At the moment, documentation of std::io::Write::write indicates that
call to it ‘represents at most one attempt to write to any wrapped
object’.  It seems that such wording was put there to contrast it
with pre-1.0 interface which attempted to write all the data (it has
since been changed in [RFC 517]).

However, the requirement puts unnecessary constraints and may complicate
adaptors which perform non-trivial transformations on the data.  For
example, they may maintain an internal buffer which needs to be written
out before the write method accepts more data.  It might be natural to
code the method such that it flushes the buffer and then grabs another
chunk of user data.  With the current wording in the documentation, the
adaptor would be forced to return Ok(0).

This commit softens the wording such that implementations can choose
code structure which makes most sense for their particular use case.

While at it, elaborate on the meaning of `Ok(0)` return pointing out
that the write_all methods interprets it as an error.

[RFC 517]: https://rust-lang.github.io/rfcs/0517-io-os-reform.html

Co-authored-by: Andrew Gallant <jamslam@gmail.com>

Co-authored-by: Jacob Lifshay <programmerjake@gmail.com>

It was already filtered out for emscripten, but wasi doesn't need dlmalloc
either since it reuses `unix/alloc.rs`.

io: soften ‘at most one write attempt’ requirement in io::Write::write

At the moment, documentation of std::io::Write::write indicates that
call to it ‘represents at most one attempt to write to any wrapped
object’.  It seems that such wording was put there to contrast it with
pre-1.0 interface which attempted to write all the data (it has since
been changed in [RFC 517]).

However, the requirement puts unnecessary constraints and may
complicate adaptors which perform non-trivial transformations on the
data.  For example, they may maintain an internal buffer which needs
to be written out before the write method accepts more data.  It might
be natural to code the method such that it flushes the buffer and then
grabs another chunk of user data.  With the current wording in the
documentation, the adaptor would be forced to return Ok(0).

This commit softens the wording such that implementations can choose
code structure which makes most sense for their particular use case.

While at it, elaborate on the meaning of `Ok(0)` return pointing out
that the write_all methods interprets it as an error.

[RFC 517]: https://rust-lang.github.io/rfcs/0517-io-os-reform.html

Launch a non-unwinding panic for misaligned pointer deref

This panic already never unwinds, but that's only because it always hits the unwind guard that's created by our `UnwindAction::Terminate`. Hitting the unwind guard generates a huge double-panic backtrace. Now we generate a normal-looking panic message when this check is hit.

r? `@thomcc`

Alter `Display` for `Ipv6Addr` for IPv4-compatible addresses

ACP: rust-lang/libs-team#239

… r=lcnr

Move WF/ConstEvaluatable goal to clause

It can show up in a param-env, so I think it needs to be a clause kind.

r? `@lcnr` or `@oli-obk`

…lacrum

std: only depend on dlmalloc for wasm*-unknown

It was already filtered out for emscripten, but wasi doesn't need dlmalloc
either since it reuses `unix/alloc.rs`.

bootstrap: check for dry run when copying env vars for msvc

The new synthetic targets for mir-opt blessing aren't added to `builder.cc` during dry runs, causing `x.py test tests/mir-opt --bless`  to crash on MSVC when it tries to copy env vars to the C compiler invocation. This PR adds a check for dry run to fix the panic.

…compiler-errors

Make `Bound::predicates`  use `Clause`

Part of rust-lang#107250

`Bound::predicates` returns an iterator over `Binder<_, Clause>` instead of `Predicate`.

I tried updating `explicit_predicates_of` as well, but it seems that it needs a lot more change than I thought. Will do it in a separate PR instead.