+ /// Given a trait bound like `Debug`, applies that trait bound the given self-type to construct
+ /// a full trait reference. The resulting trait reference is returned. This may also generate
+ /// auxiliary bounds, which are added to `bounds`.
+ ///
+ /// Example:
+ ///
+ /// ```
+ /// poly_trait_ref = Iterator<Item = u32>
+ /// self_ty = Foo
+ /// ```
+ ///
+ /// this would return `Foo: Iterator` and add `<Foo as Iterator>::Item = u32` into `bounds`.
+ ///
+ /// **A note on binders:** against our usual convention, there is an implied bounder around
+ /// the `self_ty` and `poly_trait_ref` parameters here. So they may reference bound regions.
+ /// If for example you had `for<'a> Foo<'a>: Bar<'a>`, then the `self_ty` would be `Foo<'a>`
+ /// where `'a` is a bound region at depth 0. Similarly, the `poly_trait_ref` would be
+ /// `Bar<'a>`. The returned poly-trait-ref will have this binder instantiated explicitly,
+ /// however.