Allow non-breaking config additions.

If stars aligned properly, we might imagine writing this:

    #[non_exhaustive]
    struct Config {
        pub field: Foo,
        pub other: Bar,
    }

...with semantics that would allow the defining crate (here, Rocket), to
construct the structure directly while consumers would need to use
public constructors or struct update syntax:

    Config {
        field: Foo,
        other: Bar,
        ..Default::default()
    }

Alas, this is not the way `non_exhaustive` works on structs. You cannot
use field-update syntax to construct `Config` above. You must use public
constructors. This means builder methods or mutating an already built
struct. This is not what we want.

I don't know why it works this way. I don't see why it must. Something
something Drop.

So we have this hack from the pre-non_exhaustive era.

core/lib/src/config/config.rs

@@ -92,6 +92,22 @@ pub struct Config {
     /// Whether to use colors and emoji when logging. **(default: `true`)**
     #[serde(deserialize_with = "figment::util::bool_from_str_or_int")]
     pub cli_colors: bool,
+    /// PRIVATE: This structure may grow (but never change otherwise) in a
+    /// non-breaking release. As such, constructing this structure should
+    /// _always_ be done using a public constructor or update syntax:
+    ///
+    /// ```rust
+    /// use rocket::Config;
+    ///
+    /// let config = Config {
+    ///     port: 1024,
+    ///     keep_alive: 10,
+    ///     ..Default::default()
+    /// };
+    /// ```
+    #[doc(hidden)]
+    #[serde(skip)]
+    pub __non_exhaustive: (),
 }
 
 impl Default for Config {
@@ -150,8 +166,9 @@ impl Config {
             secret_key: SecretKey::zero(),
             temp_dir: std::env::temp_dir(),
             log_level: LogLevel::Normal,
-            cli_colors: true,
             shutdown: Shutdown::default(),
+            cli_colors: true,
+            __non_exhaustive: (),
         }
     }
 

core/lib/src/config/shutdown.rs

@@ -188,6 +188,7 @@ impl fmt::Display for Sig {
 ///         grace: 10,
 ///         mercy: 5,
 ///         force: true,
+///         ..Default::default()
 ///     },
 ///     ..Config::default()
 /// };
@@ -236,7 +237,24 @@ pub struct Shutdown {
     /// cooperate.
     ///
     /// **default: `true`**
+    #[serde(deserialize_with = "figment::util::bool_from_str_or_int")]
     pub force: bool,
+    /// PRIVATE: This structure may grow (but never change otherwise) in a
+    /// non-breaking release. As such, constructing this structure should
+    /// _always_ be done using a public constructor or update syntax:
+    ///
+    /// ```rust
+    /// use rocket::config::Shutdown;
+    ///
+    /// let config = Shutdown {
+    ///     grace: 5,
+    ///     mercy: 10,
+    ///     ..Default::default()
+    /// };
+    /// ```
+    #[doc(hidden)]
+    #[serde(skip)]
+    pub __non_exhaustive: (),
 }
 
 impl fmt::Display for Shutdown {
@@ -266,6 +284,7 @@ impl Default for Shutdown {
             grace: 2,
             mercy: 3,
             force: true,
+            __non_exhaustive: (),
         }
     }
 }