Add ConstraintLocator docs

compiler/rustc_typeck/src/collect/type_of.rs

@@ -512,8 +512,15 @@ fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Ty<'_> {
 
  struct ConstraintLocator<'tcx> {
  tcx: TyCtxt<'tcx>,
+
+ /// def_id of the opaque type whose defining uses are being checked
  def_id: DefId,
- // (first found type span, actual type)
+
+ /// as we walk the defining uses, we are checking that all of them
+ /// define the same hidden type. This variable is set to `Some`
+ /// with the first type that we find, and then later types are
+ /// checked against it (we also carry the span of that first
+ /// type).
  found: Option<(Span, Ty<'tcx>)>,
  }