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.
Fix some minor issues in the guide.
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`.