diff options
| -rw-r--r-- | compiler/rustc_trait_selection/src/solve/assembly/mod.rs | 17 |
1 files changed, 13 insertions, 4 deletions
diff --git a/compiler/rustc_trait_selection/src/solve/assembly/mod.rs b/compiler/rustc_trait_selection/src/solve/assembly/mod.rs index 9e8dbd0cde2..3bf1f83df32 100644 --- a/compiler/rustc_trait_selection/src/solve/assembly/mod.rs +++ b/compiler/rustc_trait_selection/src/solve/assembly/mod.rs @@ -321,11 +321,20 @@ impl<'tcx> EvalCtxt<'_, 'tcx> { candidates } - /// If the self type of a goal is an alias, computing the relevant candidates is difficult. + /// If the self type of a goal is an alias we first try to normalize the self type + /// and compute the candidates for the normalized self type in case that succeeds. /// - /// To deal with this, we first try to normalize the self type and add the candidates for the normalized - /// self type to the list of candidates in case that succeeds. We also have to consider candidates with the - /// projection as a self type as well + /// These candidates are used in addition to the ones with the alias as a self type. + /// We do this to simplify both builtin candidates and for better performance. + /// + /// We generate the builtin candidates on the fly by looking at the self type, e.g. + /// add `FnPtr` candidates if the self type is a function pointer. Handling builtin + /// candidates while the self type is still an alias seems difficult. This is similar + /// to `try_structurally_resolve_type` during hir typeck (FIXME once implemented). + /// + /// Looking at all impls for some trait goal is prohibitively expensive. We therefore + /// only look at implementations with a matching self type. Because of this function, + /// we can avoid looking at all existing impls if the self type is an alias. #[instrument(level = "debug", skip_all)] fn assemble_candidates_after_normalizing_self_ty<G: GoalKind<'tcx>>( &mut self, |