Implement RFC 2532 – Associated Type Defaults

Author: jonas-schievink

src/librustc_infer/traits/project.rs

@@ -1054,25 +1054,40 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                     // an error when we confirm the candidate
                     // (which will ultimately lead to `normalize_to_error`
                     // being invoked).
-                    node_item.item.defaultness.has_value()
+                    false
                 } else {
+                    // If we're looking at a trait *impl*, the item is
+                    // specializable if the impl or the item are marked
+                    // `default`.
                     node_item.item.defaultness.is_default()
                         || super::util::impl_is_default(selcx.tcx(), node_item.node.def_id())
                 };
 
-                // Only reveal a specializable default if we're past type-checking
-                // and the obligations is monomorphic, otherwise passes such as
-                // transmute checking and polymorphic MIR optimizations could
-                // get a result which isn't correct for all monomorphizations.
-                if !is_default {
-                    true
-                } else if obligation.param_env.reveal == Reveal::All {
-                    // NOTE(eddyb) inference variables can resolve to parameters, so
-                    // assume `poly_trait_ref` isn't monomorphic, if it contains any.
-                    let poly_trait_ref = selcx.infcx().resolve_vars_if_possible(&poly_trait_ref);
-                    !poly_trait_ref.needs_infer() && !poly_trait_ref.needs_subst()
-                } else {
-                    false
+                match is_default {
+                    // Non-specializable items are always projectable
+                    false => true,
+
+                    // Only reveal a specializable default if we're past type-checking
+                    // and the obligation is monomorphic, otherwise passes such as
+                    // transmute checking and polymorphic MIR optimizations could
+                    // get a result which isn't correct for all monomorphizations.
+                    true if obligation.param_env.reveal == Reveal::All => {
+                        // NOTE(eddyb) inference variables can resolve to parameters, so
+                        // assume `poly_trait_ref` isn't monomorphic, if it contains any.
+                        let poly_trait_ref =
+                            selcx.infcx().resolve_vars_if_possible(&poly_trait_ref);
+                        !poly_trait_ref.needs_infer() && !poly_trait_ref.needs_subst()
+                    }
+
+                    true => {
+                        debug!(
+                            "assemble_candidates_from_impls: not eligible due to default: \
+                             assoc_ty={} predicate={}",
+                            selcx.tcx().def_path_str(node_item.item.def_id),
+                            obligation.predicate,
+                        );
+                        false
+                    }
                 }
             }
             super::VtableParam(..) => {

src/librustc_typeck/check/mod.rs

@@ -1964,7 +1964,6 @@ fn check_impl_items_against_trait<'tcx>(
 
     // Locate trait definition and items
     let trait_def = tcx.trait_def(impl_trait_ref.def_id);
-    let mut overridden_associated_type = None;
 
     let impl_items = || impl_item_refs.iter().map(|iiref| tcx.hir().impl_item(iiref.id));
 
@@ -2046,9 +2045,6 @@ fn check_impl_items_against_trait<'tcx>(
                 hir::ImplItemKind::OpaqueTy(..) | hir::ImplItemKind::TyAlias(_) => {
                     let opt_trait_span = tcx.hir().span_if_local(ty_trait_item.def_id);
                     if ty_trait_item.kind == ty::AssocKind::Type {
-                        if ty_trait_item.defaultness.has_value() {
-                            overridden_associated_type = Some(impl_item);
-                        }
                         compare_ty_impl(
                             tcx,
                             &ty_impl_item,
@@ -2082,8 +2078,6 @@ fn check_impl_items_against_trait<'tcx>(
 
     // Check for missing items from trait
     let mut missing_items = Vec::new();
-    let mut invalidated_items = Vec::new();
-    let associated_type_overridden = overridden_associated_type.is_some();
     for trait_item in tcx.associated_items(impl_trait_ref.def_id).in_definition_order() {
         let is_implemented = trait_def
             .ancestors(tcx, impl_id)
@@ -2094,28 +2088,13 @@ fn check_impl_items_against_trait<'tcx>(
         if !is_implemented && !traits::impl_is_default(tcx, impl_id) {
             if !trait_item.defaultness.has_value() {
                 missing_items.push(*trait_item);
-            } else if associated_type_overridden {
-                invalidated_items.push(trait_item.ident);
             }
         }
     }
 
     if !missing_items.is_empty() {
         missing_items_err(tcx, impl_span, &missing_items, full_impl_span);
     }
-
-    if !invalidated_items.is_empty() {
-        let invalidator = overridden_associated_type.unwrap();
-        struct_span_err!(
-            tcx.sess,
-            invalidator.span,
-            E0399,
-            "the following trait items need to be reimplemented as `{}` was overridden: `{}`",
-            invalidator.ident,
-            invalidated_items.iter().map(|name| name.to_string()).collect::<Vec<_>>().join("`, `")
-        )
-        .emit();
-    }
 }
 
 fn missing_items_err(

src/test/ui/associated-types/associated-types-overridden-default.rs

@@ -1,3 +1,8 @@
+// check-pass
+
+// Before RFC 2532, overriding one assoc. type default required overriding all
+// provided defaults.
+
 #![feature(associated_type_defaults)]
 
 pub trait Tr {
@@ -9,7 +14,6 @@ pub trait Tr {
 
 impl Tr for () {
     type Assoc = ();
-    //~^ ERROR need to be reimplemented as `Assoc` was overridden: `Assoc2`, `C`, `foo`
 }
 
 fn main() {}

src/test/ui/associated-types/associated-types-overridden-default.stderr

@@ -0,0 +1,35 @@
+#![feature(associated_type_defaults)]
+
+// Having a cycle in assoc. type defaults is okay...
+trait Tr {
+    type A = Self::B;
+    type B = Self::A;
+}
+
+// ...but is an error in any impl that doesn't override at least one of the defaults
+impl Tr for () {}
+//~^ ERROR overflow evaluating the requirement
+
+// As soon as at least one is redefined, it works:
+impl Tr for u8 {
+    type A = u8;
+}
+
+impl Tr for u32 {
+    type A = ();
+    type B = u8;
+}
+
+// ...but only if this actually breaks the cycle
+impl Tr for bool {
+//~^ ERROR overflow evaluating the requirement
+    type A = Box<Self::B>;
+    //~^ ERROR overflow evaluating the requirement
+}
+// (the error is shown twice for some reason)
+
+fn main() {
+    // Check that the overridden type propagates to the other
+    let _a: <u8 as Tr>::A = 0u8;
+    let _b: <u8 as Tr>::B = 0u8;
+}

src/test/ui/associated-types/defaults-cyclic-fail.rs

@@ -0,0 +1,21 @@
+error[E0275]: overflow evaluating the requirement `<() as Tr>::B`
+  --> $DIR/defaults-cyclic-fail.rs:10:6
+   |
+LL | impl Tr for () {}
+   |      ^^
+
+error[E0275]: overflow evaluating the requirement `<bool as Tr>::B`
+  --> $DIR/defaults-cyclic-fail.rs:24:6
+   |
+LL | impl Tr for bool {
+   |      ^^
+
+error[E0275]: overflow evaluating the requirement `<bool as Tr>::B`
+  --> $DIR/defaults-cyclic-fail.rs:26:5
+   |
+LL |     type A = Box<Self::B>;
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0275`.

src/test/ui/associated-types/defaults-cyclic-fail.stderr

@@ -0,0 +1,22 @@
+// check-pass
+
+#![feature(associated_type_defaults)]
+
+trait Tr {
+    type Item = u8;
+    type Container = Vec<Self::Item>;
+}
+
+impl Tr for () {}
+
+impl Tr for u16 {
+    type Item = u16;
+}
+
+fn main() {
+    let _container: <() as Tr>::Container = Vec::<u8>::new();
+    let _item: <() as Tr>::Item = 0u8;
+
+    let _container: <u16 as Tr>::Container = Vec::<u16>::new();
+    let _item: <u16 as Tr>::Item = 0u16;
+}

src/test/ui/associated-types/defaults-cyclic-pass.rs

@@ -0,0 +1,50 @@
+#![feature(associated_type_defaults)]
+
+// Associated type defaults may not be assumed inside the trait defining them.
+// ie. they only resolve to `<Self as Tr>::A`, not the actual type `()`
+trait Tr {
+    type A = ();
+
+    fn f(p: Self::A) {
+        let () = p;
+        //~^ ERROR mismatched types
+        //~| NOTE expected associated type, found `()`
+        //~| NOTE expected associated type `<Self as Tr>::A`
+        //~| NOTE consider constraining
+        //~| NOTE for more information, visit
+    }
+}
+
+// An impl that doesn't override the type *can* assume the default.
+impl Tr for () {
+    fn f(p: Self::A) {
+        let () = p;
+    }
+}
+
+impl Tr for u8 {
+    type A = ();
+
+    fn f(p: Self::A) {
+        let () = p;
+    }
+}
+
+trait AssocConst {
+    type Ty = u8;
+
+    // Assoc. consts also cannot assume that default types hold
+    const C: Self::Ty = 0u8;
+    //~^ ERROR mismatched types
+    //~| NOTE expected associated type, found `u8`
+    //~| NOTE expected associated type `<Self as AssocConst>::Ty`
+    //~| NOTE consider constraining
+    //~| NOTE for more information, visit
+}
+
+// An impl can, however
+impl AssocConst for () {
+    const C: Self::Ty = 0u8;
+}
+
+fn main() {}

src/test/ui/associated-types/defaults-in-other-trait-items.rs

@@ -0,0 +1,25 @@
+error[E0308]: mismatched types
+  --> $DIR/defaults-in-other-trait-items.rs:9:13
+   |
+LL |         let () = p;
+   |             ^^ expected associated type, found `()`
+   |
+   = note: expected associated type `<Self as Tr>::A`
+                    found unit type `()`
+   = note: consider constraining the associated type `<Self as Tr>::A` to `()` or calling a method that returns `<Self as Tr>::A`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+
+error[E0308]: mismatched types
+  --> $DIR/defaults-in-other-trait-items.rs:37:25
+   |
+LL |     const C: Self::Ty = 0u8;
+   |                         ^^^ expected associated type, found `u8`
+   |
+   = note: expected associated type `<Self as AssocConst>::Ty`
+                         found type `u8`
+   = note: consider constraining the associated type `<Self as AssocConst>::Ty` to `u8` or calling a method that returns `<Self as AssocConst>::Ty`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/associated-types/defaults-in-other-trait-items.stderr

@@ -23,8 +23,7 @@ mod priv_trait {
         <Pub as PrivTr>::CONST;
         //~^ ERROR associated constant `PrivTr::CONST` is private
         let _: <Pub as PrivTr>::AssocTy;
-        //~^ ERROR trait `priv_trait::PrivTr` is private
-        //~| ERROR trait `priv_trait::PrivTr` is private
+        //~^ ERROR associated type `PrivTr::AssocTy` is private
         pub type InSignatureTy = <Pub as PrivTr>::AssocTy;
         //~^ ERROR trait `priv_trait::PrivTr` is private
         pub trait InSignatureTr: PrivTr {}
@@ -116,15 +115,11 @@ mod priv_parent_substs {
         <Priv as PubTr<_>>::CONST;
         //~^ ERROR type `priv_parent_substs::Priv` is private
 
-        let _: <Pub as PubTr>::AssocTy;
-        //~^ ERROR type `priv_parent_substs::Priv` is private
-        //~| ERROR type `priv_parent_substs::Priv` is private
+        let _: <Pub as PubTr>::AssocTy;  // FIXME no longer an error?!
         let _: <Pub as PubTr<_>>::AssocTy;
         //~^ ERROR type `priv_parent_substs::Priv` is private
-        //~| ERROR type `priv_parent_substs::Priv` is private
         let _: <Priv as PubTr<_>>::AssocTy;
         //~^ ERROR type `priv_parent_substs::Priv` is private
-        //~| ERROR type `priv_parent_substs::Priv` is private
 
         pub type InSignatureTy1 = <Pub as PubTr>::AssocTy;
         //~^ ERROR type `priv_parent_substs::Priv` is private