native, rustuv: Fix spawning with empty args

There was a bug in both libnative and libuv which prevented child processes from
being spawned correctly on windows when one of the arguments was an empty
string. The libuv bug has since been fixed upstream, and the libnative bug was
fixed as part of this commit.

When updating libuv, this also includes a fix for #15149.

Closes #15149
Closes #16272
Fix typo `convering` to `converting`
lint: dead_code ignores items with leading underscores.

This generalises the behaviour with struct fields (which recieve no
dead_code warning if they have a leading _), and other similar lints, to
all items, e.g. `fn _foo() {} fn main() {}` has no warnings.
Note naming convention of lists (xs, ys, ...)

People reading the tutorial may not be familiar with the convention of naming lists, vectors and the like as xs, ys, etc. Without some explanation of the reasoning behind it, it might come off as just throwaway non-descriptive names. Languages like Haskell gets flak from using short, non-descriptive names, while in reality, there are clear conventions and reasons for using certain terse variable names.

I assumed that the convention came from a language like Haskell, so I
tailored the explanation according to that.
auto merge of #15410 : LemmingAvalanche/rust/patch-1, r=alexcrichton

People reading the tutorial may not be familiar with the convention of naming lists, vectors and the like as xs, ys, etc. Without some explanation of the reasoning behind it, it might come off as just throwaway non-descriptive names. Languages like Haskell gets flak from using short, non-descriptive names, while in reality, there are clear conventions and reasons for using certain terse variable names. 

This is just a proposed explanation of this convention, as I've interpreted it - I assumed that the convention came from a language like Haskell, so I tailored it according to that. So beware that I might have misjudged how it is used in the Rust language, or at least how it is used in the Rust tutorial.
Fix some minor issues in the guide.
auto merge of #16399 : superlogical/rust/patch-2, r=steveklabnik
auto merge of #16379 : parir/rust/guide-typos, r=kballard

This PR fixes some minor grammar and spelling issues in the guide.
core/std: squash dead_code warnings from fail! invocations.

The fail macro defines some function/static items internally, which got
a dead_code warning when `fail!()` is used inside a dead function. This
is ugly and unnecessarily reveals implementation details, so the
warnings can be squashed.

Fixes #16192.
auto merge of #16196 : huonw/rust/fail-dead-code, r=alexcrichton

The fail macro defines some function/static items internally, which got
a dead_code warning when `fail!()` is used inside a dead function. This
is ugly and unnecessarily reveals implementation details, so the
warnings can be squashed.

Fixes #16192.
Accept empty log level
API docs/examples for std::slice

 - API doc/example for next() in Permutations
 - API doc/example for permutations() in ImmutableCloneableVector
 - Moved examples for permutations and next into trait definition as
   comments on pull request #16244.
 - Fix erroneus inclusion of src/llvm in older commit.
auto merge of #16416 : nick29581/rust/log, r=pnkfelix

Previously we would accept an empty log level without an equals sign, but not with one. This addresses that minor nit. E.g., `RUST_LOG=rustc::middle::trans=` will work the same as `RUST_LOG=rustc::middle::trans`.
Update docs to use HTTPS for static.rust-lang.org addresses

cc #16123
Download snapshots using HTTPS

cc #16123
Guide: vectors
auto merge of #16429 : steveklabnik/rust/guide_vectors, r=cmr

Since https://github.com/rust-lang/rust/pull/16380 didn't get pulled in yet, I added it in here too.

This covers the very, very, very basics of vectors. I wanted to have a section that mentioned them, but I'm
unsure what else I should cover. So I just did the absolute simplest things. Feedback very welcome.
auto merge of #16427 : brson/rust/https, r=thestinger
auto merge of #16417 : jasonthompson/rust/docs/slice3, r=alexcrichton

- Moved examples for permutations and next into trait definition as
   comments on pull request #16244.
- Fixed (hopefully) issue with erronious commit of changes to src/llvm.
Fix many-crates-but-no-match test. (Issue #16348)