Fix misspelled comments for tests.
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 #16182 : jbcrail/rust/fix-test-comments, r=sfackler
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
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`.
Print stack overflow messages for Linux and OS X
Fix sigaction on OS X