Explain how nested generic args are represented

Author: khyperia

src/ty-module/generic-arguments.md

@@ -126,3 +126,30 @@ For the `MyStruct<U>` written in the `Foo` type alias, we would represent it in
 - There would be an `AdtDef` (and corresponding `DefId`) for `MyStruct`.
 - There would be a `GenericArgs` containing the list `[GenericArgKind::Type(Ty(u32))]`
 - And finally a `TyKind::Adt` with the `AdtDef` and `GenericArgs` listed above.
+
+## Nested generic args
+
+Consider:
+
+```rust,ignore
+struct MyStruct<T>(T);
+
+impl<T> MyStruct<T> {
+    fn func<T2>(t: T, t2: T2) {}
+}
+
+fn main() {
+    MyStruct::<u32>::func::<bool>(2, true);
+}
+```
+
+The construct `MyStruct::<u32>::func::<bool>` is represented by a tuple: a DefId pointing at `func`, and then a
+`GenericArgs` list that "walks" all containing generic parameters - in this case, the list would be `[u32, bool]`.
+
+The [`ty::Generics`] type (returned by the [`generics_of`] query) contains the information of how the nested hierarchy
+gets flattened down to a list, and lets you figure out which index in the `GenericArgs` list corresponds to which
+generic (handling complications such as `Self` sometimes being a generic parameter, and so whether it should be included
+in the list or not). Check how it works for exact specifics on how parameters get mapped to indices.
+
+[`ty::Generics`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Generics.html
+[`generics_of`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.generics_of