Fix cross-crate associated constant evaluation

src/librustc/middle/cstore.rs

@@ -181,6 +181,7 @@ impl<'a> InlinedItemRef<'a> {
     pub fn from_trait_item(def_id: DefId, item: &'a hir::TraitItem, _map: &hir_map::Map) -> InlinedItemRef<'a> {
         let (body, kind) = match item.node {
             hir::ConstTraitItem(ref ty, Some(ref body)) => (&**body, InlinedItemKindRef::Const(ty)),
+            hir::ConstTraitItem(_, None) => bug!("InlinedItemRef::from_trait_item called for const without body"),
             _ => bug!("InlinedItemRef::from_trait_item wrong kind")
         };
         InlinedItemRef {

src/librustc_const_eval/eval.rs

@@ -102,14 +102,15 @@ pub fn lookup_const_by_id<'a, 'tcx: 'a>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                 _ => None
             },
             Some(ast_map::NodeTraitItem(ti)) => match ti.node {
-                hir::ConstTraitItem(..) => {
+                hir::ConstTraitItem(ref ty, ref expr_option) => {
                     if let Some(substs) = substs {
                         // If we have a trait item and the substitutions for it,
                         // `resolve_trait_associated_const` will select an impl
                         // or the default.
                         let trait_id = tcx.map.get_parent(node_id);
                         let trait_id = tcx.map.local_def_id(trait_id);
-                        resolve_trait_associated_const(tcx, ti, trait_id, substs)
+                        let default_value = expr_option.as_ref().map(|expr| (&**expr, tcx.ast_ty_to_prim_ty(ty)));
+                        resolve_trait_associated_const(tcx, def_id, default_value, trait_id, substs)
                     } else {
                         // Technically, without knowing anything about the
                         // expression that generates the obligation, we could
@@ -145,6 +146,27 @@ pub fn lookup_const_by_id<'a, 'tcx: 'a>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             }
             _ => None
         };
+        let expr_ty = match tcx.sess.cstore.describe_def(def_id) {
+            Some(Def::AssociatedConst(_)) => {
+                let trait_id = tcx.sess.cstore.trait_of_item(def_id);
+                // As mentioned in the comments above for in-crate
+                // constants, we only try to find the expression for a
+                // trait-associated const if the caller gives us the
+                // substitutions for the reference to it.
+                if let Some(trait_id) = trait_id {
+                    used_substs = true;
+
+                    if let Some(substs) = substs {
+                        resolve_trait_associated_const(tcx, def_id, expr_ty, trait_id, substs)
+                    } else {
+                        None
+                    }
+                } else {
+                    expr_ty
+                }
+            }
+            _ => expr_ty
+        };
         // If we used the substitutions, particularly to choose an impl
         // of a trait-associated const, don't cache that, because the next
         // lookup with the same def_id may yield a different result.
@@ -1036,7 +1058,8 @@ fn infer<'a, 'tcx>(i: ConstInt,
 }
 
 fn resolve_trait_associated_const<'a, 'tcx: 'a>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
-                                                ti: &'tcx hir::TraitItem,
+                                                trait_item_id: DefId,
+                                                default_value: Option<(&'tcx Expr, Option<ty::Ty<'tcx>>)>,
                                                 trait_id: DefId,
                                                 rcvr_substs: &'tcx Substs<'tcx>)
                                                 -> Option<(&'tcx Expr, Option<ty::Ty<'tcx>>)>
@@ -1070,21 +1093,16 @@ fn resolve_trait_associated_const<'a, 'tcx: 'a>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         // when constructing the inference context above.
         match selection {
             traits::VtableImpl(ref impl_data) => {
+                let name = tcx.associated_item(trait_item_id).name;
                 let ac = tcx.associated_items(impl_data.impl_def_id)
-                    .find(|item| item.kind == ty::AssociatedKind::Const && item.name == ti.name);
+                    .find(|item| item.kind == ty::AssociatedKind::Const && item.name == name);
                 match ac {
                     Some(ic) => lookup_const_by_id(tcx, ic.def_id, None),
-                    None => match ti.node {
-                        hir::ConstTraitItem(ref ty, Some(ref expr)) => {
-                            Some((&*expr, tcx.ast_ty_to_prim_ty(ty)))
-                        },
-                        _ => None,
-                    },
+                    None => default_value,
                 }
             }
             _ => {
-            span_bug!(ti.span,
-                      "resolve_trait_associated_const: unexpected vtable type")
+                bug!("resolve_trait_associated_const: unexpected vtable type")
             }
         }
     })

src/librustc_metadata/encoder.rs

@@ -516,7 +516,9 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             generics: Some(self.encode_generics(def_id)),
             predicates: Some(self.encode_predicates(def_id)),
 
-            ast: if trait_item.kind == ty::AssociatedKind::Const {
+            ast: if let hir::ConstTraitItem(_, Some(_)) = ast_item.node {
+                // We only save the HIR for associated consts with bodies
+                // (InlinedItemRef::from_trait_item panics otherwise)
                 let trait_def_id = trait_item.container.id();
                 Some(self.encode_inlined_item(InlinedItemRef::from_trait_item(trait_def_id, ast_item, &tcx.map)))
             } else {

src/test/run-pass/associated-const-const-eval.rs

@@ -0,0 +1,30 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(associated_consts)]
+
+trait Foo {
+    const NUM: usize;
+}
+
+impl Foo for i32 {
+    const NUM: usize = 1;
+}
+
+const FOO: usize = <i32 as Foo>::NUM;
+
+fn main() {
+    assert_eq!(1, FOO);
+
+    match 1 {
+        <i32 as Foo>::NUM => {},
+        _ => assert!(false)
+    }
+}

src/test/run-pass/associated-const-cross-crate-const-eval.rs

@@ -0,0 +1,38 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// aux-build:associated-const-cc-lib.rs
+
+#![feature(associated_consts)]
+
+extern crate associated_const_cc_lib as foolib;
+
+pub struct LocalFoo;
+
+impl foolib::Foo for LocalFoo {
+    const BAR: usize = 1;
+}
+
+const FOO_1: usize = <foolib::FooNoDefault as foolib::Foo>::BAR;
+const FOO_2: usize = <LocalFoo as foolib::Foo>::BAR;
+const FOO_3: usize = foolib::InherentBar::BAR;
+
+fn main() {
+    assert_eq!(0, FOO_1);
+    assert_eq!(1, FOO_2);
+    assert_eq!(3, FOO_3);
+
+    match 0 {
+        <foolib::FooNoDefault as foolib::Foo>::BAR => {},
+        <LocalFoo as foolib::Foo>::BAR => assert!(false),
+        foolib::InherentBar::BAR => assert!(false),
+        _ => assert!(false)
+    }
+}