Make various fixes:

- add feature gate
- add basic tests
- adjust parser to eliminate conflict between `const fn` and associated
constants
- allow `const fn` in traits/trait-impls, but forbid later in type check
- correct some merge conflicts

src/librustc/diagnostics.rs

@@ -845,5 +845,6 @@ register_diagnostics! {
     E0314, // closure outlives stack frame
     E0315, // cannot invoke closure outside of its lifetime
     E0316, // nested quantification of lifetimes
-    E0370  // discriminant overflow
+    E0370, // discriminant overflow
+    E0378  // method calls limited to constant inherent methods
 }

src/librustc/metadata/encoder.rs

@@ -879,12 +879,11 @@ fn encode_info_for_method<'a, 'tcx>(ecx: &EncodeContext<'a, 'tcx>,
             let any_types = !scheme.generics.types.is_empty();
             let needs_inline = any_types || is_default_impl ||
                                attr::requests_inline(&impl_item.attrs);
-            let constness = ast_method.pe_constness();
-            if needs_inline || constness == ast::Constness::Const {
+            if needs_inline || sig.constness == ast::Constness::Const {
                 encode_inlined_item(ecx, rbml_w, IIImplItemRef(local_def(parent_id),
                                                                impl_item));
             }
-            encode_constness(rbml_w, constness);
+            encode_constness(rbml_w, sig.constness);
             if !any_types {
                 encode_symbol(ecx, rbml_w, m.def_id.node);
             }

src/librustc/middle/check_const.rs

@@ -36,7 +36,6 @@ use util::nodemap::NodeMap;
 use util::ppaux::Repr;
 
 use syntax::ast;
-use syntax::ast_util::PostExpansionMethod;
 use syntax::codemap::Span;
 use syntax::visit::{self, Visitor};
 
@@ -149,16 +148,16 @@ impl<'a, 'tcx> CheckCrateVisitor<'a, 'tcx> {
             Entry::Occupied(entry) => return *entry.get(),
             Entry::Vacant(entry) => {
                 // Prevent infinite recursion on re-entry.
-                entry.insert(PURE_CONST);
+                entry.insert(ConstQualif::empty());
             }
         }
 
         let mode = match fk {
-            visit::FkItemFn(_, _, _, ast::Constness::Const, _) => {
+            visit::FkItemFn(_, _, _, ast::Constness::Const, _, _) => {
                 Mode::ConstFn
             }
-            visit::FkMethod(_, _, m) => {
-                if m.pe_constness() == ast::Constness::Const {
+            visit::FkMethod(_, m, _) => {
+                if m.constness == ast::Constness::Const {
                     Mode::ConstFn
                 } else {
                     Mode::Var
@@ -189,7 +188,7 @@ impl<'a, 'tcx> CheckCrateVisitor<'a, 'tcx> {
 
         // Keep only bits that aren't affected by function body (NON_ZERO_SIZED),
         // and bits that don't change semantics, just optimizations (PREFER_IN_PLACE).
-        let qualif = qualif & (NON_ZERO_SIZED | PREFER_IN_PLACE);
+        let qualif = qualif & (ConstQualif::NON_ZERO_SIZED | ConstQualif::PREFER_IN_PLACE);
 
         self.tcx.const_qualif_map.borrow_mut().insert(fn_id, qualif);
         qualif
@@ -210,7 +209,7 @@ impl<'a, 'tcx> CheckCrateVisitor<'a, 'tcx> {
             self.add_qualif(qualif);
 
             if ty::type_contents(self.tcx, ret_ty).interior_unsafe() {
-                self.add_qualif(MUTABLE_MEM);
+                self.add_qualif(ConstQualif::MUTABLE_MEM);
             }
 
             true
@@ -366,7 +365,7 @@ impl<'a, 'tcx, 'v> Visitor<'v> for CheckCrateVisitor<'a, 'tcx> {
                                                        macro in const?!")
                 }
             };
-            self.add_qualif(NOT_CONST);
+            self.add_qualif(ConstQualif::NOT_CONST);
             if self.mode != Mode::Var {
                 span_err!(self.tcx.sess, span, E0016,
                           "blocks in {}s are limited to items and \
@@ -602,7 +601,7 @@ fn check_expr<'a, 'tcx>(v: &mut CheckCrateVisitor<'a, 'tcx>,
                 }
                 Some(def::DefLocal(_)) if v.mode == Mode::ConstFn => {
                     // Sadly, we can't determine whether the types are zero-sized.
-                    v.add_qualif(NOT_CONST | NON_ZERO_SIZED);
+                    v.add_qualif(ConstQualif::NOT_CONST | ConstQualif::NON_ZERO_SIZED);
                 }
                 def => {
                     v.add_qualif(ConstQualif::NOT_CONST);
@@ -651,20 +650,8 @@ fn check_expr<'a, 'tcx>(v: &mut CheckCrateVisitor<'a, 'tcx>,
                 }
             }
         }
-        ast::ExprBlock(ref block) => {
-            // Check all statements in the block
-            let mut block_span_err = |span| {
-                v.add_qualif(ConstQualif::NOT_CONST);
-                if v.mode != Mode::Var {
-                    span_err!(v.tcx.sess, e.span, E0015,
-                              "function calls in {}s are limited to \
-                               constant functions, \
-                               struct and enum constructors", v.msg());
-                }
-            }
-        }
         ast::ExprMethodCall(..) => {
-            let method_did = match v.tcx.method_map.borrow()[method_call].origin {
+            let method_did = match v.tcx.method_map.borrow()[&method_call].origin {
                 ty::MethodStatic(did) => Some(did),
                 _ => None
             };
@@ -673,9 +660,9 @@ fn check_expr<'a, 'tcx>(v: &mut CheckCrateVisitor<'a, 'tcx>,
                 None => false
             };
             if !is_const {
-                v.add_qualif(NOT_CONST);
+                v.add_qualif(ConstQualif::NOT_CONST);
                 if v.mode != Mode::Var {
-                    span_err!(v.tcx.sess, e.span, E0021,
+                    span_err!(v.tcx.sess, e.span, E0378,
                               "method calls in {}s are limited to \
                                constant inherent methods", v.msg());
                 }

src/librustc/middle/const_eval.rs

@@ -25,7 +25,7 @@ use util::ppaux::Repr;
 
 use syntax::ast::{self, Expr};
 use syntax::ast_map::blocks::FnLikeNode;
-use syntax::ast_util::{self, PostExpansionMethod};
+use syntax::ast_util;
 use syntax::codemap::Span;
 use syntax::feature_gate;
 use syntax::parse::token::InternedString;
@@ -216,17 +216,17 @@ fn inline_const_fn_from_external_crate(tcx: &ty::ctxt, def_id: ast::DefId)
     let fn_id = match csearch::maybe_get_item_ast(tcx, def_id,
         box |a, b, c, d| astencode::decode_inlined_item(a, b, c, d)) {
         csearch::FoundAst::Found(&ast::IIItem(ref item)) => Some(item.id),
-        csearch::FoundAst::Found(&ast::IIImplItem(_, ast::MethodImplItem(ref m))) => Some(m.id),
+        csearch::FoundAst::Found(&ast::IIImplItem(_, ref item)) => Some(item.id),
         _ => None
     };
     tcx.extern_const_fns.borrow_mut().insert(def_id,
                                              fn_id.unwrap_or(ast::DUMMY_NODE_ID));
     fn_id
 }
 
-pub fn lookup_const_fn_by_id<'a>(tcx: &'a ty::ctxt, def_id: ast::DefId)
-                                 -> Option<FnLikeNode<'a>> {
-
+pub fn lookup_const_fn_by_id<'tcx>(tcx: &ty::ctxt<'tcx>, def_id: ast::DefId)
+                                   -> Option<FnLikeNode<'tcx>>
+{
     let fn_id = if !ast_util::is_local(def_id) {
         if let Some(fn_id) = inline_const_fn_from_external_crate(tcx, def_id) {
             fn_id
@@ -243,11 +243,11 @@ pub fn lookup_const_fn_by_id<'a>(tcx: &'a ty::ctxt, def_id: ast::DefId)
     };
 
     match fn_like.kind() {
-        visit::FkItemFn(_, _, _, ast::Constness::Const, _) => {
+        visit::FkItemFn(_, _, _, ast::Constness::Const, _, _) => {
             Some(fn_like)
         }
-        visit::FkMethod(_, _, m) => {
-            if m.pe_constness() == ast::Constness::Const {
+        visit::FkMethod(_, m, _) => {
+            if m.constness == ast::Constness::Const {
                 Some(fn_like)
             } else {
                 None

src/librustc/middle/effect.rs

@@ -87,7 +87,7 @@ impl<'a, 'tcx, 'v> Visitor<'v> for EffectCheckVisitor<'a, 'tcx> {
                 block: &'v ast::Block, span: Span, _: ast::NodeId) {
 
         let (is_item_fn, is_unsafe_fn) = match fn_kind {
-            visit::FkItemFn(_, _, unsafety, _, _) =>
+            visit::FkItemFn(_, _, unsafety, _, _, _) =>
                 (true, unsafety == ast::Unsafety::Unsafe),
             visit::FkMethod(_, sig, _) =>
                 (true, sig.unsafety == ast::Unsafety::Unsafe),

src/librustc/middle/resolve_lifetime.rs

@@ -447,7 +447,7 @@ impl<'a> LifetimeContext<'a> {
                    fb: &'b ast::Block,
                    _span: Span) {
         match fk {
-            visit::FkItemFn(_, generics, _, _, _) => {
+            visit::FkItemFn(_, generics, _, _, _, _) => {
                 visit::walk_fn_decl(self, fd);
                 self.visit_generics(generics);
             }

src/librustc/middle/stability.rs

@@ -23,7 +23,7 @@ use syntax::{attr, visit};
 use syntax::ast;
 use syntax::ast::{Attribute, Block, Crate, DefId, FnDecl, NodeId, Variant};
 use syntax::ast::{Item, Generics, StructField};
-use syntax::ast_util::{is_local, PostExpansionMethod};
+use syntax::ast_util::is_local;
 use syntax::attr::{Stability, AttrMetaMethods};
 use syntax::visit::{FnKind, Visitor};
 use syntax::feature_gate::emit_feature_err;

src/librustc_lint/builtin.rs

@@ -1324,7 +1324,7 @@ impl LintPass for UnsafeCode {
     fn check_fn(&mut self, cx: &Context, fk: visit::FnKind, _: &ast::FnDecl,
                 _: &ast::Block, span: Span, _: ast::NodeId) {
         match fk {
-            visit::FkItemFn(_, _, ast::Unsafety::Unsafe, _, _) =>
+            visit::FkItemFn(_, _, ast::Unsafety::Unsafe, _, _, _) =>
                 cx.span_lint(UNSAFE_CODE, span, "declaration of an `unsafe` function"),
 
             visit::FkMethod(_, sig, _) => {

src/librustc_resolve/lib.rs

@@ -245,7 +245,7 @@ impl<'a, 'v, 'tcx> Visitor<'v> for Resolver<'a, 'tcx> {
                 _: Span,
                 node_id: NodeId) {
         let rib_kind = match function_kind {
-            visit::FkItemFn(_, generics, _, _, _) => {
+            visit::FkItemFn(_, generics, _, _, _, _) => {
                 self.visit_generics(generics);
                 ItemRibKind
             }

src/librustc_trans/save/dump_csv.rs

@@ -1167,7 +1167,7 @@ impl<'l, 'tcx, 'v> Visitor<'v> for DumpCsvVisitor<'l, 'tcx> {
                                           &location[..],
                                           self.cur_scope);
             }
-            ast::ItemFn(ref decl, _, _, ref ty_params, ref body) =>
+            ast::ItemFn(ref decl, _, _, _, ref ty_params, ref body) =>
                 self.process_fn(item, &**decl, ty_params, &**body),
             ast::ItemStatic(ref typ, _, ref expr) =>
                 self.process_static_or_const_item(item, typ, expr),

src/librustc_trans/trans/consts.rs

@@ -41,7 +41,7 @@ use syntax::{ast, ast_util};
 use syntax::parse::token;
 use syntax::ptr::P;
 
-type FnArgMap<'a> = Option<&'a NodeMap<ValueRef>>;
+pub type FnArgMap<'a> = Option<&'a NodeMap<ValueRef>>;
 
 pub fn const_lit(cx: &CrateContext, e: &ast::Expr, lit: &ast::Lit)
     -> ValueRef {
@@ -863,7 +863,7 @@ fn const_expr_unadjusted<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>,
                       _ => break
                   };
               }
-              let def = cx.tcx().def_map.borrow()[callee.id].full_def();
+              let def = cx.tcx().def_map.borrow()[&callee.id].full_def();
               let arg_vals = map_list(args);
               match def {
                   def::DefFn(did, _) | def::DefMethod(did, _) => {
@@ -893,7 +893,7 @@ fn const_expr_unadjusted<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>,
           ast::ExprMethodCall(_, _, ref args) => {
               let arg_vals = map_list(args);
               let method_call = ty::MethodCall::expr(e.id);
-              let method_did = match cx.tcx().method_map.borrow()[method_call].origin {
+              let method_did = match cx.tcx().method_map.borrow()[&method_call].origin {
                   ty::MethodStatic(did) => did,
                   _ => cx.sess().span_bug(e.span, "expected a const method def")
               };

src/librustc_trans/trans/debuginfo/mod.rs

@@ -232,7 +232,7 @@ pub fn create_function_debug_context<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>,
             }
 
             match item.node {
-                ast::ItemFn(ref fn_decl, _, _, ref generics, ref top_level_block) => {
+                ast::ItemFn(ref fn_decl, _, _, _, ref generics, ref top_level_block) => {
                     (item.ident.name, fn_decl, generics, top_level_block, item.span, true)
                 }
                 _ => {

src/librustc_typeck/check/mod.rs

@@ -830,11 +830,15 @@ pub fn check_item_body<'a,'tcx>(ccx: &CrateCtxt<'a,'tcx>, it: &'tcx ast::Item) {
                     check_const(ccx, trait_item.span, &*expr, trait_item.id)
                 }
                 ast::MethodTraitItem(ref sig, Some(ref body)) => {
+                    check_trait_fn_not_const(ccx, trait_item.span, sig.constness);
+
                     check_method_body(ccx, &trait_def.generics, sig, body,
                                       trait_item.id, trait_item.span);
                 }
+                ast::MethodTraitItem(ref sig, None) => {
+                    check_trait_fn_not_const(ccx, trait_item.span, sig.constness);
+                }
                 ast::ConstTraitItem(_, None) |
-                ast::MethodTraitItem(_, None) |
                 ast::TypeTraitItem(..) => {
                     // Nothing to do.
                 }
@@ -845,6 +849,20 @@ pub fn check_item_body<'a,'tcx>(ccx: &CrateCtxt<'a,'tcx>, it: &'tcx ast::Item) {
     }
 }
 
+fn check_trait_fn_not_const<'a,'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
+                                     span: Span,
+                                     constness: ast::Constness)
+{
+    match constness {
+        ast::Constness::NotConst => {
+            // good
+        }
+        ast::Constness::Const => {
+            span_err!(ccx.tcx.sess, span, E0379, "trait fns cannot be declared const");
+        }
+    }
+}
+
 fn check_trait_on_unimplemented<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
                                generics: &ast::Generics,
                                item: &ast::Item) {
@@ -966,7 +984,9 @@ fn check_impl_items_against_trait<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
                     }
                 }
             }
-            ast::MethodImplItem(_, ref body) => {
+            ast::MethodImplItem(ref sig, ref body) => {
+                check_trait_fn_not_const(ccx, impl_item.span, sig.constness);
+
                 let impl_method_def_id = local_def(impl_item.id);
                 let impl_item_ty = ty::impl_or_trait_item(ccx.tcx,
                                                           impl_method_def_id);

src/librustc_typeck/diagnostics.rs

@@ -1050,6 +1050,7 @@ register_diagnostics! {
            // fields need coercions
     E0376, // the trait `CoerceUnsized` may only be implemented for a coercion
            // between structures
-    E0377  // the trait `CoerceUnsized` may only be implemented for a coercion
+    E0377, // the trait `CoerceUnsized` may only be implemented for a coercion
            // between structures with the same definition
+    E0379  // trait fns cannot be const
 }

src/librustdoc/clean/mod.rs

@@ -1352,15 +1352,18 @@ impl<'tcx> Clean<Item> for ty::Method<'tcx> {
                 generics: generics,
                 self_: self_,
                 decl: decl,
-                abi: self.fty.abi
+                abi: self.fty.abi,
+
+                // trait methods canot (currently, at least) be const
+                constness: ast::Constness::NotConst,
             })
         } else {
             TyMethodItem(TyMethod {
                 unsafety: self.fty.unsafety,
                 generics: generics,
                 self_: self_,
                 decl: decl,
-                abi: self.fty.abi
+                abi: self.fty.abi,
             })
         };
 

src/librustdoc/html/format.rs

@@ -37,7 +37,7 @@ pub struct VisSpace(pub Option<ast::Visibility>);
 pub struct UnsafetySpace(pub ast::Unsafety);
 /// Similarly to VisSpace, this structure is used to render a function constness
 /// with a space after it.
-#[derive(Copy)]
+#[derive(Copy, Clone)]
 pub struct ConstnessSpace(pub ast::Constness);
 /// Wrapper struct for properly emitting a method declaration.
 pub struct Method<'a>(pub &'a clean::SelfTy, pub &'a clean::FnDecl);

src/librustdoc/visit_ast.rs

@@ -125,7 +125,7 @@ impl<'a, 'tcx> RustdocVisitor<'a, 'tcx> {
                     name: ast::Ident, fd: &ast::FnDecl,
                     unsafety: &ast::Unsafety,
                     constness: ast::Constness,
-                    _abi: &abi::Abi,
+                    abi: &abi::Abi,
                     gen: &ast::Generics) -> Function {
         debug!("Visiting fn");
         Function {
@@ -294,7 +294,7 @@ impl<'a, 'tcx> RustdocVisitor<'a, 'tcx> {
                 om.enums.push(self.visit_enum_def(item, name, ed, gen)),
             ast::ItemStruct(ref sd, ref gen) =>
                 om.structs.push(self.visit_struct_def(item, name, &**sd, gen)),
-            ast::ItemFn(ref fd, unsafety, constness, ref abi, ref gen, _) =>
+            ast::ItemFn(ref fd, ref unsafety, constness, ref abi, ref gen, _) =>
                 om.fns.push(self.visit_fn(item, name, &**fd, unsafety,
                                           constness, abi, gen)),
             ast::ItemTy(ref ty, ref gen) => {

src/libsyntax/ast.rs

@@ -1215,6 +1215,7 @@ pub struct TypeField {
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub struct MethodSig {
     pub unsafety: Unsafety,
+    pub constness: Constness,
     pub abi: Abi,
     pub decl: P<FnDecl>,
     pub generics: Generics,
@@ -1549,7 +1550,6 @@ pub enum ExplicitSelf_ {
 pub type ExplicitSelf = Spanned<ExplicitSelf_>;
 
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
-             Constness,
 pub struct Mod {
     /// A span from the first token past `{` to the last token until `}`.
     /// For `mod foo;`, the inner span ranges from the first token