rustc: qualify expressions in check_const for potential promotion.

5 files changed, 454 insertions, 140 deletions

diff --git a/src/librustc/diagnostics.rs b/src/librustc/diagnostics.rs
index efcc3715d3e..133bef30e40 100644
--- a/src/librustc/diagnostics.rs
+++ b/src/librustc/diagnostics.rs
@@ -59,6 +59,7 @@ register_diagnostics! {
     E0010,
     E0011,
     E0012,
+    E0013,
     E0014,
     E0015,
     E0016,
diff --git a/src/librustc/metadata/common.rs b/src/librustc/metadata/common.rs
index 1de0c018d42..4930eddb35a 100644
--- a/src/librustc/metadata/common.rs
+++ b/src/librustc/metadata/common.rs
@@ -143,6 +143,7 @@ pub enum astencode_tag { // Reserves 0x40 -- 0x5f
     tag_table_upvar_capture_map = 0x56,
     tag_table_capture_modes = 0x57,
     tag_table_object_cast_map = 0x58,
+    tag_table_const_qualif = 0x59,
 }
 
 pub const tag_item_trait_item_sort: uint = 0x60;
diff --git a/src/librustc/middle/astencode.rs b/src/librustc/middle/astencode.rs
index edb14f7ed4c..c3302debdfa 100644
--- a/src/librustc/middle/astencode.rs
+++ b/src/librustc/middle/astencode.rs
@@ -23,6 +23,7 @@ use metadata::tydecode;
 use metadata::tydecode::{DefIdSource, NominalType, TypeWithId, TypeParameter};
 use metadata::tydecode::{RegionParameter, ClosureSource};
 use metadata::tyencode;
+use middle::check_const::ConstQualif;
 use middle::mem_categorization::Typer;
 use middle::subst;
 use middle::subst::VecPerParamSpace;
@@ -1306,6 +1307,15 @@ fn encode_side_tables_for_id(ecx: &e::EncodeContext,
             })
         })
     }
+
+    for &qualif in tcx.const_qualif_map.borrow().get(&id).iter() {
+        rbml_w.tag(c::tag_table_const_qualif, |rbml_w| {
+            rbml_w.id(id);
+            rbml_w.tag(c::tag_table_val, |rbml_w| {
+                qualif.encode(rbml_w).unwrap()
+            })
+        })
+    }
 }
 
 trait doc_decoder_helpers {
@@ -1920,6 +1930,10 @@ fn decode_side_tables(dcx: &DecodeContext,
                         dcx.tcx.closure_kinds.borrow_mut().insert(ast_util::local_def(id),
                                                                   closure_kind);
                     }
+                    c::tag_table_const_qualif => {
+                        let qualif: ConstQualif = Decodable::decode(val_dsr).unwrap();
+                        dcx.tcx.const_qualif_map.borrow_mut().insert(id, qualif);
+                    }
                     _ => {
                         dcx.tcx.sess.bug(
                             &format!("unknown tag found in side tables: {:x}",
diff --git a/src/librustc/middle/check_const.rs b/src/librustc/middle/check_const.rs
index 34d1654ec4b..41d425cd2f6 100644
--- a/src/librustc/middle/check_const.rs
+++ b/src/librustc/middle/check_const.rs
@@ -24,14 +24,14 @@
 // - It's not possible to take the address of a static item with unsafe interior. This is enforced
 // by borrowck::gather_loans
 
-use self::Mode::*;
-
+use middle::const_eval;
 use middle::def;
 use middle::expr_use_visitor as euv;
 use middle::infer;
 use middle::mem_categorization as mc;
 use middle::traits;
-use middle::ty;
+use middle::ty::{self, Ty};
+use util::nodemap::NodeMap;
 use util::ppaux;
 
 use syntax::ast;
@@ -39,34 +39,127 @@ use syntax::codemap::Span;
 use syntax::print::pprust;
 use syntax::visit::{self, Visitor};
 
+use std::collections::hash_map::Entry;
+
+// Const qualification, from partial to completely promotable.
+bitflags! {
+    #[derive(RustcEncodable, RustcDecodable)]
+    flags ConstQualif: u8 {
+        // Const rvalue which can be placed behind a reference.
+        const PURE_CONST          = 0b000000,
+        // Inner mutability (can not be placed behind a reference) or behind
+        // &mut in a non-global expression. Can be copied from static memory.
+        const MUTABLE_MEM         = 0b000001,
+        // Constant value with a type that implements Drop. Can be copied
+        // from static memory, similar to MUTABLE_MEM.
+        const NEEDS_DROP          = 0b000010,
+        // Even if the value can be placed in static memory, copying it from
+        // there is more expensive than in-place instantiation, and/or it may
+        // be too large. This applies to [T; N] and everything containing it.
+        // N.B.: references need to clear this flag to not end up on the stack.
+        const PREFER_IN_PLACE     = 0b000100,
+        // May use more than 0 bytes of memory, doesn't impact the constness
+        // directly, but is not allowed to be borrowed mutably in a constant.
+        const NON_ZERO_SIZED      = 0b001000,
+        // Actually borrowed, has to always be in static memory. Does not
+        // propagate, and requires the expression to behave like a 'static
+        // lvalue. The set of expressions with this flag is the minimum
+        // that have to be promoted.
+        const HAS_STATIC_BORROWS  = 0b010000,
+        // Invalid const for miscellaneous reasons (e.g. not implemented).
+        const NOT_CONST           = 0b100000,
+
+        // Borrowing the expression won't produce &'static T if any of these
+        // bits are set, though the value could be copied from static memory
+        // if `NOT_CONST` isn't set.
+        const NON_STATIC_BORROWS = MUTABLE_MEM.bits | NEEDS_DROP.bits | NOT_CONST.bits
+    }
+}
+
 #[derive(Copy, Eq, PartialEq)]
 enum Mode {
-    InConstant,
-    InStatic,
-    InStaticMut,
-    InNothing,
+    Const,
+    Static,
+    StaticMut,
+
+    // An expression that occurs outside of any constant context
+    // (i.e. `const`, `static`, array lengths, etc.). The value
+    // can be variable at runtime, but will be promotable to
+    // static memory if we can prove it is actually constant.
+    Var,
 }
 
 struct CheckCrateVisitor<'a, 'tcx: 'a> {
     tcx: &'a ty::ctxt<'tcx>,
     mode: Mode,
+    qualif: ConstQualif,
+    rvalue_borrows: NodeMap<ast::Mutability>
 }
 
 impl<'a, 'tcx> CheckCrateVisitor<'a, 'tcx> {
-    fn with_mode<F>(&mut self, mode: Mode, f: F) where
-        F: FnOnce(&mut CheckCrateVisitor<'a, 'tcx>),
+    fn with_mode<F, R>(&mut self, mode: Mode, f: F) -> R where
+        F: FnOnce(&mut CheckCrateVisitor<'a, 'tcx>) -> R,
     {
-        let old = self.mode;
+        let (old_mode, old_qualif) = (self.mode, self.qualif);
         self.mode = mode;
-        f(self);
-        self.mode = old;
+        self.qualif = PURE_CONST;
+        let r = f(self);
+        self.mode = old_mode;
+        self.qualif = old_qualif;
+        r
+    }
+
+    fn with_euv<'b, F, R>(&'b mut self, item_id: Option<ast::NodeId>, f: F) -> R where
+        F: for<'t> FnOnce(&mut euv::ExprUseVisitor<'b, 't, 'tcx,
+                                    ty::ParameterEnvironment<'a, 'tcx>>) -> R,
+    {
+        let param_env = match item_id {
+            Some(item_id) => ty::ParameterEnvironment::for_item(self.tcx, item_id),
+            None => ty::empty_parameter_environment(self.tcx)
+        };
+        f(&mut euv::ExprUseVisitor::new(self, &param_env))
+    }
+
+    fn global_expr(&mut self, mode: Mode, expr: &ast::Expr) -> ConstQualif {
+        assert!(mode != Mode::Var);
+        match self.tcx.const_qualif_map.borrow_mut().entry(expr.id) {
+            Entry::Occupied(entry) => return *entry.get(),
+            Entry::Vacant(entry) => {
+                // Prevent infinite recursion on re-entry.
+                entry.insert(PURE_CONST);
+            }
+        }
+        self.with_mode(mode, |this| {
+            this.with_euv(None, |euv| euv.consume_expr(expr));
+            this.visit_expr(expr);
+            this.qualif
+        })
+    }
+
+    fn add_qualif(&mut self, qualif: ConstQualif) {
+        self.qualif = self.qualif | qualif;
+    }
+
+    fn record_borrow(&mut self, id: ast::NodeId, mutbl: ast::Mutability) {
+        match self.rvalue_borrows.entry(id) {
+            Entry::Occupied(mut entry) => {
+                // Merge the two borrows, taking the most demanding
+                // one, mutability-wise.
+                if mutbl == ast::MutMutable {
+                    entry.insert(mutbl);
+                }
+            }
+            Entry::Vacant(entry) => {
+                entry.insert(mutbl);
+            }
+        }
     }
 
     fn msg(&self) -> &'static str {
         match self.mode {
-            InConstant => "constant",
-            InStaticMut | InStatic => "static",
-            InNothing => unreachable!(),
+            Mode::Const => "constant",
+            Mode::StaticMut | Mode::Static => "static",
+            Mode::Var => unreachable!(),
         }
     }
 
@@ -108,43 +201,169 @@ impl<'a, 'tcx, 'v> Visitor<'v> for CheckCrateVisitor<'a, 'tcx> {
         match i.node {
             ast::ItemStatic(_, ast::MutImmutable, ref expr) => {
                 self.check_static_type(&**expr);
-                self.with_mode(InStatic, |v| v.visit_expr(&**expr));
+                self.global_expr(Mode::Static, &**expr);
             }
             ast::ItemStatic(_, ast::MutMutable, ref expr) => {
                 self.check_static_mut_type(&**expr);
-                self.with_mode(InStaticMut, |v| v.visit_expr(&**expr));
+                self.global_expr(Mode::StaticMut, &**expr);
             }
             ast::ItemConst(_, ref expr) => {
-                self.with_mode(InConstant, |v| v.visit_expr(&**expr));
+                self.global_expr(Mode::Const, &**expr);
             }
             ast::ItemEnum(ref enum_definition, _) => {
-                self.with_mode(InConstant, |v| {
-                    for var in &enum_definition.variants {
-                        if let Some(ref ex) = var.node.disr_expr {
-                            v.visit_expr(&**ex);
-                        }
+                for var in &enum_definition.variants {
+                    if let Some(ref ex) = var.node.disr_expr {
+                        self.global_expr(Mode::Const, &**ex);
                     }
-                });
+                }
             }
             _ => {
-                self.with_mode(InNothing, |v| visit::walk_item(v, i));
+                self.with_mode(Mode::Var, |v| visit::walk_item(v, i));
             }
         }
     }
 
+    fn visit_fn(&mut self,
+                fk: visit::FnKind<'v>,
+                fd: &'v ast::FnDecl,
+                b: &'v ast::Block,
+                s: Span,
+                fn_id: ast::NodeId) {
+        assert!(self.mode == Mode::Var);
+        self.with_euv(Some(fn_id), |euv| euv.walk_fn(fd, b));
+        visit::walk_fn(self, fk, fd, b, s);
+    }
+
     fn visit_pat(&mut self, p: &ast::Pat) {
-        let mode = match p.node {
-            ast::PatLit(_) | ast::PatRange(..) => InConstant,
-            _ => InNothing
-        };
-        self.with_mode(mode, |v| visit::walk_pat(v, p))
+        match p.node {
+            ast::PatLit(ref lit) => {
+                self.global_expr(Mode::Const, &**lit);
+            }
+            ast::PatRange(ref start, ref end) => {
+                self.global_expr(Mode::Const, &**start);
+                self.global_expr(Mode::Const, &**end);
+            }
+            _ => visit::walk_pat(self, p)
+        }
     }
 
     fn visit_expr(&mut self, ex: &ast::Expr) {
-        if self.mode != InNothing {
-            check_expr(self, ex);
+        let mut outer = self.qualif;
+        self.qualif = PURE_CONST;
+
+        let node_ty = ty::node_id_to_type(self.tcx, ex.id);
+        check_expr(self, ex, node_ty);
+
+        // Special-case some expressions to avoid certain flags bubbling up.
+        match ex.node {
+            ast::ExprCall(ref callee, ref args) => {
+                for arg in args.iter() {
+                    self.visit_expr(&**arg)
+                }
+
+                let inner = self.qualif;
+                self.visit_expr(&**callee);
+                // The callee's size doesn't count in the call.
+                let added = self.qualif - inner;
+                self.qualif = inner | (added - NON_ZERO_SIZED);
+            }
+            ast::ExprRepeat(ref element, _) => {
+                self.visit_expr(&**element);
+                // The count is checked elsewhere (typeck).
+                let count = match node_ty.sty {
+                    ty::ty_vec(_, Some(n)) => n,
+                    _ => unreachable!()
+                };
+                // [element; 0] is always zero-sized.
+                if count == 0 {
+                    self.qualif = self.qualif - (NON_ZERO_SIZED | PREFER_IN_PLACE);
+                }
+            }
+            ast::ExprMatch(ref discr, ref arms, _) => {
+                // Compute the most demanding borrow from all the arms'
+                // patterns and set that on the discriminator.
+                let mut borrow = None;
+                for pat in arms.iter().flat_map(|arm| arm.pats.iter()) {
+                    let pat_borrow = self.rvalue_borrows.remove(&pat.id);
+                    match (borrow, pat_borrow) {
+                        (None, _) | (_, Some(ast::MutMutable)) => {
+                            borrow = pat_borrow;
+                        }
+                        _ => {}
+                    }
+                }
+                if let Some(mutbl) = borrow {
+                    self.record_borrow(discr.id, mutbl);
+                }
+                visit::walk_expr(self, ex);
+            }
+            // Division by zero and overflow checking.
+            ast::ExprBinary(op, _, _) => {
+                visit::walk_expr(self, ex);
+                let div_or_rem = op.node == ast::BiDiv || op.node == ast::BiRem;
+                match node_ty.sty {
+                    ty::ty_uint(_) | ty::ty_int(_) if div_or_rem => {
+                        if !self.qualif.intersects(NOT_CONST) {
+                            match const_eval::eval_const_expr_partial(self.tcx, ex, None) {
+                                Ok(_) => {}
+                                Err(msg) => {
+                                    span_err!(self.tcx.sess, ex.span, E0020,
+                                              "{} in a constant expression", msg)
+                                }
+                            }
+                        }
+                    }
+                    _ => {}
+                }
+            }
+            _ => visit::walk_expr(self, ex)
+        }
+
+        // Handle borrows on (or inside the autorefs of) this expression.
+        match self.rvalue_borrows.remove(&ex.id) {
+            Some(ast::MutImmutable) => {
+                // Constants cannot be borrowed if they contain interior mutability as
+                // it means that our "silent insertion of statics" could change
+                // initializer values (very bad).
+                // If the type doesn't have interior mutability, then `MUTABLE_MEM` has
+                // propagated from another error, so erroring again would be just noise.
+                let tc = ty::type_contents(self.tcx, node_ty);
+                if self.qualif.intersects(MUTABLE_MEM) && tc.interior_unsafe() {
+                    outer = outer | NOT_CONST;
+                    if self.mode != Mode::Var {
+                        self.tcx.sess.span_err(ex.span,
+                            "cannot borrow a constant which contains \
+                             interior mutability, create a static instead");
+                    }
+                }
+                // If the reference has to be 'static, avoid in-place initialization
+                // as that will end up pointing to the stack instead.
+                if !self.qualif.intersects(NON_STATIC_BORROWS) {
+                    self.qualif = self.qualif - PREFER_IN_PLACE;
+                    self.add_qualif(HAS_STATIC_BORROWS);
+                }
+            }
+            Some(ast::MutMutable) => {
+                // `&mut expr` means expr could be mutated, unless it's zero-sized.
+                if self.qualif.intersects(NON_ZERO_SIZED) {
+                    if self.mode == Mode::Var {
+                        outer = outer | NOT_CONST;
+                        self.add_qualif(MUTABLE_MEM);
+                    } else {
+                        span_err!(self.tcx.sess, ex.span, E0017,
+                            "references in {}s may only refer \
+                             to immutable values", self.msg())
+                    }
+                }
+                if !self.qualif.intersects(NON_STATIC_BORROWS) {
+                    self.add_qualif(HAS_STATIC_BORROWS);
+                }
+            }
+            None => {}
         }
-        visit::walk_expr(self, ex);
+        self.tcx.const_qualif_map.borrow_mut().insert(ex.id, self.qualif);
+        // Don't propagate certain flags.
+        self.qualif = outer | (self.qualif - HAS_STATIC_BORROWS);
     }
 }
 
@@ -152,32 +371,49 @@ impl<'a, 'tcx, 'v> Visitor<'v> for CheckCrateVisitor<'a, 'tcx> {
 /// const/static items. It walks through the *value*
 /// of the item walking down the expression and evaluating
 /// every nested expression. If the expression is not part
-/// of a const/static item, this function does nothing but
-/// walking down through it.
-fn check_expr(v: &mut CheckCrateVisitor, e: &ast::Expr) {
-    let node_ty = ty::node_id_to_type(v.tcx, e.id);
-
+/// of a const/static item, it is qualified for promotion
+/// instead of producing errors.
+fn check_expr<'a, 'tcx>(v: &mut CheckCrateVisitor<'a, 'tcx>,
+                        e: &ast::Expr, node_ty: Ty<'tcx>) {
     match node_ty.sty {
         ty::ty_struct(did, _) |
         ty::ty_enum(did, _) if ty::has_dtor(v.tcx, did) => {
-            v.tcx.sess.span_err(e.span,
-                                &format!("{}s are not allowed to have destructors",
-                                         v.msg())[])
+            v.add_qualif(NEEDS_DROP);
+            if v.mode != Mode::Var {
+                v.tcx.sess.span_err(e.span,
+                                    &format!("{}s are not allowed to have destructors",
+                                             v.msg())[]);
+            }
         }
         _ => {}
     }
 
+    let method_call = ty::MethodCall::expr(e.id);
     match e.node {
+        ast::ExprUnary(..) |
+        ast::ExprBinary(..) |
+        ast::ExprIndex(..) if v.tcx.method_map.borrow().contains_key(&method_call) => {
+            v.add_qualif(NOT_CONST);
+            if v.mode != Mode::Var {
+                span_err!(v.tcx.sess, e.span, E0011,
+                            "user-defined operators are not allowed in {}s", v.msg());
+            }
+        }
         ast::ExprBox(..) |
         ast::ExprUnary(ast::UnUniq, _) => {
-            span_err!(v.tcx.sess, e.span, E0010,
-                      "allocations are not allowed in {}s", v.msg());
+            v.add_qualif(NOT_CONST);
+            if v.mode != Mode::Var {
+                span_err!(v.tcx.sess, e.span, E0010,
+                          "allocations are not allowed in {}s", v.msg());
+            }
         }
-        ast::ExprBinary(..) | ast::ExprUnary(..) => {
-            let method_call = ty::MethodCall::expr(e.id);
-            if v.tcx.method_map.borrow().contains_key(&method_call) {
-                span_err!(v.tcx.sess, e.span, E0011,
-                          "user-defined operators are not allowed in {}s", v.msg());
+        ast::ExprUnary(ast::UnDeref, ref ptr) => {
+            match ty::node_id_to_type(v.tcx, ptr.id).sty {
+                ty::ty_ptr(_) => {
+                    // This shouldn't be allowed in constants at all.
+                    v.add_qualif(NOT_CONST);
+                }
+                _ => {}
             }
         }
         ast::ExprCast(ref from, _) => {
@@ -188,51 +424,110 @@ fn check_expr(v: &mut CheckCrateVisitor, e: &ast::Expr) {
                 ty::type_is_unsafe_ptr(toty) ||
                 (ty::type_is_bare_fn(toty) && ty::type_is_bare_fn_item(fromty));
             if !is_legal_cast {
-                span_err!(v.tcx.sess, e.span, E0012,
-                          "can not cast to `{}` in {}s",
-                          ppaux::ty_to_string(v.tcx, toty), v.msg());
+                v.add_qualif(NOT_CONST);
+                if v.mode != Mode::Var {
+                    span_err!(v.tcx.sess, e.span, E0012,
+                              "can not cast to `{}` in {}s",
+                              ppaux::ty_to_string(v.tcx, toty), v.msg());
+                }
             }
             if ty::type_is_unsafe_ptr(fromty) && ty::type_is_numeric(toty) {
-                span_err!(v.tcx.sess, e.span, E0018,
-                          "can not cast a pointer to an integer in {}s", v.msg());
+                v.add_qualif(NOT_CONST);
+                if v.mode != Mode::Var {
+                    span_err!(v.tcx.sess, e.span, E0018,
+                              "can not cast a pointer to an integer in {}s", v.msg());
+                }
             }
         }
         ast::ExprPath(_) | ast::ExprQPath(_) => {
-            match v.tcx.def_map.borrow()[e.id] {
-                def::DefStatic(..) if v.mode == InConstant => {
-                    span_err!(v.tcx.sess, e.span, E0013,
-                              "constants cannot refer to other statics, \
-                               insert an intermediate constant instead");
+            let def = v.tcx.def_map.borrow().get(&e.id).cloned();
+            match def {
+                Some(def::DefVariant(_, _, _)) => {
+                    // Count the discriminator or function pointer.
+                    v.add_qualif(NON_ZERO_SIZED);
+                }
+                Some(def::DefStruct(_)) => {
+                    if let ty::ty_bare_fn(..) = node_ty.sty {
+                        // Count the function pointer.
+                        v.add_qualif(NON_ZERO_SIZED);
+                    }
+                }
+                Some(def::DefFn(..)) |
+                Some(def::DefStaticMethod(..)) | Some(def::DefMethod(..)) => {
+                    // Count the function pointer.
+                    v.add_qualif(NON_ZERO_SIZED);
+                }
+                Some(def::DefStatic(..)) => {
+                    match v.mode {
+                        Mode::Static | Mode::StaticMut => {}
+                        Mode::Const => {
+                            span_err!(v.tcx.sess, e.span, E0013,
+                                "constants cannot refer to other statics, \
+                                 insert an intermediate constant instead");
+                        }
+                        Mode::Var => v.add_qualif(NOT_CONST)
+                    }
+                }
+                Some(def::DefConst(did)) => {
+                    if let Some(expr) = const_eval::lookup_const_by_id(v.tcx, did) {
+                        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");
+                    }
                 }
-                def::DefStatic(..) | def::DefConst(..) |
-                def::DefFn(..) | def::DefStaticMethod(..) | def::DefMethod(..) |
-                def::DefStruct(_) | def::DefVariant(_, _, _) => {}
-
                 def => {
-                    debug!("(checking const) found bad def: {:?}", def);
-                    span_err!(v.tcx.sess, e.span, E0014,
-                              "paths in constants may only refer to constants \
-                               or functions");
+                    v.add_qualif(NOT_CONST);
+                    if v.mode != Mode::Var {
+                        debug!("(checking const) found bad def: {:?}", def);
+                        span_err!(v.tcx.sess, e.span, E0014,
+                                  "paths in {}s may only refer to constants \
+                                   or functions", v.msg());
+                    }
                 }
             }
         }
         ast::ExprCall(ref callee, _) => {
-            match v.tcx.def_map.borrow()[callee.id] {
-                def::DefStruct(..) | def::DefVariant(..) => {}    // OK.
+            let mut callee = &**callee;
+            loop {
+                callee = match callee.node {
+                    ast::ExprParen(ref inner) => &**inner,
+                    ast::ExprBlock(ref block) => match block.expr {
+                        Some(ref tail) => &**tail,
+                        None => break
+                    },
+                    _ => break
+                };
+            }
+            let def = v.tcx.def_map.borrow().get(&callee.id).cloned();
+            match def {
+                Some(def::DefStruct(..)) => {}
+                Some(def::DefVariant(..)) => {
+                    // Count the discriminator.
+                    v.add_qualif(NON_ZERO_SIZED);
+                }
                 _ => {
-                    span_err!(v.tcx.sess, e.span, E0015,
-                              "function calls in constants are limited to \
-                               struct and enum constructors");
+                    v.add_qualif(NOT_CONST);
+                    if v.mode != Mode::Var {
+                        span_err!(v.tcx.sess, e.span, E0015,
+                                  "function calls in {}s are limited to \
+                                   struct and enum constructors", v.msg());
+                    }
                 }
             }
         }
         ast::ExprBlock(ref block) => {
             // Check all statements in the block
-            for stmt in &block.stmts {
-                let block_span_err = |span|
+            let mut block_span_err = |span| {
+                v.add_qualif(NOT_CONST);
+                if v.mode != Mode::Var {
                     span_err!(v.tcx.sess, span, E0016,
-                              "blocks in constants are limited to items and \
-                               tail expressions");
+                              "blocks in {}s are limited to items and \
+                               tail expressions", v.msg());
+                }
+            };
+            for stmt in &block.stmts {
                 match stmt.node {
                     ast::StmtDecl(ref decl, _) => {
                         match decl.node {
@@ -251,26 +546,40 @@ fn check_expr(v: &mut CheckCrateVisitor, e: &ast::Expr) {
                 }
             }
         }
-        ast::ExprAddrOf(ast::MutMutable, ref inner) => {
-            match inner.node {
-                // Mutable slices are allowed. Only in `static mut`.
-                ast::ExprVec(_) if v.mode == InStaticMut => {}
-                _ => span_err!(v.tcx.sess, e.span, E0017,
-                               "references in {}s may only refer \
-                                to immutable values", v.msg())
+        ast::ExprStruct(..) => {
+            let did = v.tcx.def_map.borrow().get(&e.id).map(|def| def.def_id());
+            if did == v.tcx.lang_items.unsafe_cell_type() {
+                v.add_qualif(MUTABLE_MEM);
             }
         }
 
         ast::ExprLit(_) |
-        ast::ExprVec(_) |
-        ast::ExprAddrOf(ast::MutImmutable, _) |
-        ast::ExprParen(..) |
+        ast::ExprAddrOf(..) => {
+            v.add_qualif(NON_ZERO_SIZED);
+        }
+
+        ast::ExprRepeat(..) => {
+            v.add_qualif(PREFER_IN_PLACE);
+        }
+
+        ast::ExprClosure(..) => {
+            // Paths in constant constexts cannot refer to local variables,
+            // as there are none, and thus closures can't have upvars there.
+            if ty::with_freevars(v.tcx, e.id, |fv| !fv.is_empty()) {
+                assert!(v.mode == Mode::Var,
+                        "global closures can't capture anything");
+                v.add_qualif(NOT_CONST);
+            }
+        }
+
+        ast::ExprUnary(..) |
+        ast::ExprBinary(..) |
+        ast::ExprIndex(..) |
         ast::ExprField(..) |
         ast::ExprTupField(..) |
-        ast::ExprIndex(..) |
-        ast::ExprTup(..) |
-        ast::ExprRepeat(..) |
-        ast::ExprStruct(..) => {}
+        ast::ExprVec(_) |
+        ast::ExprParen(..) |
+        ast::ExprTup(..) => {}
 
         // Conditional control flow (possible to implement).
         ast::ExprMatch(..) |
@@ -289,7 +598,6 @@ fn check_expr(v: &mut CheckCrateVisitor, e: &ast::Expr) {
         ast::ExprRet(_) |
 
         // Miscellaneous expressions that could be implemented.
-        ast::ExprClosure(..) |
         ast::ExprRange(..) |
 
         // Various other expressions.
@@ -298,50 +606,27 @@ fn check_expr(v: &mut CheckCrateVisitor, e: &ast::Expr) {
         ast::ExprAssignOp(..) |
         ast::ExprInlineAsm(_) |
         ast::ExprMac(_) => {
-            span_err!(v.tcx.sess, e.span, E0019,
-                      "{} contains unimplemented expression type", v.msg());
+            v.add_qualif(NOT_CONST);
+            if v.mode != Mode::Var {
+                span_err!(v.tcx.sess, e.span, E0019,
+                          "{} contains unimplemented expression type", v.msg());
+            }
         }
     }
 }
 
-struct GlobalVisitor<'a,'b,'tcx:'a+'b>(
-    euv::ExprUseVisitor<'a,'b,'tcx,ty::ParameterEnvironment<'b,'tcx>>);
-
-struct GlobalChecker<'a,'tcx:'a> {
-    tcx: &'a ty::ctxt<'tcx>
-}
-
 pub fn check_crate(tcx: &ty::ctxt) {
-    let param_env = ty::empty_parameter_environment(tcx);
-    let mut checker = GlobalChecker {
-        tcx: tcx
-    };
-    let visitor = euv::ExprUseVisitor::new(&mut checker, &param_env);
-    visit::walk_crate(&mut GlobalVisitor(visitor), tcx.map.krate());
-
     visit::walk_crate(&mut CheckCrateVisitor {
         tcx: tcx,
-        mode: InNothing,
+        mode: Mode::Var,
+        qualif: NOT_CONST,
+        rvalue_borrows: NodeMap()
     }, tcx.map.krate());
 
     tcx.sess.abort_if_errors();
 }
 
-impl<'a,'b,'t,'v> Visitor<'v> for GlobalVisitor<'a,'b,'t> {
-    fn visit_item(&mut self, item: &ast::Item) {
-        match item.node {
-            ast::ItemConst(_, ref e) |
-            ast::ItemStatic(_, _, ref e) => {
-                let GlobalVisitor(ref mut v) = *self;
-                v.consume_expr(&**e);
-            }
-            _ => {}
-        }
-        visit::walk_item(self, item);
-    }
-}
-
-impl<'a, 'tcx> euv::Delegate<'tcx> for GlobalChecker<'a, 'tcx> {
+impl<'a, 'tcx> euv::Delegate<'tcx> for CheckCrateVisitor<'a, 'tcx> {
     fn consume(&mut self,
                _consume_id: ast::NodeId,
                consume_span: Span,
@@ -351,12 +636,14 @@ impl<'a, 'tcx> euv::Delegate<'tcx> for GlobalChecker<'a, 'tcx> {
         loop {
             match cur.cat {
                 mc::cat_static_item => {
-                    // statics cannot be consumed by value at any time, that would imply
-                    // that they're an initializer (what a const is for) or kept in sync
-                    // over time (not feasible), so deny it outright.
-                    self.tcx.sess.span_err(consume_span,
-                        "cannot refer to other statics by value, use the \
-                         address-of operator or a constant instead");
+                    if self.mode != Mode::Var {
+                        // statics cannot be consumed by value at any time, that would imply
+                        // that they're an initializer (what a const is for) or kept in sync
+                        // over time (not feasible), so deny it outright.
+                        self.tcx.sess.span_err(consume_span,
+                            "cannot refer to other statics by value, use the \
+                             address-of operator or a constant instead");
+                    }
                     break;
                 }
                 mc::cat_deref(ref cmt, _, _) |
@@ -370,29 +657,35 @@ impl<'a, 'tcx> euv::Delegate<'tcx> for GlobalChecker<'a, 'tcx> {
         }
     }
     fn borrow(&mut self,
-              _borrow_id: ast::NodeId,
+              borrow_id: ast::NodeId,
               borrow_span: Span,
               cmt: mc::cmt<'tcx>,
               _loan_region: ty::Region,
-              _bk: ty::BorrowKind,
-              _loan_cause: euv::LoanCause) {
+              bk: ty::BorrowKind,
+              loan_cause: euv::LoanCause) {
         let mut cur = &cmt;
         let mut is_interior = false;
         loop {
             match cur.cat {
                 mc::cat_rvalue(..) => {
-                    // constants cannot be borrowed if they contain interior mutability as
-                    // it means that our "silent insertion of statics" could change
-                    // initializer values (very bad).
-                    if ty::type_contents(self.tcx, cur.ty).interior_unsafe() {
-                        self.tcx.sess.span_err(borrow_span,
-                            "cannot borrow a constant which contains \
-                            interior mutability, create a static instead");
+                    if loan_cause == euv::MatchDiscriminant {
+                        // Ignore the dummy immutable borrow created by EUV.
+                        break;
                     }
+                    let mutbl = bk.to_mutbl_lossy();
+                    if mutbl == ast::MutMutable && self.mode == Mode::StaticMut {
+                        // Mutable slices are the only `&mut` allowed in globals,
+                        // but only in `static mut`, nowhere else.
+                        match cmt.ty.sty {
+                            ty::ty_vec(_, _) => break,
+                            _ => {}
+                        }
+                    }
+                    self.record_borrow(borrow_id, mutbl);
                     break;
                 }
                 mc::cat_static_item => {
-                    if is_interior {
+                    if is_interior && self.mode != Mode::Var {
                         // Borrowed statics can specifically *only* have their address taken,
                         // not any number of other borrows such as borrowing fields, reading
                         // elements of an array, etc.
@@ -433,4 +726,4 @@ impl<'a, 'tcx> euv::Delegate<'tcx> for GlobalChecker<'a, 'tcx> {
                    _consume_pat: &ast::Pat,
                    _cmt: mc::cmt,
                    _mode: euv::ConsumeMode) {}
-}
\ No newline at end of file
+}
diff --git a/src/librustc/middle/ty.rs b/src/librustc/middle/ty.rs
index 6e14f7b09f8..fe5c81bf2c0 100644
--- a/src/librustc/middle/ty.rs
+++ b/src/librustc/middle/ty.rs
@@ -44,6 +44,7 @@ use session::Session;
 use lint;
 use metadata::csearch;
 use middle;
+use middle::check_const;
 use middle::const_eval;
 use middle::def::{self, DefMap, ExportMap};
 use middle::dependency_format;
@@ -838,6 +839,9 @@ pub struct ctxt<'tcx> {
 
     /// Caches whether traits are object safe
     pub object_safety_cache: RefCell<DefIdMap<bool>>,
+
+    /// Maps Expr NodeId's to their constant qualification.
+    pub const_qualif_map: RefCell<NodeMap<check_const::ConstQualif>>,
 }
 
 // Flags that we track on types. These flags are propagated upwards
@@ -2472,6 +2476,7 @@ pub fn mk_ctxt<'tcx>(s: Session,
         type_impls_copy_cache: RefCell::new(HashMap::new()),
         type_impls_sized_cache: RefCell::new(HashMap::new()),
         object_safety_cache: RefCell::new(DefIdMap()),
+        const_qualif_map: RefCell::new(NodeMap()),
    }
 }