diff options
| -rw-r--r-- | src/librustc/middle/traits/specialize/mod.rs | 32 |
1 files changed, 29 insertions, 3 deletions
diff --git a/src/librustc/middle/traits/specialize/mod.rs b/src/librustc/middle/traits/specialize/mod.rs index 2dc4926736e..93505987633 100644 --- a/src/librustc/middle/traits/specialize/mod.rs +++ b/src/librustc/middle/traits/specialize/mod.rs @@ -64,9 +64,35 @@ pub fn translate_substs<'tcx>(tcx: &ty::ctxt<'tcx>, /// When we have selected one impl, but are actually using item definitions from /// a parent impl providing a default, we need a way to translate between the /// type parameters of the two impls. Here the `source_impl` is the one we've -/// selected, and `source_substs` is a substitution of its generics (and possibly -/// some relevant `FnSpace` variables as well). And `target_impl` is the impl -/// we're actually going to get the definition from. +/// selected, and `source_substs` is a substitution of its generics (and +/// possibly some relevant `FnSpace` variables as well). And `target_impl` is +/// the impl we're actually going to get the definition from. The resulting +/// substitution will map from `target_impl`'s generics to `source_impl`'s +/// generics as instantiated by `source_subst`. +/// +/// For example, consider the following scenario: +/// +/// ```rust +/// trait Foo { ... } +/// impl<T, U> Foo for (T, U) { ... } // target impl +/// impl<V> Foo for (V, V) { ... } // source impl +/// ``` +/// +/// Suppose we have selected "source impl" with `V` instantiated with `u32`. +/// This function will produce a substitution with `T` and `U` both mapping to `u32`. +/// +/// Where clauses add some trickiness here, because they can be used to "define" +/// an argument indirectly: +/// +/// ```rust +/// impl<'a, I, T: 'a> Iterator for Cloned<I> +/// where I: Iterator<Item=&'a T>, T: Clone +/// ``` +/// +/// In a case like this, the substitution for `T` is determined indirectly, +/// through associated type projection. We deal with such cases by using +/// *fulfillment* to relate the two impls, requiring that all projections are +/// resolved. fn translate_substs_between_impls<'tcx>(tcx: &ty::ctxt<'tcx>, source_impl: DefId, source_substs: Substs<'tcx>, |