diff options
| -rw-r--r-- | src/librustc/ty/sty.rs | 26 |
1 files changed, 25 insertions, 1 deletions
diff --git a/src/librustc/ty/sty.rs b/src/librustc/ty/sty.rs index 10f89c965d1..05e10784e2d 100644 --- a/src/librustc/ty/sty.rs +++ b/src/librustc/ty/sty.rs @@ -569,6 +569,31 @@ impl<'a, 'gcx, 'tcx> Binder<ExistentialPredicate<'tcx>> { impl<'tcx> serialize::UseSpecializedDecodable for &'tcx List<ExistentialPredicate<'tcx>> {} impl<'tcx> List<ExistentialPredicate<'tcx>> { + /// Returns the "principal def id" of this set of existential predicates. + /// + /// A Rust trait object type consists (in addition to a lifetime bound) + /// of a set of trait bounds, which are separated into any number + /// of auto-trait bounds, and at most 1 non-auto-trait bound. The + /// non-auto-trait bound is called the "principal" of the trait + /// object. + /// + /// Only the principal can have methods or type parameters (because + /// auto traits can have neither of them). This is important, because + /// it means the auto traits can be treated as an unordered set (methods + /// would force an order for the vtable, while relating traits with + /// type parameters without knowing the order to relate them in is + /// a rather non-trivial task). + /// + /// For example, in the trait object `dyn fmt::Debug + Sync`, the + /// principal bound is `Some(fmt::Debug)`, while the auto-trait bounds + /// are the set `{Sync}`. + /// + /// It is also possible to have a "trivial" trait object that + /// consists only of auto traits, with no principal - for example, + /// `dyn Send + Sync`. In that case, the set of auto-trait bounds + /// is `{Send, Sync}`, while there is no principal. These trait objects + /// have a "trivial" vtable consisting of just the size, alignment, + /// and destructor. pub fn principal(&self) -> Option<ExistentialTraitRef<'tcx>> { match self[0] { ExistentialPredicate::Trait(tr) => Some(tr), @@ -576,7 +601,6 @@ impl<'tcx> List<ExistentialPredicate<'tcx>> { } } - pub fn principal_def_id(&self) -> Option<DefId> { self.principal().map(|d| d.def_id) } |