Document the ordering constraint on Binders and Substitution

Author: lowr

Diff for: crates/hir-ty/src/lib.rs

@@ -81,7 +81,20 @@ pub type PlaceholderIndex = chalk_ir::PlaceholderIndex;
 pub type VariableKind = chalk_ir::VariableKind<Interner>;
 pub type VariableKinds = chalk_ir::VariableKinds<Interner>;
 pub type CanonicalVarKinds = chalk_ir::CanonicalVarKinds<Interner>;
+/// Represents generic parameters and an item bound by them. When the item has parent, the binders
+/// also contain the generic parameters for its parent. See chalk's documentation for details.
+///
+/// One thing to keep in mind when working with `Binders` (and `Substitution`s, which represent
+/// generic arguments) in rust-analyzer is that the ordering within *is* significant - the generic
+/// parameters/arguments for an item MUST come before those for its parent. This is to facilitate
+/// the integration with chalk-solve, which mildly puts constraints as such. See #13335 for its
+/// motivation in detail.
 pub type Binders<T> = chalk_ir::Binders<T>;
+/// Interned list of generic arguments for an item. When an item has parent, the `Substitution` for
+/// it contains generic arguments for both its parent and itself. See chalk's documentation for
+/// details.
+///
+/// See `Binders` for the constraint on the ordering.
 pub type Substitution = chalk_ir::Substitution<Interner>;
 pub type GenericArg = chalk_ir::GenericArg<Interner>;
 pub type GenericArgData = chalk_ir::GenericArgData<Interner>;