Change unused_labels from allow to warn
Fixes#66324, making the unused_labels lint warn instead of allow by default. I'm told @rust-lang/lang will need to review this, and perhaps will want to do a crater run.
Deleted unused labels from compiler and fixed or allowed
unused labels in tests. This patch removes some gratuitous
unused labels and turns off the warning for unused labels
that are a necessary part of tests. This will permit
setting the `unused_labels` lint to `warn`.
This commit gives each stable keyword a short entry in the "Keywords"
section in the docs for `std`. The newly added entries are a single
summary line and a note that the documentation is not yet complete. I
changed some of the existing summary lines for consistency's sake. Each
line is either a verb phrase ("name the type of a trait object" for
`dyn`), or an object ("A value of type `bool` representing logical true"
for `true`). I tried to avoid using the keyword itself or the word
"keyword" in the summary.
Later PRs can flesh out each keyword with an example of each
context in which a keyword can appear and a link to the rust book.
Keywords which are not close to stable rust such as `box` (which is
getting unstabilized) or `try` are ignored in this PR.
`const fn` is no longer coming soon (const keyword docs)
The `const` keyword [documentation](https://doc.rust-lang.org/std/keyword.const.html) mentions that `const fn`s are coming soon, but they have already been added.
Most of these will eventually be filled, but right now travis-ci enjoys
complaining about the fact that there's links that lead nowhere, so
they're gone. Hopefully someone remembers to re-add them later.
This commit also splits out linky-line-thingies into two lines, which
judging from the source code for tidy, should be enough to make it shut
up and accept me for who I am, dammit.
