Auto merge of #43932 - eddyb:const-scoping, r=nikomatsakis

Forward-compatibly deny drops in constants if they *could* actually run.

This is part of #40036, specifically the checks for user-defined destructor invocations on locals which *may not* have been moved away, the motivating example being:
```rust
const FOO: i32 = (HasDrop {...}, 0).1;
```
The evaluation of constant MIR will continue to create `'static` slots for more locals than is necessary (if `Storage{Live,Dead}` statements are ignored), but it shouldn't be misusable.

r? @nikomatsakis

src/librustc/middle/region.rs

@@ -223,7 +223,9 @@ pub struct RegionMaps {
     /// table, the appropriate cleanup scope is the innermost
     /// enclosing statement, conditional expression, or repeating
     /// block (see `terminating_scopes`).
-    rvalue_scopes: NodeMap<CodeExtent>,
+    /// In constants, None is used to indicate that certain expressions
+    /// escape into 'static and should have no local cleanup scope.
+    rvalue_scopes: NodeMap<Option<CodeExtent>>,
 
     /// Encodes the hierarchy of fn bodies. Every fn body (including
     /// closures) forms its own distinct region hierarchy, rooted in
@@ -358,9 +360,11 @@ impl<'tcx> RegionMaps {
         self.var_map.insert(var, lifetime);
     }
 
-    fn record_rvalue_scope(&mut self, var: ast::NodeId, lifetime: CodeExtent) {
+    fn record_rvalue_scope(&mut self, var: ast::NodeId, lifetime: Option<CodeExtent>) {
         debug!("record_rvalue_scope(sub={:?}, sup={:?})", var, lifetime);
-        assert!(var != lifetime.node_id());
+        if let Some(lifetime) = lifetime {
+            assert!(var != lifetime.node_id());
+        }
         self.rvalue_scopes.insert(var, lifetime);
     }
 
@@ -389,7 +393,7 @@ impl<'tcx> RegionMaps {
         // check for a designated rvalue scope
         if let Some(&s) = self.rvalue_scopes.get(&expr_id) {
             debug!("temporary_scope({:?}) = {:?} [custom]", expr_id, s);
-            return Some(s);
+            return s;
         }
 
         // else, locate the innermost terminating scope
@@ -803,16 +807,11 @@ fn resolve_expr<'a, 'tcx>(visitor: &mut RegionResolutionVisitor<'a, 'tcx>, expr:
 }
 
 fn resolve_local<'a, 'tcx>(visitor: &mut RegionResolutionVisitor<'a, 'tcx>,
-                           local: &'tcx hir::Local) {
-    debug!("resolve_local(local.id={:?},local.init={:?})",
-           local.id,local.init.is_some());
+                           pat: Option<&'tcx hir::Pat>,
+                           init: Option<&'tcx hir::Expr>) {
+    debug!("resolve_local(pat={:?}, init={:?})", pat, init);
 
-    // For convenience in trans, associate with the local-id the var
-    // scope that will be used for any bindings declared in this
-    // pattern.
     let blk_scope = visitor.cx.var_parent;
-    let blk_scope = blk_scope.expect("locals must be within a block");
-    visitor.region_maps.record_var_scope(local.id, blk_scope);
 
     // As an exception to the normal rules governing temporary
     // lifetimes, initializers in a let have a temporary lifetime
@@ -872,15 +871,22 @@ fn resolve_local<'a, 'tcx>(visitor: &mut RegionResolutionVisitor<'a, 'tcx>,
     //
     // FIXME(#6308) -- Note that `[]` patterns work more smoothly post-DST.
 
-    if let Some(ref expr) = local.init {
+    if let Some(expr) = init {
         record_rvalue_scope_if_borrow_expr(visitor, &expr, blk_scope);
 
-        if is_binding_pat(&local.pat) {
-            record_rvalue_scope(visitor, &expr, blk_scope);
+        if let Some(pat) = pat {
+            if is_binding_pat(pat) {
+                record_rvalue_scope(visitor, &expr, blk_scope);
+            }
         }
     }
 
-    intravisit::walk_local(visitor, local);
+    if let Some(pat) = pat {
+        visitor.visit_pat(pat);
+    }
+    if let Some(expr) = init {
+        visitor.visit_expr(expr);
+    }
 
     /// True if `pat` match the `P&` nonterminal:
     ///
@@ -954,7 +960,7 @@ fn resolve_local<'a, 'tcx>(visitor: &mut RegionResolutionVisitor<'a, 'tcx>,
     fn record_rvalue_scope_if_borrow_expr<'a, 'tcx>(
         visitor: &mut RegionResolutionVisitor<'a, 'tcx>,
         expr: &hir::Expr,
-        blk_id: CodeExtent)
+        blk_id: Option<CodeExtent>)
     {
         match expr.node {
             hir::ExprAddrOf(_, ref subexpr) => {
@@ -1004,7 +1010,7 @@ fn resolve_local<'a, 'tcx>(visitor: &mut RegionResolutionVisitor<'a, 'tcx>,
     /// Note: ET is intended to match "rvalues or lvalues based on rvalues".
     fn record_rvalue_scope<'a, 'tcx>(visitor: &mut RegionResolutionVisitor<'a, 'tcx>,
                                      expr: &hir::Expr,
-                                     blk_scope: CodeExtent) {
+                                     blk_scope: Option<CodeExtent>) {
         let mut expr = expr;
         loop {
             // Note: give all the expressions matching `ET` with the
@@ -1077,12 +1083,7 @@ impl<'a, 'tcx> Visitor<'tcx> for RegionResolutionVisitor<'a, 'tcx> {
 
         let outer_cx = self.cx;
         let outer_ts = mem::replace(&mut self.terminating_scopes, NodeSet());
-
-        // Only functions have an outer terminating (drop) scope,
-        // while temporaries in constant initializers are 'static.
-        if let MirSource::Fn(_) = MirSource::from_node(self.tcx, owner_id) {
-            self.terminating_scopes.insert(body_id.node_id);
-        }
+        self.terminating_scopes.insert(body_id.node_id);
 
         if let Some(root_id) = self.cx.root_id {
             self.region_maps.record_fn_parent(body_id.node_id, root_id);
@@ -1100,7 +1101,30 @@ impl<'a, 'tcx> Visitor<'tcx> for RegionResolutionVisitor<'a, 'tcx> {
 
         // The body of the every fn is a root scope.
         self.cx.parent = self.cx.var_parent;
-        self.visit_expr(&body.value);
+        if let MirSource::Fn(_) = MirSource::from_node(self.tcx, owner_id) {
+            self.visit_expr(&body.value);
+        } else {
+            // Only functions have an outer terminating (drop) scope, while
+            // temporaries in constant initializers may be 'static, but only
+            // according to rvalue lifetime semantics, using the same
+            // syntactical rules used for let initializers.
+            //
+            // E.g. in `let x = &f();`, the temporary holding the result from
+            // the `f()` call lives for the entirety of the surrounding block.
+            //
+            // Similarly, `const X: ... = &f();` would have the result of `f()`
+            // live for `'static`, implying (if Drop restrictions on constants
+            // ever get lifted) that the value *could* have a destructor, but
+            // it'd get leaked instead of the destructor running during the
+            // evaluation of `X` (if at all allowed by CTFE).
+            //
+            // However, `const Y: ... = g(&f());`, like `let y = g(&f());`,
+            // would *not* let the `f()` temporary escape into an outer scope
+            // (i.e. `'static`), which means that after `g` returns, it drops,
+            // and all the associated destruction scope rules apply.
+            self.cx.var_parent = None;
+            resolve_local(self, None, Some(&body.value));
+        }
 
         // Restore context we had at the start.
         self.cx = outer_cx;
@@ -1120,7 +1144,7 @@ impl<'a, 'tcx> Visitor<'tcx> for RegionResolutionVisitor<'a, 'tcx> {
         resolve_expr(self, ex);
     }
     fn visit_local(&mut self, l: &'tcx Local) {
-        resolve_local(self, l);
+        resolve_local(self, Some(&l.pat), l.init.as_ref().map(|e| &**e));
     }
 }
 

src/librustc_mir/transform/qualify_consts.rs

@@ -120,6 +120,7 @@ struct Qualifier<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
     return_qualif: Option<Qualif>,
     qualif: Qualif,
     const_fn_arg_vars: BitVector,
+    local_needs_drop: IndexVec<Local, Option<Span>>,
     temp_promotion_state: IndexVec<Local, TempState>,
     promotion_candidates: Vec<Candidate>
 }
@@ -146,6 +147,7 @@ impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
             return_qualif: None,
             qualif: Qualif::empty(),
             const_fn_arg_vars: BitVector::new(mir.local_decls.len()),
+            local_needs_drop: IndexVec::from_elem(None, &mir.local_decls),
             temp_promotion_state: temps,
             promotion_candidates: vec![]
         }
@@ -193,16 +195,26 @@ impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
         self.add(original);
     }
 
+    /// Check for NEEDS_DROP (from an ADT or const fn call) and
+    /// error, unless we're in a function.
+    fn always_deny_drop(&self) {
+        self.deny_drop_with_feature_gate_override(false);
+    }
+
     /// Check for NEEDS_DROP (from an ADT or const fn call) and
     /// error, unless we're in a function, or the feature-gate
     /// for globals with destructors is enabled.
     fn deny_drop(&self) {
+        self.deny_drop_with_feature_gate_override(true);
+    }
+
+    fn deny_drop_with_feature_gate_override(&self, allow_gate: bool) {
         if self.mode == Mode::Fn || !self.qualif.intersects(Qualif::NEEDS_DROP) {
             return;
         }
 
         // Static and const fn's allow destructors, but they're feature-gated.
-        let msg = if self.mode != Mode::Const {
+        let msg = if allow_gate && self.mode != Mode::Const {
             // Feature-gate for globals with destructors is enabled.
             if self.tcx.sess.features.borrow().drop_types_in_const {
                 return;
@@ -223,15 +235,16 @@ impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
         let mut err =
             struct_span_err!(self.tcx.sess, self.span, E0493, "{}", msg);
 
-        if self.mode != Mode::Const {
+        if allow_gate && self.mode != Mode::Const {
             help!(&mut err,
                   "in Nightly builds, add `#![feature(drop_types_in_const)]` \
                    to the crate attributes to enable");
         } else {
             self.find_drop_implementation_method_span()
                 .map(|span| err.span_label(span, "destructor defined here"));
 
-            err.span_label(self.span, "constants cannot have destructors");
+            err.span_label(self.span,
+                format!("{}s cannot have destructors", self.mode));
         }
 
         err.emit();
@@ -314,6 +327,15 @@ impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
             return;
         }
 
+        // When initializing a local, record whether the *value* being
+        // stored in it needs dropping, which it may not, even if its
+        // type does, e.g. `None::<String>`.
+        if let Lvalue::Local(local) = *dest {
+            if qualif.intersects(Qualif::NEEDS_DROP) {
+                self.local_needs_drop[local] = Some(self.span);
+            }
+        }
+
         match *dest {
             Lvalue::Local(index) if self.mir.local_kind(index) == LocalKind::Temp => {
                 debug!("store to temp {:?}", index);
@@ -360,7 +382,6 @@ impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
 
             let target = match mir[bb].terminator().kind {
                 TerminatorKind::Goto { target } |
-                // Drops are considered noops.
                 TerminatorKind::Drop { target, .. } |
                 TerminatorKind::Assert { target, .. } |
                 TerminatorKind::Call { destination: Some((_, target)), .. } => {
@@ -560,22 +581,32 @@ impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
 
     fn visit_operand(&mut self, operand: &Operand<'tcx>, location: Location) {
         match *operand {
-            Operand::Consume(_) => {
+            Operand::Consume(ref lvalue) => {
                 self.nest(|this| {
                     this.super_operand(operand, location);
                     this.try_consume();
                 });
+
+                // Mark the consumed locals to indicate later drops are noops.
+                if let Lvalue::Local(local) = *lvalue {
+                    self.local_needs_drop[local] = None;
+                }
             }
             Operand::Constant(ref constant) => {
-                if let Literal::Item { def_id, substs } = constant.literal {
-                    // Don't peek inside generic (associated) constants.
-                    if substs.types().next().is_some() {
+                if let Literal::Item { def_id, substs: _ } = constant.literal {
+                    // Don't peek inside trait associated constants.
+                    if self.tcx.trait_of_item(def_id).is_some() {
                         self.add_type(constant.ty);
                     } else {
                         let bits = self.tcx.at(constant.span).mir_const_qualif(def_id);
 
                         let qualif = Qualif::from_bits(bits).expect("invalid mir_const_qualif");
                         self.add(qualif);
+
+                        // Just in case the type is more specific than
+                        // the definition, e.g. impl associated const
+                        // with type parameters, take it into account.
+                        self.qualif.restrict(constant.ty, self.tcx, self.param_env);
                     }
 
                     // Let `const fn` transitively have destructors,
@@ -866,6 +897,30 @@ impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
                 }
                 self.assign(dest, location);
             }
+        } else if let TerminatorKind::Drop { location: ref lvalue, .. } = *kind {
+            self.super_terminator_kind(bb, kind, location);
+
+            // Deny *any* live drops anywhere other than functions.
+            if self.mode != Mode::Fn {
+                // HACK(eddyb) Emulate a bit of dataflow analysis,
+                // conservatively, that drop elaboration will do.
+                let needs_drop = if let Lvalue::Local(local) = *lvalue {
+                    self.local_needs_drop[local]
+                } else {
+                    None
+                };
+
+                if let Some(span) = needs_drop {
+                    let ty = lvalue.ty(self.mir, self.tcx).to_ty(self.tcx);
+                    self.add_type(ty);
+
+                    // Use the original assignment span to be more precise.
+                    let old_span = self.span;
+                    self.span = span;
+                    self.always_deny_drop();
+                    self.span = old_span;
+                }
+            }
         } else {
             // Qualify any operands inside other terminators.
             self.super_terminator_kind(bb, kind, location);

src/librustc_passes/consts.rs

@@ -87,19 +87,14 @@ impl<'a, 'gcx> CheckCrateVisitor<'a, 'gcx> {
         }
     }
 
-    // Adds the worst effect out of all the values of one type.
-    fn add_type(&mut self, ty: Ty<'gcx>) {
-        if !ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) {
-            self.promotable = false;
-        }
-
-        if ty.needs_drop(self.tcx, self.param_env) {
-            self.promotable = false;
-        }
+    // Returns true iff all the values of the type are promotable.
+    fn type_has_only_promotable_values(&mut self, ty: Ty<'gcx>) -> bool {
+        ty.is_freeze(self.tcx, self.param_env, DUMMY_SP) &&
+        !ty.needs_drop(self.tcx, self.param_env)
     }
 
     fn handle_const_fn_call(&mut self, def_id: DefId, ret_ty: Ty<'gcx>) {
-        self.add_type(ret_ty);
+        self.promotable &= self.type_has_only_promotable_values(ret_ty);
 
         self.promotable &= if let Some(fn_id) = self.tcx.hir.as_local_node_id(def_id) {
             FnLikeNode::from_node(self.tcx.hir.get(fn_id)).map_or(false, |fn_like| {
@@ -333,20 +328,30 @@ fn check_expr<'a, 'tcx>(v: &mut CheckCrateVisitor<'a, 'tcx>, e: &hir::Expr, node
             match def {
                 Def::VariantCtor(..) | Def::StructCtor(..) |
                 Def::Fn(..) | Def::Method(..) => {}
-                Def::AssociatedConst(_) => v.add_type(node_ty),
-                Def::Const(did) => {
-                    v.promotable &= if let Some(node_id) = v.tcx.hir.as_local_node_id(did) {
-                        match v.tcx.hir.expect_item(node_id).node {
-                            hir::ItemConst(_, body) => {
+
+                Def::Const(did) |
+                Def::AssociatedConst(did) => {
+                    let promotable = if v.tcx.trait_of_item(did).is_some() {
+                        // Don't peek inside trait associated constants.
+                        false
+                    } else if let Some(node_id) = v.tcx.hir.as_local_node_id(did) {
+                        match v.tcx.hir.maybe_body_owned_by(node_id) {
+                            Some(body) => {
                                 v.visit_nested_body(body);
                                 v.tcx.rvalue_promotable_to_static.borrow()[&body.node_id]
                             }
-                            _ => false
+                            None => false
                         }
                     } else {
                         v.tcx.const_is_rvalue_promotable_to_static(did)
                     };
+
+                    // Just in case the type is more specific than the definition,
+                    // e.g. impl associated const with type parameters, check it.
+                    // Also, trait associated consts are relaxed by this.
+                    v.promotable &= promotable || v.type_has_only_promotable_values(node_ty);
                 }
+
                 _ => {
                     v.promotable = false;
                 }

src/test/compile-fail/check-static-values-constraints.rs

@@ -86,8 +86,9 @@ static STATIC8: SafeStruct = SafeStruct{field1: SafeEnum::Variant1,
 // This example should fail because field1 in the base struct is not safe
 static STATIC9: SafeStruct = SafeStruct{field1: SafeEnum::Variant1,
                                         ..SafeStruct{field1: SafeEnum::Variant3(WithDtor),
+//~^ ERROR destructors in statics are an unstable feature
+//~| ERROR statics are not allowed to have destructors
                                                      field2: SafeEnum::Variant1}};
-//~^^ ERROR destructors in statics are an unstable feature
 
 struct UnsafeStruct;