Auto merge of #23606 - quantheory:associated_const, r=nikomatsakis

Closes #17841.

The majority of the work should be done, e.g. trait and inherent impls, different forms of UFCS syntax, defaults, and cross-crate usage. It's probably enough to replace the constants in `f32`, `i8`, and so on, or close to good enough.

There is still some significant functionality missing from this commit:

 - ~~Associated consts can't be used in match patterns at all. This is simply because I haven't updated the relevant bits in the parser or `resolve`, but it's *probably* not hard to get working.~~
 - Since you can't select an impl for trait-associated consts until partway through type-checking, there are some problems with code that assumes that you can check constants earlier. Associated consts that are not in inherent impls cause ICEs if you try to use them in array sizes or match ranges. For similar reasons, `check_static_recursion` doesn't check them properly, so the stack goes ka-blooey if you use an associated constant that's recursively defined. That's a bit trickier to solve; I'm not entirely sure what the best approach is yet.
 - Dealing with consts associated with type parameters will raise some new issues (e.g. if you have a `T: Int` type parameter and want to use `<T>::ZERO`). See rust-lang/rfcs#865.
 - ~~Unused associated consts don't seem to trigger the `dead_code` lint when they should. Probably easy to fix.~~

Also, this is the first time I've been spelunking in rustc to such a large extent, so I've probably done some silly things in a couple of places.

Author: bors

src/doc/reference.md

@@ -2135,7 +2135,10 @@ The currently implemented features of the reference compiler are:
           semantics are likely to change, so this macro usage must be opted
           into.
 
-* `associated_types` - Allows type aliases in traits. Experimental.
+* `associated_consts` - Allows constants to be defined in `impl` and `trait`
+                        blocks, so that they can be associated with a type or
+                        trait in a similar manner to methods and associated
+                        types.
 
 * `box_patterns` - Allows `box` patterns, the exact semantics of which
                    is subject to change.

src/grammar/parser-lalr.y

@@ -505,10 +505,20 @@ trait_items
 ;
 
 trait_item
-: trait_type
+: trait_const
+| trait_type
 | trait_method
 ;
 
+trait_const
+: maybe_outer_attrs CONST ident maybe_const_default ';' { $$ = mk_node("ConstTraitItem", 3, $1, $3, $4); }
+;
+
+maybe_const_default
+: '=' expr { $$ = mk_node("ConstDefault", 1, $2); }
+| %empty   { $$ = mk_none(); }
+;
+
 trait_type
 : maybe_outer_attrs TYPE ty_param ';' { $$ = mk_node("TypeTraitItem", 2, $1, $3); }
 ;
@@ -611,7 +621,16 @@ impl_items
 impl_item
 : impl_method
 | item_macro
-| trait_type
+| impl_const
+| impl_type
+;
+
+impl_const
+: attrs_and_vis item_const { $$ = mk_node("ImplConst", 1, $1, $2); }
+;
+
+impl_type
+: attrs_and_vis TYPE ident generic_params '=' ty_sum ';'  { $$ = mk_node("ImplType", 4, $1, $3, $4, $6); }
 ;
 
 item_fn

src/librustc/metadata/csearch.rs

@@ -175,6 +175,13 @@ pub fn get_provided_trait_methods<'tcx>(tcx: &ty::ctxt<'tcx>,
     decoder::get_provided_trait_methods(cstore.intr.clone(), &*cdata, def.node, tcx)
 }
 
+pub fn get_associated_consts<'tcx>(tcx: &ty::ctxt<'tcx>, def: ast::DefId)
+                                   -> Vec<Rc<ty::AssociatedConst<'tcx>>> {
+    let cstore = &tcx.sess.cstore;
+    let cdata = cstore.get_crate_data(def.krate);
+    decoder::get_associated_consts(cstore.intr.clone(), &*cdata, def.node, tcx)
+}
+
 pub fn get_type_name_if_impl(cstore: &cstore::CStore, def: ast::DefId)
                           -> Option<ast::Name> {
     let cdata = cstore.get_crate_data(def.krate);

src/librustc/metadata/decoder.rs

@@ -305,7 +305,25 @@ fn item_to_def_like(item: rbml::Doc, did: ast::DefId, cnum: ast::CrateNum)
     -> DefLike {
     let fam = item_family(item);
     match fam {
-        Constant  => DlDef(def::DefConst(did)),
+        Constant  => {
+            // Check whether we have an associated const item.
+            if item_sort(item) == Some('C') {
+                // Check whether the associated const is from a trait or impl.
+                // See the comment for methods below.
+                let provenance = if reader::maybe_get_doc(
+                      item, tag_item_trait_parent_sort).is_some() {
+                    def::FromTrait(item_reqd_and_translated_parent_item(cnum,
+                                                                        item))
+                } else {
+                    def::FromImpl(item_reqd_and_translated_parent_item(cnum,
+                                                                       item))
+                };
+                DlDef(def::DefAssociatedConst(did, provenance))
+            } else {
+                // Regular const item.
+                DlDef(def::DefConst(did))
+            }
+        }
         ImmStatic => DlDef(def::DefStatic(did, false)),
         MutStatic => DlDef(def::DefStatic(did, true)),
         Struct    => DlDef(def::DefStruct(did)),
@@ -826,6 +844,7 @@ pub fn get_impl_items(cdata: Cmd, impl_id: ast::NodeId)
                         tag_item_impl_item, |doc| {
         let def_id = item_def_id(doc, cdata);
         match item_sort(doc) {
+            Some('C') => impl_items.push(ty::ConstTraitItemId(def_id)),
             Some('r') | Some('p') => {
                 impl_items.push(ty::MethodTraitItemId(def_id))
             }
@@ -877,6 +896,18 @@ pub fn get_impl_or_trait_item<'tcx>(intr: Rc<IdentInterner>,
     let vis = item_visibility(method_doc);
 
     match item_sort(method_doc) {
+        Some('C') => {
+            let ty = doc_type(method_doc, tcx, cdata);
+            let default = get_provided_source(method_doc, cdata);
+            ty::ConstTraitItem(Rc::new(ty::AssociatedConst {
+                name: name,
+                ty: ty,
+                vis: vis,
+                def_id: def_id,
+                container: container,
+                default: default,
+            }))
+        }
         Some('r') | Some('p') => {
             let generics = doc_generics(method_doc, tcx, cdata, tag_method_ty_generics);
             let predicates = doc_predicates(method_doc, tcx, cdata, tag_method_ty_generics);
@@ -914,6 +945,7 @@ pub fn get_trait_item_def_ids(cdata: Cmd, id: ast::NodeId)
     reader::tagged_docs(item, tag_item_trait_item, |mth| {
         let def_id = item_def_id(mth, cdata);
         match item_sort(mth) {
+            Some('C') => result.push(ty::ConstTraitItemId(def_id)),
             Some('r') | Some('p') => {
                 result.push(ty::MethodTraitItemId(def_id));
             }
@@ -951,11 +983,8 @@ pub fn get_provided_trait_methods<'tcx>(intr: Rc<IdentInterner>,
                                                     cdata,
                                                     did.node,
                                                     tcx);
-            match trait_item {
-                ty::MethodTraitItem(ref method) => {
-                    result.push((*method).clone())
-                }
-                ty::TypeTraitItem(_) => {}
+            if let ty::MethodTraitItem(ref method) = trait_item {
+                result.push((*method).clone())
             }
         }
         true
@@ -964,6 +993,36 @@ pub fn get_provided_trait_methods<'tcx>(intr: Rc<IdentInterner>,
     return result;
 }
 
+pub fn get_associated_consts<'tcx>(intr: Rc<IdentInterner>,
+                                   cdata: Cmd,
+                                   id: ast::NodeId,
+                                   tcx: &ty::ctxt<'tcx>)
+                                   -> Vec<Rc<ty::AssociatedConst<'tcx>>> {
+    let data = cdata.data();
+    let item = lookup_item(id, data);
+    let mut result = Vec::new();
+
+    for &tag in &[tag_item_trait_item, tag_item_impl_item] {
+        reader::tagged_docs(item, tag, |ac_id| {
+            let did = item_def_id(ac_id, cdata);
+            let ac_doc = lookup_item(did.node, data);
+
+            if item_sort(ac_doc) == Some('C') {
+                let trait_item = get_impl_or_trait_item(intr.clone(),
+                                                        cdata,
+                                                        did.node,
+                                                        tcx);
+                if let ty::ConstTraitItem(ref ac) = trait_item {
+                    result.push((*ac).clone())
+                }
+            }
+            true
+        });
+    }
+
+    return result;
+}
+
 pub fn get_type_name_if_impl(cdata: Cmd,
                              node_id: ast::NodeId) -> Option<ast::Name> {
     let item = lookup_item(node_id, cdata.data());

src/librustc/metadata/encoder.rs

@@ -378,14 +378,11 @@ fn encode_reexported_static_base_methods(ecx: &EncodeContext,
                     let impl_item = ty::impl_or_trait_item(
                         ecx.tcx,
                         method_did.def_id());
-                    match impl_item {
-                        ty::MethodTraitItem(ref m) => {
-                            encode_reexported_static_method(rbml_w,
-                                                            exp,
-                                                            m.def_id,
-                                                            m.name);
-                        }
-                        ty::TypeTraitItem(_) => {}
+                    if let ty::MethodTraitItem(ref m) = impl_item {
+                        encode_reexported_static_method(rbml_w,
+                                                        exp,
+                                                        m.def_id,
+                                                        m.name);
                     }
                 }
             }
@@ -802,6 +799,43 @@ fn encode_method_ty_fields<'a, 'tcx>(ecx: &EncodeContext<'a, 'tcx>,
     encode_provided_source(rbml_w, method_ty.provided_source);
 }
 
+fn encode_info_for_associated_const(ecx: &EncodeContext,
+                                    rbml_w: &mut Encoder,
+                                    associated_const: &ty::AssociatedConst,
+                                    impl_path: PathElems,
+                                    parent_id: NodeId,
+                                    impl_item_opt: Option<&ast::ImplItem>) {
+    debug!("encode_info_for_associated_const({:?},{:?})",
+           associated_const.def_id,
+           token::get_name(associated_const.name));
+
+    rbml_w.start_tag(tag_items_data_item);
+
+    encode_def_id(rbml_w, associated_const.def_id);
+    encode_name(rbml_w, associated_const.name);
+    encode_visibility(rbml_w, associated_const.vis);
+    encode_family(rbml_w, 'C');
+    encode_provided_source(rbml_w, associated_const.default);
+
+    encode_parent_item(rbml_w, local_def(parent_id));
+    encode_item_sort(rbml_w, 'C');
+
+    encode_bounds_and_type_for_item(rbml_w, ecx, associated_const.def_id.local_id());
+
+    let stab = stability::lookup(ecx.tcx, associated_const.def_id);
+    encode_stability(rbml_w, stab);
+
+    let elem = ast_map::PathName(associated_const.name);
+    encode_path(rbml_w, impl_path.chain(Some(elem).into_iter()));
+
+    if let Some(ii) = impl_item_opt {
+        encode_attributes(rbml_w, &ii.attrs);
+        encode_inlined_item(ecx, rbml_w, IIImplItemRef(local_def(parent_id), ii));
+    }
+
+    rbml_w.end_tag();
+}
+
 fn encode_info_for_method<'a, 'tcx>(ecx: &EncodeContext<'a, 'tcx>,
                                     rbml_w: &mut Encoder,
                                     m: &ty::Method<'tcx>,
@@ -1195,6 +1229,10 @@ fn encode_info_for_item(ecx: &EncodeContext,
         for &item_def_id in items {
             rbml_w.start_tag(tag_item_impl_item);
             match item_def_id {
+                ty::ConstTraitItemId(item_def_id) => {
+                    encode_def_id(rbml_w, item_def_id);
+                    encode_item_sort(rbml_w, 'C');
+                }
                 ty::MethodTraitItemId(item_def_id) => {
                     encode_def_id(rbml_w, item_def_id);
                     encode_item_sort(rbml_w, 'r');
@@ -1232,6 +1270,14 @@ fn encode_info_for_item(ecx: &EncodeContext,
             });
 
             match ty::impl_or_trait_item(tcx, trait_item_def_id.def_id()) {
+                ty::ConstTraitItem(ref associated_const) => {
+                    encode_info_for_associated_const(ecx,
+                                                     rbml_w,
+                                                     &*associated_const,
+                                                     path.clone(),
+                                                     item.id,
+                                                     ast_item)
+                }
                 ty::MethodTraitItem(ref method_type) => {
                     encode_info_for_method(ecx,
                                            rbml_w,
@@ -1276,6 +1322,10 @@ fn encode_info_for_item(ecx: &EncodeContext,
         for &method_def_id in &*ty::trait_item_def_ids(tcx, def_id) {
             rbml_w.start_tag(tag_item_trait_item);
             match method_def_id {
+                ty::ConstTraitItemId(const_def_id) => {
+                    encode_def_id(rbml_w, const_def_id);
+                    encode_item_sort(rbml_w, 'C');
+                }
                 ty::MethodTraitItemId(method_def_id) => {
                     encode_def_id(rbml_w, method_def_id);
                     encode_item_sort(rbml_w, 'r');
@@ -1321,6 +1371,25 @@ fn encode_info_for_item(ecx: &EncodeContext,
                 ty::impl_or_trait_item(tcx, item_def_id.def_id());
             let is_nonstatic_method;
             match trait_item_type {
+                ty::ConstTraitItem(associated_const) => {
+                    encode_name(rbml_w, associated_const.name);
+                    encode_def_id(rbml_w, associated_const.def_id);
+                    encode_visibility(rbml_w, associated_const.vis);
+
+                    encode_provided_source(rbml_w, associated_const.default);
+
+                    let elem = ast_map::PathName(associated_const.name);
+                    encode_path(rbml_w,
+                                path.clone().chain(Some(elem).into_iter()));
+
+                    encode_item_sort(rbml_w, 'C');
+                    encode_family(rbml_w, 'C');
+
+                    encode_bounds_and_type_for_item(rbml_w, ecx,
+                                                    associated_const.def_id.local_id());
+
+                    is_nonstatic_method = false;
+                }
                 ty::MethodTraitItem(method_ty) => {
                     let method_def_id = item_def_id.def_id();
 
@@ -1365,6 +1434,10 @@ fn encode_info_for_item(ecx: &EncodeContext,
             let trait_item = &*ms[i];
             encode_attributes(rbml_w, &trait_item.attrs);
             match trait_item.node {
+                ast::ConstTraitItem(_, _) => {
+                    encode_inlined_item(ecx, rbml_w,
+                                        IITraitItemRef(def_id, trait_item));
+                }
                 ast::MethodTraitItem(ref sig, ref body) => {
                     // If this is a static method, we've already
                     // encoded this.
@@ -1384,9 +1457,7 @@ fn encode_info_for_item(ecx: &EncodeContext,
                     encode_method_argument_names(rbml_w, &sig.decl);
                 }
 
-                ast::TypeTraitItem(..) => {
-                    encode_item_sort(rbml_w, 't');
-                }
+                ast::TypeTraitItem(..) => {}
             }
 
             rbml_w.end_tag();

src/librustc/middle/astencode.rs

@@ -465,6 +465,9 @@ impl tr for def::Def {
           def::DefForeignMod(did) => { def::DefForeignMod(did.tr(dcx)) }
           def::DefStatic(did, m) => { def::DefStatic(did.tr(dcx), m) }
           def::DefConst(did) => { def::DefConst(did.tr(dcx)) }
+          def::DefAssociatedConst(did, p) => {
+              def::DefAssociatedConst(did.tr(dcx), p.map(|did2| did2.tr(dcx)))
+          }
           def::DefLocal(nid) => { def::DefLocal(dcx.tr_id(nid)) }
           def::DefVariant(e_did, v_did, is_s) => {
             def::DefVariant(e_did.tr(dcx), v_did.tr(dcx), is_s)

src/librustc/middle/cfg/construct.rs

@@ -105,6 +105,7 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
         match pat.node {
             ast::PatIdent(_, _, None) |
             ast::PatEnum(_, None) |
+            ast::PatQPath(..) |
             ast::PatLit(..) |
             ast::PatRange(..) |
             ast::PatWild(_) => {

src/librustc/middle/check_const.rs

@@ -223,6 +223,28 @@ impl<'a, 'tcx, 'v> Visitor<'v> for CheckCrateVisitor<'a, 'tcx> {
         }
     }
 
+    fn visit_trait_item(&mut self, t: &'v ast::TraitItem) {
+        match t.node {
+            ast::ConstTraitItem(_, ref default) => {
+                if let Some(ref expr) = *default {
+                    self.global_expr(Mode::Const, &*expr);
+                } else {
+                    visit::walk_trait_item(self, t);
+                }
+            }
+            _ => self.with_mode(Mode::Var, |v| visit::walk_trait_item(v, t)),
+        }
+    }
+
+    fn visit_impl_item(&mut self, i: &'v ast::ImplItem) {
+        match i.node {
+            ast::ConstImplItem(_, ref expr) => {
+                self.global_expr(Mode::Const, &*expr);
+            }
+            _ => self.with_mode(Mode::Var, |v| visit::walk_impl_item(v, i)),
+        }
+    }
+
     fn visit_fn(&mut self,
                 fk: visit::FnKind<'v>,
                 fd: &'v ast::FnDecl,
@@ -468,13 +490,16 @@ fn check_expr<'a, 'tcx>(v: &mut CheckCrateVisitor<'a, 'tcx>,
                         Mode::Var => v.add_qualif(NOT_CONST)
                     }
                 }
-                Some(def::DefConst(did)) => {
-                    if let Some(expr) = const_eval::lookup_const_by_id(v.tcx, did) {
+                Some(def::DefConst(did)) |
+                Some(def::DefAssociatedConst(did, _)) => {
+                    if let Some(expr) = const_eval::lookup_const_by_id(v.tcx, did,
+                                                                       Some(e.id)) {
                         let inner = v.global_expr(Mode::Const, expr);
                         v.add_qualif(inner);
                     } else {
-                        v.tcx.sess.span_bug(e.span, "DefConst doesn't point \
-                                                     to an ItemConst");
+                        v.tcx.sess.span_bug(e.span,
+                                            "DefConst or DefAssociatedConst \
+                                             doesn't point to a constant");
                     }
                 }
                 def => {