Auto merge of #30446 - michaelwu:associated-const-type-params-pt1, r=nikomatsakis

This provides limited support for using associated consts on type parameters. It generally works on things that can be figured out at trans time. This doesn't work for array lengths or match arms. I have another patch to make it work in const expressions.

CC @eddyb @nikomatsakis

Author: bors

src/librustc/middle/check_const.rs

@@ -657,13 +657,10 @@ fn check_expr<'a, 'tcx>(v: &mut CheckCrateVisitor<'a, 'tcx>,
                 Some(def::DefConst(did)) |
                 Some(def::DefAssociatedConst(did)) => {
                     if let Some(expr) = const_eval::lookup_const_by_id(v.tcx, did,
-                                                                       Some(e.id)) {
+                                                                       Some(e.id),
+                                                                       None) {
                         let inner = v.global_expr(Mode::Const, expr);
                         v.add_qualif(inner);
-                    } else {
-                        v.tcx.sess.span_bug(e.span,
-                                            "DefConst or DefAssociatedConst \
-                                             doesn't point to a constant");
                     }
                 }
                 Some(def::DefLocal(..)) if v.mode == Mode::ConstFn => {

src/librustc/middle/check_match.rs

@@ -455,7 +455,8 @@ impl<'a, 'tcx> Folder for StaticInliner<'a, 'tcx> {
                 let def = self.tcx.def_map.borrow().get(&pat.id).map(|d| d.full_def());
                 match def {
                     Some(DefAssociatedConst(did)) |
-                    Some(DefConst(did)) => match lookup_const_by_id(self.tcx, did, Some(pat.id)) {
+                    Some(DefConst(did)) => match lookup_const_by_id(self.tcx, did,
+                                                                    Some(pat.id), None) {
                         Some(const_expr) => {
                             const_expr_to_pat(self.tcx, const_expr, pat.span).map(|new_pat| {
 

src/librustc/middle/const_eval.rs

@@ -18,6 +18,7 @@ use front::map as ast_map;
 use front::map::blocks::FnLikeNode;
 use middle::cstore::{self, CrateStore, InlinedItem};
 use middle::{def, infer, subst, traits};
+use middle::subst::Subst;
 use middle::def_id::DefId;
 use middle::pat_util::def_to_path;
 use middle::ty::{self, Ty};
@@ -48,7 +49,7 @@ fn lookup_const<'a>(tcx: &'a ty::ctxt, e: &Expr) -> Option<&'a Expr> {
     match opt_def {
         Some(def::DefConst(def_id)) |
         Some(def::DefAssociatedConst(def_id)) => {
-            lookup_const_by_id(tcx, def_id, Some(e.id))
+            lookup_const_by_id(tcx, def_id, Some(e.id), None)
         }
         Some(def::DefVariant(enum_def, variant_def, _)) => {
             lookup_variant_by_id(tcx, enum_def, variant_def)
@@ -88,9 +89,17 @@ fn lookup_variant_by_id<'a>(tcx: &'a ty::ctxt,
     }
 }
 
+/// * `def_id` is the id of the constant.
+/// * `maybe_ref_id` is the id of the expr referencing the constant.
+/// * `param_substs` is the monomorphization substitution for the expression.
+///
+/// `maybe_ref_id` and `param_substs` are optional and are used for
+/// finding substitutions in associated constants. This generally
+/// happens in late/trans const evaluation.
 pub fn lookup_const_by_id<'a, 'tcx: 'a>(tcx: &'a ty::ctxt<'tcx>,
                                         def_id: DefId,
-                                        maybe_ref_id: Option<ast::NodeId>)
+                                        maybe_ref_id: Option<ast::NodeId>,
+                                        param_substs: Option<&'tcx subst::Substs<'tcx>>)
                                         -> Option<&'tcx Expr> {
     if let Some(node_id) = tcx.map.as_local_node_id(def_id) {
         match tcx.map.find(node_id) {
@@ -111,8 +120,11 @@ pub fn lookup_const_by_id<'a, 'tcx: 'a>(tcx: &'a ty::ctxt<'tcx>,
                         Some(ref_id) => {
                             let trait_id = tcx.trait_of_item(def_id)
                                               .unwrap();
-                            let substs = tcx.node_id_item_substs(ref_id)
-                                            .substs;
+                            let mut substs = tcx.node_id_item_substs(ref_id)
+                                                .substs;
+                            if let Some(param_substs) = param_substs {
+                                substs = substs.subst(tcx, param_substs);
+                            }
                             resolve_trait_associated_const(tcx, ti, trait_id,
                                                            substs)
                         }
@@ -158,8 +170,11 @@ pub fn lookup_const_by_id<'a, 'tcx: 'a>(tcx: &'a ty::ctxt<'tcx>,
                         // a trait-associated const if the caller gives us
                         // the expression that refers to it.
                         Some(ref_id) => {
-                            let substs = tcx.node_id_item_substs(ref_id)
-                                            .substs;
+                            let mut substs = tcx.node_id_item_substs(ref_id)
+                                                .substs;
+                            if let Some(param_substs) = param_substs {
+                                substs = substs.subst(tcx, param_substs);
+                            }
                             resolve_trait_associated_const(tcx, ti, trait_id,
                                                            substs).map(|e| e.id)
                         }
@@ -1013,7 +1028,7 @@ pub fn eval_const_expr_partial<'tcx>(tcx: &ty::ctxt<'tcx>,
                           _ => (None, None)
                       }
                   } else {
-                      (lookup_const_by_id(tcx, def_id, Some(e.id)), None)
+                      (lookup_const_by_id(tcx, def_id, Some(e.id), None), None)
                   }
               }
               Some(def::DefAssociatedConst(def_id)) => {
@@ -1048,7 +1063,7 @@ pub fn eval_const_expr_partial<'tcx>(tcx: &ty::ctxt<'tcx>,
                           },
                       }
                   } else {
-                      (lookup_const_by_id(tcx, def_id, Some(e.id)), None)
+                      (lookup_const_by_id(tcx, def_id, Some(e.id), None), None)
                   }
               }
               Some(def::DefVariant(enum_def, variant_def, _)) => {
@@ -1260,20 +1275,16 @@ fn resolve_trait_associated_const<'a, 'tcx: 'a>(tcx: &'a ty::ctxt<'tcx>,
         Ok(None) => {
             return None
         }
-        Err(e) => {
-            tcx.sess.span_bug(ti.span,
-                              &format!("Encountered error `{:?}` when trying \
-                                        to select an implementation for \
-                                        constant trait item reference.",
-                                       e))
+        Err(_) => {
+            return None
         }
     };
 
     match selection {
         traits::VtableImpl(ref impl_data) => {
             match tcx.associated_consts(impl_data.impl_def_id)
                      .iter().find(|ic| ic.name == ti.name) {
-                Some(ic) => lookup_const_by_id(tcx, ic.def_id, None),
+                Some(ic) => lookup_const_by_id(tcx, ic.def_id, None, None),
                 None => match ti.node {
                     hir::ConstTraitItem(_, Some(ref expr)) => Some(&*expr),
                     _ => None,

src/librustc_mir/hair/cx/pattern.rs

@@ -85,7 +85,8 @@ impl<'patcx, 'cx, 'tcx> PatCx<'patcx, 'cx, 'tcx> {
                 let def = self.cx.tcx.def_map.borrow().get(&pat.id).unwrap().full_def();
                 match def {
                     def::DefConst(def_id) | def::DefAssociatedConst(def_id) =>
-                        match const_eval::lookup_const_by_id(self.cx.tcx, def_id, Some(pat.id)) {
+                        match const_eval::lookup_const_by_id(self.cx.tcx, def_id,
+                                                             Some(pat.id), None) {
                             Some(const_expr) => {
                                 let pat = const_eval::const_expr_to_pat(self.cx.tcx, const_expr,
                                                                         pat.span);

src/librustc_trans/trans/consts.rs

@@ -217,7 +217,8 @@ fn const_fn_call<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
 
 pub fn get_const_expr<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
                                 def_id: DefId,
-                                ref_expr: &hir::Expr)
+                                ref_expr: &hir::Expr,
+                                param_substs: &'tcx Substs<'tcx>)
                                 -> &'tcx hir::Expr {
     let def_id = inline::maybe_instantiate_inline(ccx, def_id);
 
@@ -226,7 +227,7 @@ pub fn get_const_expr<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
                             "cross crate constant could not be inlined");
     }
 
-    match const_eval::lookup_const_by_id(ccx.tcx(), def_id, Some(ref_expr.id)) {
+    match const_eval::lookup_const_by_id(ccx.tcx(), def_id, Some(ref_expr.id), Some(param_substs)) {
         Some(ref expr) => expr,
         None => {
             ccx.sess().span_bug(ref_expr.span, "constant item not found")
@@ -264,10 +265,12 @@ pub enum TrueConst {
 
 use self::ConstEvalFailure::*;
 
-fn get_const_val(ccx: &CrateContext,
-                 def_id: DefId,
-                 ref_expr: &hir::Expr) -> Result<ValueRef, ConstEvalFailure> {
-    let expr = get_const_expr(ccx, def_id, ref_expr);
+fn get_const_val<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
+                           def_id: DefId,
+                           ref_expr: &hir::Expr,
+                           param_substs: &'tcx Substs<'tcx>)
+                           -> Result<ValueRef, ConstEvalFailure> {
+    let expr = get_const_expr(ccx, def_id, ref_expr, param_substs);
     let empty_substs = ccx.tcx().mk_substs(Substs::trans_empty());
     match get_const_expr_as_global(ccx, expr, check_const::ConstQualif::empty(),
                                    empty_substs, TrueConst::Yes) {
@@ -297,7 +300,7 @@ pub fn get_const_expr_as_global<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
                 if !ccx.tcx().tables.borrow().adjustments.contains_key(&expr.id) {
                     debug!("get_const_expr_as_global ({:?}): found const {:?}",
                            expr.id, def_id);
-                    return get_const_val(ccx, def_id, expr);
+                    return get_const_val(ccx, def_id, expr, param_substs);
                 }
             },
             _ => {},
@@ -888,7 +891,7 @@ fn const_expr_unadjusted<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>,
                     expr::trans_def_fn_unadjusted(cx, e, def, param_substs).val
                 }
                 def::DefConst(def_id) | def::DefAssociatedConst(def_id) => {
-                    const_deref_ptr(cx, try!(get_const_val(cx, def_id, e)))
+                    const_deref_ptr(cx, try!(get_const_val(cx, def_id, e, param_substs)))
                 }
                 def::DefVariant(enum_did, variant_did, _) => {
                     let vinfo = cx.tcx().lookup_adt_def(enum_did).variant_with_id(variant_did);

src/librustc_trans/trans/expr.rs

@@ -165,7 +165,9 @@ pub fn trans_into<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
                 hir::ExprPath(..) => {
                     match bcx.def(expr.id) {
                         def::DefConst(did) => {
-                            let const_expr = consts::get_const_expr(bcx.ccx(), did, expr);
+                            let empty_substs = bcx.tcx().mk_substs(Substs::trans_empty());
+                            let const_expr = consts::get_const_expr(bcx.ccx(), did, expr,
+                                                                    empty_substs);
                             // Temporarily get cleanup scopes out of the way,
                             // as they require sub-expressions to be contained
                             // inside the current AST scope.

src/librustc_trans/trans/mir/did.rs

@@ -50,7 +50,7 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
             },
             ItemKind::Constant => {
                 let did = inline::maybe_instantiate_inline(bcx.ccx(), did);
-                let expr = const_eval::lookup_const_by_id(bcx.tcx(), did, None)
+                let expr = const_eval::lookup_const_by_id(bcx.tcx(), did, None, Some(substs))
                             .expect("def was const, but lookup_const_by_id failed");
                 // FIXME: this is falling back to translating from HIR. This is not easy to fix,
                 // because we would have somehow adapt const_eval to work on MIR rather than HIR.

src/librustc_typeck/check/mod.rs

@@ -3785,35 +3785,8 @@ pub fn resolve_ty_and_def_ufcs<'a, 'b, 'tcx>(fcx: &FnCtxt<'b, 'tcx>,
                                                         def::Def)>
 {
 
-    // Associated constants can't depend on generic types.
-    fn have_disallowed_generic_consts<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
-                                                def: def::Def,
-                                                ty: Ty<'tcx>,
-                                                span: Span,
-                                                node_id: ast::NodeId) -> bool {
-        match def {
-            def::DefAssociatedConst(..) => {
-                if ty.has_param_types() || ty.has_self_ty() {
-                    span_err!(fcx.sess(), span, E0329,
-                              "Associated consts cannot depend \
-                               on type parameters or Self.");
-                    fcx.write_error(node_id);
-                    return true;
-                }
-            }
-            _ => {}
-        }
-        false
-    }
-
     // If fully resolved already, we don't have to do anything.
     if path_res.depth == 0 {
-        if let Some(ty) = opt_self_ty {
-            if have_disallowed_generic_consts(fcx, path_res.full_def(), ty,
-                                              span, node_id) {
-                return None;
-            }
-        }
         Some((opt_self_ty, &path.segments, path_res.base_def))
     } else {
         let mut def = path_res.base_def;
@@ -3829,9 +3802,6 @@ pub fn resolve_ty_and_def_ufcs<'a, 'b, 'tcx>(fcx: &FnCtxt<'b, 'tcx>,
         let item_name = item_segment.identifier.name;
         match method::resolve_ufcs(fcx, span, item_name, ty, node_id) {
             Ok((def, lp)) => {
-                if have_disallowed_generic_consts(fcx, def, ty, span, node_id) {
-                    return None;
-                }
                 // Write back the new resolution.
                 fcx.ccx.tcx.def_map.borrow_mut()
                        .insert(node_id, def::PathResolution {

src/librustdoc/clean/inline.rs

@@ -321,7 +321,7 @@ pub fn build_impl(cx: &DocContext,
                 let did = assoc_const.def_id;
                 let type_scheme = tcx.lookup_item_type(did);
                 let default = if assoc_const.has_value {
-                    Some(const_eval::lookup_const_by_id(tcx, did, None)
+                    Some(const_eval::lookup_const_by_id(tcx, did, None, None)
                          .unwrap().span.to_src(cx))
                 } else {
                     None
@@ -479,7 +479,7 @@ fn build_const(cx: &DocContext, tcx: &ty::ctxt,
     use rustc::middle::const_eval;
     use rustc_front::print::pprust;
 
-    let expr = const_eval::lookup_const_by_id(tcx, did, None).unwrap_or_else(|| {
+    let expr = const_eval::lookup_const_by_id(tcx, did, None, None).unwrap_or_else(|| {
         panic!("expected lookup_const_by_id to succeed for {:?}", did);
     });
     debug!("converting constant expr {:?} to snippet", expr);

src/test/compile-fail/associated-const-array-len.rs

@@ -0,0 +1,21 @@
+// Copyright 2016 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 ID: usize;
+}
+
+const X: [i32; <i32 as Foo>::ID] = [0, 1, 2]; //~ ERROR E0250
+
+fn main() {
+    assert_eq!(1, X);
+}

src/test/compile-fail/associated-const-no-item.rs

@@ -0,0 +1,22 @@
+// Copyright 2016 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 ID: i32;
+}
+
+const X: i32 = <i32>::ID;
+//~^ ERROR no associated item named `ID` found for type `i32`
+
+fn main() {
+    assert_eq!(1, X);
+}

src/test/compile-fail/associated-const-type-parameter-arms.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.
+
+#![feature(associated_consts)]
+
+pub enum EFoo { A, B, C, D }
+
+pub trait Foo {
+    const X: EFoo;
+}
+
+struct Abc;
+impl Foo for Abc {
+    const X: EFoo = EFoo::B;
+}
+
+struct Def;
+impl Foo for Def {
+    const X: EFoo = EFoo::D;
+}
+
+pub fn test<A: Foo, B: Foo>(arg: EFoo) {
+    match arg {
+        A::X => println!("A::X"), //~ error: statics cannot be referenced in patterns [E0158]
+        B::X => println!("B::X"), //~ error: statics cannot be referenced in patterns [E0158]
+        _ => (),
+    }
+}
+
+fn main() {
+}

src/test/compile-fail/associated-const-type-parameter-arrays-2.rs

@@ -0,0 +1,32 @@
+// 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)]
+
+pub trait Foo {
+    const Y: usize;
+}
+
+struct Abc;
+impl Foo for Abc {
+    const Y: usize = 8;
+}
+
+struct Def;
+impl Foo for Def {
+    const Y: usize = 33;
+}
+
+pub fn test<A: Foo, B: Foo>() {
+    let _array = [4; <A as Foo>::Y]; //~ error: expected constant integer
+}
+
+fn main() {
+}