This has the following positive effects:
1) The lifetime retrieved through 'Deref' is now long-lived.
2) An '&State<T>` can be created via an '&T'.
3) '&State<T>' is shorter to type than 'State<'_, T>'.
Sentinels resolve a long-standing usability and functional correctness
issue in Rocket: starting an application with guards and/or responders
that depend on state that isn't available. The canonical example is the
'State' guard. Prior to this commit, an application with routes that
queried unmanaged state via 'State' would fail at runtime. With this
commit, the application refuses to launch with a detailed error message.
The 'Sentinel' docs explains it as:
A sentinel, automatically run on ignition, can trigger a launch
abort should an instance fail to meet arbitrary conditions. Every
type that appears in a mounted route's type signature is eligible to
be a sentinel. Of these, those that implement 'Sentinel' have their
'abort()' method invoked automatically, immediately after ignition,
once for each unique type. Sentinels inspect the finalized instance
of 'Rocket' and can trigger a launch abort by returning 'true'.
The following types are now sentinels:
* 'contrib::databases::Connection' (any '#[database]' type)
* 'contrib::templates::Metadata'
* 'contrib::templates::Template'
* 'core::State'
The following are "specialized" sentinels, which allow sentinel
discovery even through type aliases:
* 'Option<T>', 'Debug<T>' if 'T: Sentinel'
* 'Result<T, E>', 'Either<T, E>' if 'T: Sentinel', 'E: Sentinel'
Closes#464.
The core 'Rocket' type is parameterized: 'Rocket<P: Phase>', where
'Phase' is a newly introduced, sealed marker trait. The trait is
implemented by three new marker types representing the three launch
phases: 'Build', 'Ignite', and 'Orbit'. Progression through these three
phases, in order, is enforced, as are the invariants guaranteed by each
phase. In particular, an instance of 'Rocket' is guaranteed to be in its
final configuration after the 'Build' phase and represent a running
local or public server in the 'Orbit' phase. The 'Ignite' phase serves
as an intermediate, enabling inspection of a finalized but stationary
instance. Transition between phases validates the invariants required
by the transition.
All APIs have been adjusted appropriately, requiring either an instance
of 'Rocket' in a particular phase ('Rocket<Build>', 'Rocket<Ignite>', or
'Rocket<Orbit>') or operating generically on a 'Rocket<P>'.
Documentation is also updated and substantially improved to mention
required and guaranteed invariants.
Additionally, this commit makes the following relevant changes:
* 'Rocket::ignite()' is now a public interface.
* 'Rocket::{build,custom}' methods can no longer panic.
* 'Launch' fairings are now 'ignite' fairings.
* 'Liftoff' fairings are always run, even in local mode.
* All 'ignite' fairings run concurrently at ignition.
* Launch logging occurs on launch, not any point prior.
* Launch log messages have improved formatting.
* A new launch error kind, 'Config', was added.
* A 'fairing::Result' type alias was introduced.
* 'Shutdown::shutdown()' is now 'Shutdown::notify()'.
Some internal changes were also introduced:
* Fairing 'Info' name for 'Templates' is now 'Templating'.
* Shutdown is implemented using 'tokio::sync::Notify'.
* 'Client::debug()' is used nearly universally in tests.
Resolves#1154.
Resolves#1136.
...because loading up a Rocket while it's ignited is a bad idea.
More seriously, because 'Rocket.ignite()' will become an "execute
everything up to here" method.
The new examples directory...
* Contains a `README.md` explaining each example.
* Consolidates examples into more complete chunks.
* Is just better.
Resolves#1447.
