Correcting the two mistakes in the README.md

copy_pointer -> copy_borrowed_ptr

Fixes rust-lang#41053.

With `-mcpu=power4`, code might use instructions like `fcfid`, excluding
older CPUs like the PowerPC G4, which apparently some users would like
to use.  The generic `-mcpu=powerpc` should stick to pure 32-bit PowerPC
instructions.

Fixes rust-lang/cargo#3852.

similar to GCC's __attribute((used))__. This attribute prevents LLVM from
optimizing away a non-exported symbol, within a compilation unit (object file),
when there are no references to it.

This is better explained with an example:

```
#[used]
static LIVE: i32 = 0;

static REFERENCED: i32 = 0;

static DEAD: i32 = 0;

fn internal() {}

pub fn exported() -> &'static i32 {
    &REFERENCED
}
```

Without optimizations, LLVM pretty much preserves all the static variables and
functions within the compilation unit.

```
$ rustc --crate-type=lib --emit=obj symbols.rs && nm -C symbols.o
0000000000000000 t drop::h1be0f8f27a2ba94a
0000000000000000 r symbols::REFERENCED::hb3bdfd46050bc84c
0000000000000000 r symbols::DEAD::hc2ea8f9bd06f380b
0000000000000000 r symbols::LIVE::h0970cf9889edb56e
0000000000000000 T symbols::exported::h6f096c2b1fc292b2
0000000000000000 t symbols::internal::h0ac1aadbc1e3a494
```

With optimizations, LLVM will drop dead code. Here `internal` is dropped because
it's not a exported function/symbol (i.e. not `pub`lic). `DEAD` is dropped for
the same reason. `REFERENCED` is preserved, even though it's not exported,
because it's referenced by the `exported` function. Finally, `LIVE` survives
because of the `#[used]` attribute even though it's not exported or referenced.

```
$ rustc --crate-type=lib -C opt-level=3 --emit=obj symbols.rs && nm -C symbols.o
0000000000000000 r symbols::REFERENCED::hb3bdfd46050bc84c
0000000000000000 r symbols::LIVE::h0970cf9889edb56e
0000000000000000 T symbols::exported::h6f096c2b1fc292b2
```

Note that the linker knows nothing about `#[used]` and will drop `LIVE`
because no other object references to it.

```
$ echo 'fn main() {}' >> symbols.rs
$ rustc symbols.rs && nm -C symbols | grep LIVE
```

At this time, `#[used]` only works on `static` variables.

to match the type signature of the llvm.used variable

it's not related to this feature

as it's specific to ELF and won't pass on macOS / Windows

(For an explanation of what this feature does, read the commit message)

I'd like to propose landing this as an experimental feature (experimental as in:
no clear stabilization path -- like `asm!`, `#[linkage]`) as it's low
maintenance (I think) and relevant to the "Usage in resource-constrained
environments" exploration area.

The main use case I see is running code before `main`. This could be used, for
instance, to cheaply initialize an allocator before `main` where the alternative
is to use `lazy_static` to initialize the allocator on its first use which it's
more expensive (atomics) and doesn't work on ARM Cortex-M0 microcontrollers (no
`AtomicUsize` on that platform)

Here's a `std` example of that:

``` rust

unsafe extern "C" fn before_main_1() {
    println!("Hello");
}

unsafe extern "C" fn before_main_2() {
    println!("World");
}

static INIT_ARRAY: [unsafe extern "C" fn(); 2] = [before_main_1, before_main_2];

fn main() {
    println!("Goodbye");
}
```

```
$ rustc -C lto -C opt-level=3 before_main.rs
$ ./before_main
Hello
World
Goodbye
```

In general, this pattern could be used to let *dependencies* run code before
`main` (which sounds like it could go very wrong in some cases). There are
probably other use cases; I hope that the people I have cc-ed can comment on
those.

Note that I'm personally unsure if the above pattern is something we want to
promote / allow and that's why I'm proposing this feature as experimental. If
this leads to more footguns than benefits then we can just axe the feature.

cc @nikomatsakis ^ I know you have some thoughts on having a process for
experimental features though I'm fine with writing an RFC before landing this.

- `dead_code` lint will have to be updated to special case `#[used]` symbols.

- Should we extend `#[used]` to work on non-generic functions?

cc rust-lang/rfcs#1002
cc rust-lang/rfcs#1459
cc @dpc @JinShil

Correcting mistakes in the README.md

Correcting the two mistakes in the README.md (issue rust-lang#40793)

Only use cargo-vendor if building from git sources

The only time we need to vendor sources is when building from git.  If one is
building from a rustc source tarball, everything should already be in place.
This also matters for distros which do offline builds, as they can't install
cargo-vendor this way.

This adds a common `Build::src_is_git` flag, and then uses it in the dist-src
target to decide whether to install or use `cargo-vendor` at all.

Fixes rust-lang#41042.

cstore: return an immutable borrow from `visible_parent_map`

This prevents an ICE when `visible_parent_map` is called multiple times, for example when an item referenced in an impl signature is imported from an  `extern crate` statement occurs within an impl.

Fixes rust-lang#41053.

r? @eddyb

… r=alexcrichton

Re-enable appveyor cache

After breaking the queue last time, I'm cautiously back with a PR to re-enable caching on appveyor.

If you look at https://ci.appveyor.com/project/rust-lang/rust/build/1.0.2623/job/46o90by4ari6gege (one of the multiple runs that started failed, there are actually two errors - one for restoring the cache, one right at the bottom for creating a directory. I only noticed the restore error at the time as I was a bit rushed to revert and didn't stop to wonder why it continued - turns out appveyor [does not abort on cache restore failure](appveyor/ci#723).

Turns out the cause of the build failures was the cache directory existing and me being thinking that because mkdir on windows is [recursive by default](http://stackoverflow.com/a/905239/2352259), it ignores the error if the directory already exists. Apparently this is not true, so now it checks if the directory exists before attempting to create.

In addition, I've added some more paranoia to double check everything is sane.

…hton

dist-powerpc-linux: use a pure 32-bit CPU profile

With `-mcpu=power4`, code might use instructions like `fcfid`, excluding
older CPUs like the PowerPC G4, which apparently some users would like
to use.  The generic `-mcpu=powerpc` should stick to pure 32-bit PowerPC
instructions.

Fixes rust-lang/cargo#3852.