item_impl holds an Option<> to the trait ref, not a list of trait refs. Therefore, we should not iterate over it.
std::io: Add tests and benchmarks for u64_from_be_bytes()
"As long an iterator" => "As long as an iterator"
Fix #11421
auto merge of #11451 : kud1ing/rust/patch-1, r=cmr
auto merge of #11452 : derekchiang/rust/fix-11421, r=cmr

Closes #11421.  A pretty trivial fix.
std::io: Optimize u64_from_be_bytes()

Instead of reading a byte at a time in a loop we copy the relevant bytes into
a temporary vector of size eight. We can then read the value from the temporary
vector using a single u64 read. LLVM seems to be able to optimize this
almost scarily good.
Improve invalid operator assignment handling.
auto merge of #11449 : rcatolino/rust/assign-binop-handling, r=alexcrichton

So far the following code
```
struct Foo;

fn main() {
  let mut t = Foo;
  let ref b = Foo;
  a += *b;
}
```
errors with 
```
test.rs:15:3: 13:11 error: binary operation + cannot be applied to type `Foo`
test.rs:15   *a += *b;
```
Since assignment-operators are no longer expanded to ```left = left OP right``` but are independents operators it should be 
```
test.rs:15:3: 13:11 error: binary operation += cannot be applied to type `Foo`
test.rs:15   *a += *b;
```
to make it clear that implementing Add for Foo is not gonna work. (cf issues #11143, #11344)

Besides that, we also need to typecheck the rhs expression even if the operator has no implementation, or we end up with unknown types for the nodes of the rhs and an ICE later on while resolving types. (once again cf #11143 and #11344).

This probably would get fixed with #5992, but in the meantime it's a confusing error to stumble upon.
@pcwalton, you wrote the original code, what do you think?
(closes #11143 and #11344)
auto merge of #11448 : c-a/rust/u64_from_be_bytes, r=alexcrichton

Instead of reading a byte at a time in a loop we hardcode how to read each size.
We also try to do as few reads as possible by reading as big primitive types as
possible. For example if size is eight we do a single read of a u64 value and
if size is seven we read it as [u32|u16|u8].

Timings on a Intel(R) Core(TM) i5-3570K CPU @ 3.40GHz

-- Before --
running 7 tests
test io::extensions::test::test_u64_from_be_bytes ... ok
test io::extensions::bench::u64_from_be_bytes_4_aligned         ... bench:       386 ns/iter (+/- 5)
test io::extensions::bench::u64_from_be_bytes_4_unaligned       ... bench:       387 ns/iter (+/- 2)
test io::extensions::bench::u64_from_be_bytes_7_aligned         ... bench:       628 ns/iter (+/- 1)
test io::extensions::bench::u64_from_be_bytes_7_unaligned       ... bench:       637 ns/iter (+/- 3)
test io::extensions::bench::u64_from_be_bytes_8_aligned         ... bench:       727 ns/iter (+/- 18)
test io::extensions::bench::u64_from_be_bytes_8_unaligned       ... bench:       723 ns/iter (+/- 22)

callgrind rustc -S  rust/src/test/bench/sudoku.rs
    u64_from_be_bytes self: 4.37%

-- After --
running 7 tests
test io::extensions::test::test_u64_from_be_bytes ... ok
test io::extensions::bench::u64_from_be_bytes_4_aligned         ... bench:       162 ns/iter (+/- 7)
test io::extensions::bench::u64_from_be_bytes_4_unaligned       ... bench:       164 ns/iter (+/- 7)
test io::extensions::bench::u64_from_be_bytes_7_aligned         ... bench:       201 ns/iter (+/- 7)
test io::extensions::bench::u64_from_be_bytes_7_unaligned       ... bench:       210 ns/iter (+/- 9)
test io::extensions::bench::u64_from_be_bytes_8_aligned         ... bench:       163 ns/iter (+/- 7)
test io::extensions::bench::u64_from_be_bytes_8_unaligned       ... bench:       163 ns/iter (+/- 10)

callgrind rustc -S  rust/src/test/bench/sudoku.rs
    u64_from_be_bytes self: 1.78%
Remove re-exports of std::io::stdio::{print, println} in the prelude.

The `print!` and `println!` macros are now the preferred method of printing, and so there is no reason to export the `stdio` functions in the prelude. The functions have also been replaced by their macro counterparts in the tutorial and other documentation so that newcomers don't get confused about what they should be using.
auto merge of #11415 : nick29581/rust/visit_trait_option, r=alexcrichton

...Therefore, we should not iterate over it.
auto merge of #11416 : bjz/rust/remove-print-fns, r=alexcrichton

The `print!` and `println!` macros are now the preferred method of printing, and so there is no reason to export the `stdio` functions in the prelude. The functions have also been replaced by their macro counterparts in the tutorial and other documentation so that newcomers don't get confused about what they should be using.