chain
cargo dev fmt
fix map import to rustc_middle
dev update_lints
fix internal clippy warnings
change visitor name to OppVisitor
use Visitor api to find Mutex::lock calls
add note about update-all-refs script, revert redundant pat to master
move closures to seperate fns, remove known problems
use span_lint_and_help, cargo dev fmt
creating suggestion
progress work on suggestion for auto fix
Implement unsafe_derive_deserialize lint
Update empty_enum.stderr
Formatting and naming
Formatting and naming
Cleanup: `node_id` -> `hir_id`
Fix issue #2907.
Don't trigger toplevel_ref_arg for `for` loops
Cleanup: future_not_send: use `return_ty` method
Remove badge FIXME from Cargo.toml
Change note_span argument for span_lint_and_note.
Add an Option<Span> argument to span_lint_and_help.
Fixes internal lint warning in code base.
Implement collapsible_span_lint_calls lint.
````

Fixes #71453

Will clean up in a separate PR

Fix stable(since) attribute for BTreeMap::remove_entry

Stabilized in #70712.

Maybe checking that the since attributes are added correctly should be automated through tidy? This is the third PR I'm opening that fixes a stable(since) attribute for something meant to be stabilized in 1.43 / 1.44 initially but then only stabilized in 1.45. (the other two are #71571, #71574)

proc_macro: Fix since attributes for new Span methods

Added in #69041, took a while to merge so the since attributes went out of date.

Fix since attribute for nonzero_bitor impl's

Added in #69813, took a while to merge so the since attributes went out of date.

fix more clippy warnings

clippy::{redundant_pattern_matching, clone_on_copy, iter_cloned_collect, option_as_ref_deref, match_ref_pats}

r? @Dylan-DPC

Cleanup and document `-C relocation-model`

As the title says, this is mostly a refactoring and documentation.

One potentially observable change here is that `-C relocation-model=default` now takes the default from the Rust target, rather than from the underlying LLVM target. In other words, `-C relocation-model=default` is now equivalent to not specifying the relocation model on command line at all.
Apparently no one used that option because it has other bugs as well, e.g. PIC `default` wasn't treated as PIC in some places.

When working with an arbitrary reader or writer, code that uses vectored
operations may end up being slower than code that copies into a single
buffer when the underlying reader or writer doesn't actually support
vectored operations. These new methods allow you to ask the reader or
witer up front if vectored operations are efficiently supported.

Currently, you have to use some heuristics to guess by e.g. checking if
the read or write only accessed the first buffer. Hyper is one concrete
example of a library that has to do this dynamically:
https://github.com/hyperium/hyper/blob/0eaf304644a396895a4ce1f0146e596640bb666a/src/proto/h1/io.rs#L582-L594