[wgpu] Document dispatch_type! macro. (#8483)

Add more documentation for the `dispatch_type` macro, and provide a
more detailed explanation of how it enables devirtualization in the
single-backend case.

wgpu/src/dispatch.rs

@@ -5,8 +5,11 @@
 //!
 //! The interface traits are all object safe and listed in the `InterfaceTypes` trait.
 //!
-//! The method for dispatching should optimize well if only one backend is compiled in,
-//! as-if there was no dispatching at all.
+//! The method for dispatching should optimize well if only one backend is
+//! compiled in, as-if there was no dispatching at all. See the comments on
+//! [`dispatch_types`] for details.
+//!
+//! [`dispatch_types`]: macro.dispatch_types.html
 
 #![allow(drop_bounds)] // This exists to remind implementors to impl drop.
 #![allow(clippy::too_many_arguments)] // It's fine.
@@ -581,16 +584,77 @@ pub trait BufferMappedRangeInterface: CommonTraits {
     fn as_uint8array(&self) -> &js_sys::Uint8Array;
 }
 
-/// Generates Dispatch types for each of the interfaces. Each type is a wrapper around the
-/// wgpu_core and webgpu types, and derefs to the appropriate interface trait-object.
+/// Generates a dispatch type for some `wgpu` API type.
 ///
-/// When there is only one backend, devirtualization fires and all dispatches should turn into
-/// direct calls. If there are multiple, some dispatching will occur.
+/// Invocations of this macro take one of the following forms:
 ///
-/// This also provides `as_*` methods so that the backend implementations can dereference other
-/// arguments. These are similarly free when there is only one backend.
+/// ```ignore
+/// dispatch_types! {mut type D: I = Core, Web, Dyn }
+/// dispatch_types! {ref type D: I = Core, Web, Dyn }
+/// ```
 ///
-/// In the future, we may want a truly generic backend, which could be extended from this enum.
+/// This defines `D` as a type that dereferences to a `dyn I` trait object. Most uses of
+/// `D` in the rest of this crate just call the methods from the `dyn I` object, not from
+/// `D` itself.
+///
+/// Internally, `D` is an enum with up to three variants holding values of type `Core`,
+/// `Web`, and `Dyn`, all of which must implement `I`. `Core`, `Web` and `Dyn` are the
+/// types from the `wgpu_core`, `webgpu`, and `custom` submodules of `wgpu::backend` that
+/// correspond to `D`. The macro generates `Deref` and `DerefMut` implementations that
+/// match on this enum and produce a `dyn I` reference for each variant.
+///
+/// The macro's `mut type` form defines `D` as the unique owner of the backend type, with
+/// a `DerefMut` implementation, and `as_*_mut` methods that return `&mut` references.
+/// This `D` does not implement `Clone`.
+///
+/// The macro's `ref type` form defines `D` to hold an `Arc` pointing to the backend type,
+/// permitting `Clone` and `Deref`, but losing exclusive, mutable access.
+///
+/// For example:
+///
+/// ```ignore
+/// dispatch_types! {ref type DispatchBuffer: BufferInterface =
+///                  CoreBuffer, WebBuffer, DynBuffer}
+/// ```
+///
+/// This defines `DispatchBuffer` as a type that dereferences to `&dyn BufferInterface`,
+/// which has methods like `map_async` and `destroy`. The enum would be:
+///
+/// ```ignore
+/// pub enum DispatchBuffer {
+///     #[cfg(wgpu_core)]
+///     Core(Arc<CoreBuffer>),
+///     #[cfg(webgpu)]
+///     WebGPU(WebBuffer),
+///     #[cfg(custom)]
+///     Custom(DynBuffer),
+/// }
+/// ```
+///
+/// This macro also defines `as_*` methods so that the backend implementations can
+/// dereference other arguments.
+///
+/// ## Devirtualization
+///
+/// The dispatch types generated by this macro are carefully designed to allow the
+/// compiler to completely devirtualize calls in most circumstances.
+///
+/// Note that every variant of the enum generated by this macro is under a `#[cfg]`.
+/// Naturally, the `match` expressions in the `Deref` and `DerefMut` implementations have
+/// matching `#[cfg]` attributes on each match arm.
+///
+/// In practice, when `wgpu`'s `"custom"` feature is not enabled, there is usually only
+/// one variant in the `enum`, making it effectively a newtype around the sole variant's
+/// data: it has no discriminant to branch on, and the `match` expressions are removed
+/// entirely by the compiler.
+///
+/// In this case, when we invoke a method from the interface trait `I` on a dispatch type,
+/// the `Deref` and `DerefMut` implementations' `match` statements build a `&dyn I` for
+/// the data, on which we immediately invoke a method. The vtable is a constant, allowing
+/// the Rust compiler to turn the `dyn` method call into an ordinary method call. This
+/// creates opportunities for inlining.
+///
+/// Similarly, the `as_*` methods are free when there is only one backend.
 macro_rules! dispatch_types {
     (
         ref type $name:ident: $interface:ident = $core_type:ident,$webgpu_type:ident,$custom_type:ident