Make type macros inelgible as sentinels.

core/codegen/src/syn_ext.rs

@@ -137,7 +137,7 @@ impl TypeExt for syn::Type {
                         self.0 = false;
                         return;
                     }
-                    ImplTrait(_) | Infer(_) => {
+                    ImplTrait(_) | Infer(_) | Macro(_) => {
                         self.0 = false;
                         return;
                     }

core/lib/src/sentinel.rs

@@ -152,14 +152,14 @@ use crate::{Rocket, Ignite};
 ///
 /// **Note:** _Rocket actively discourages using `impl Trait` in route
 /// signatures. In addition to impeding sentinel discovery, doing so decreases
-/// the ability to gleam handler functionality based on its type signature._
+/// the ability to gleam a handler's functionality based on its type signature._
 ///
 /// The return type of the route `f` depends on its implementation. At present,
 /// it is not possible to name the underlying concrete type of an `impl Trait`
 /// at compile-time and thus not possible to determine if it implements
-/// `Sentinel`. As such, existentials _are not_ eligible to be sentinels. This
-/// limitation applies per embedded type: the directly named `AnotherSentinel`
-/// type continues to be eligible to be a sentinel.
+/// `Sentinel`. As such, existentials _are not_ eligible to be sentinels.
+/// However, this limitation applies per embedding, so the inner, directly named
+/// `AnotherSentinel` type continues to be eligible to be a sentinel.
 ///
 /// When possible, prefer to name all types:
 ///
@@ -202,6 +202,33 @@ use crate::{Rocket, Ignite};
 /// cases, a type alias _will_ "consider" embeddings. Nevertheless, prefer to
 /// write concrete types when possible.
 ///
+/// ## Type Macros
+///
+/// It is impossible to determine, a priori, what a type macro will expand to.
+/// As such, Rocket is unable to determine which sentinels, if any, a type macro
+/// references, and thus no sentinels are discovered from type macros.
+///
+/// Even approximations are impossible. For example, consider the following:
+///
+/// ```rust
+/// # use rocket::*;
+/// macro_rules! MyType {
+///     (State<'_, u32>) => (&'_ rocket::Config)
+/// }
+///
+/// #[get("/")]
+/// fn f(guard: MyType![State<'_, u32>]) {
+///     /* ... */
+/// }
+/// ```
+///
+/// While the `MyType![State<'_, u32>]` type _appears_ to contain a `State`
+/// sentinel, the macro actually expands to `&'_ rocket::Config`, which is _not_
+/// the `State` sentinel.
+///
+/// You should prefer not to use type macros, or if necessary, restrict your use
+/// to those that always expand to types without sentinels.
+///
 /// # Custom Sentinels
 ///
 /// Any type can implement `Sentinel`, and the implementation can arbitrarily