Auto merge of #31307 - nagisa:mir-drop-terminator, r=nikomatsakis

The scope of these refactorings is a little bit bigger than the title implies. See each commit for details.

I’m submitting this for nitpicking now (the first 4 commits), because I feel the basic idea/implementation is sound and should work. I will eventually expand this PR to cover the translator changes necessary for all this to work (+ tests), ~~and perhaps implement a dynamic dropping scheme while I’m at it as well.~~

r? @nikomatsakis

25 files changed, 547 insertions, 332 deletions

diff --git a/src/doc/book/lang-items.md b/src/doc/book/lang-items.md
index e492bd3e782..b948567ac5b 100644
--- a/src/doc/book/lang-items.md
+++ b/src/doc/book/lang-items.md
@@ -39,11 +39,17 @@ unsafe fn allocate(size: usize, _align: usize) -> *mut u8 {
 
     p
 }
+
 #[lang = "exchange_free"]
 unsafe fn deallocate(ptr: *mut u8, _size: usize, _align: usize) {
     libc::free(ptr as *mut libc::c_void)
 }
 
+#[lang = "box_free"]
+unsafe fn box_free<T>(ptr: *mut T) {
+    deallocate(ptr as *mut u8, ::core::mem::size_of::<T>(), ::core::mem::align_of::<T>());
+}
+
 #[start]
 fn main(argc: isize, argv: *const *const u8) -> isize {
     let x = box 1;
diff --git a/src/liballoc/heap.rs b/src/liballoc/heap.rs
index 7e7e3c619cb..08b403a60f3 100644
--- a/src/liballoc/heap.rs
+++ b/src/liballoc/heap.rs
@@ -16,6 +16,8 @@
             issue = "27700")]
 
 use core::{isize, usize};
+#[cfg(not(test))]
+use core::intrinsics::{size_of, min_align_of};
 
 #[allow(improper_ctypes)]
 extern "C" {
@@ -147,6 +149,17 @@ unsafe fn exchange_free(ptr: *mut u8, old_size: usize, align: usize) {
     deallocate(ptr, old_size, align);
 }
 
+#[cfg(not(test))]
+#[lang = "box_free"]
+#[inline]
+unsafe fn box_free<T>(ptr: *mut T) {
+    let size = size_of::<T>();
+    // We do not allocate for Box<T> when T is ZST, so deallocation is also not necessary.
+    if size != 0 {
+        deallocate(ptr as *mut u8, size, min_align_of::<T>());
+    }
+}
+
 #[cfg(test)]
 mod tests {
     extern crate test;
diff --git a/src/librustc/middle/dead.rs b/src/librustc/middle/dead.rs
index 1fa4093853b..7ed931265f2 100644
--- a/src/librustc/middle/dead.rs
+++ b/src/librustc/middle/dead.rs
@@ -435,7 +435,7 @@ impl<'a, 'tcx> DeadVisitor<'a, 'tcx> {
         let is_named = node.name().is_some();
         let field_type = self.tcx.node_id_to_type(node.id);
         let is_marker_field = match field_type.ty_to_def_id() {
-            Some(def_id) => self.tcx.lang_items.items().any(|(_, item)| *item == Some(def_id)),
+            Some(def_id) => self.tcx.lang_items.items().iter().any(|item| *item == Some(def_id)),
             _ => false
         };
         is_named
diff --git a/src/librustc/middle/lang_items.rs b/src/librustc/middle/lang_items.rs
index f77ca10e88f..a22ad7a0707 100644
--- a/src/librustc/middle/lang_items.rs
+++ b/src/librustc/middle/lang_items.rs
@@ -37,9 +37,6 @@ use syntax::parse::token::InternedString;
 use rustc_front::intravisit::Visitor;
 use rustc_front::hir;
 
-use std::iter::Enumerate;
-use std::slice;
-
 // The actual lang items defined come at the end of this file in one handy table.
 // So you probably just want to nip down to the end.
 macro_rules! lets_do_this {
@@ -70,8 +67,8 @@ impl LanguageItems {
         }
     }
 
-    pub fn items<'a>(&'a self) -> Enumerate<slice::Iter<'a, Option<DefId>>> {
-        self.items.iter().enumerate()
+    pub fn items(&self) -> &[Option<DefId>] {
+        &*self.items
     }
 
     pub fn item_name(index: usize) -> &'static str {
@@ -336,6 +333,7 @@ lets_do_this! {
 
     ExchangeMallocFnLangItem,        "exchange_malloc",         exchange_malloc_fn;
     ExchangeFreeFnLangItem,          "exchange_free",           exchange_free_fn;
+    BoxFreeFnLangItem,               "box_free",                box_free_fn;
     StrDupUniqFnLangItem,            "strdup_uniq",             strdup_uniq_fn;
 
     StartFnLangItem,                 "start",                   start_fn;
diff --git a/src/librustc/middle/reachable.rs b/src/librustc/middle/reachable.rs
index 6373bfbc55e..6dd447e3b68 100644
--- a/src/librustc/middle/reachable.rs
+++ b/src/librustc/middle/reachable.rs
@@ -362,7 +362,7 @@ pub fn find_reachable(tcx: &ty::ctxt,
     for (id, _) in &access_levels.map {
         reachable_context.worklist.push(*id);
     }
-    for (_, item) in tcx.lang_items.items() {
+    for item in tcx.lang_items.items().iter() {
         if let Some(did) = *item {
             if let Some(node_id) = tcx.map.as_local_node_id(did) {
                 reachable_context.worklist.push(node_id);
diff --git a/src/librustc/mir/repr.rs b/src/librustc/mir/repr.rs
index 988bf2c76a8..783c58469a1 100644
--- a/src/librustc/mir/repr.rs
+++ b/src/librustc/mir/repr.rs
@@ -8,21 +8,20 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use graphviz::IntoCow;
 use middle::const_eval::ConstVal;
 use middle::def_id::DefId;
 use middle::subst::Substs;
 use middle::ty::{self, AdtDef, ClosureSubsts, FnOutput, Region, Ty};
 use rustc_back::slice;
-use rustc_data_structures::tuple_slice::TupleSlice;
 use rustc_front::hir::InlineAsm;
-use syntax::ast::{self, Name};
-use syntax::codemap::Span;
-use graphviz::IntoCow;
 use std::ascii;
-use std::borrow::Cow;
+use std::borrow::{Cow};
 use std::fmt::{self, Debug, Formatter, Write};
 use std::{iter, u32};
 use std::ops::{Index, IndexMut};
+use syntax::ast::{self, Name};
+use syntax::codemap::Span;
 
 /// Lowered representation of a single function.
 #[derive(Clone, RustcEncodable, RustcDecodable)]
@@ -263,101 +262,63 @@ pub enum Terminator<'tcx> {
     /// `END_BLOCK`.
     Return,
 
+    /// Drop the Lvalue
+    Drop {
+        value: Lvalue<'tcx>,
+        target: BasicBlock,
+        unwind: Option<BasicBlock>
+    },
+
     /// Block ends with a call of a converging function
     Call {
         /// The function that’s being called
         func: Operand<'tcx>,
         /// Arguments the function is called with
         args: Vec<Operand<'tcx>>,
-        /// The kind of call with associated information
-        kind: CallKind<'tcx>,
+        /// Destination for the return value. If some, the call is converging.
+        destination: Option<(Lvalue<'tcx>, BasicBlock)>,
+        /// Cleanups to be done if the call unwinds.
+        cleanup: Option<BasicBlock>
     },
 }
 
-#[derive(Clone, RustcEncodable, RustcDecodable)]
-pub enum CallKind<'tcx> {
-    /// Diverging function without associated cleanup
-    Diverging,
-    /// Diverging function with associated cleanup
-    DivergingCleanup(BasicBlock),
-    /// Converging function without associated cleanup
-    Converging {
-        /// Destination where the call result is written
-        destination: Lvalue<'tcx>,
-        /// Block to branch into on successful return
-        target: BasicBlock,
-    },
-    ConvergingCleanup {
-        /// Destination where the call result is written
-        destination: Lvalue<'tcx>,
-        /// First target is branched to on successful return.
-        /// Second block contains the cleanups to do on unwind.
-        targets: (BasicBlock, BasicBlock)
-    }
-}
-
-impl<'tcx> CallKind<'tcx> {
-    pub fn successors(&self) -> &[BasicBlock] {
-        match *self {
-            CallKind::Diverging => &[],
-            CallKind::DivergingCleanup(ref b) |
-            CallKind::Converging { target: ref b, .. } => slice::ref_slice(b),
-            CallKind::ConvergingCleanup { ref targets, .. } => targets.as_slice(),
-        }
-    }
-
-    pub fn successors_mut(&mut self) -> &mut [BasicBlock] {
-        match *self {
-            CallKind::Diverging => &mut [],
-            CallKind::DivergingCleanup(ref mut b) |
-            CallKind::Converging { target: ref mut b, .. } => slice::mut_ref_slice(b),
-            CallKind::ConvergingCleanup { ref mut targets, .. } => targets.as_mut_slice(),
-        }
-    }
-
-    pub fn destination(&self) -> Option<&Lvalue<'tcx>> {
-        match *self {
-            CallKind::Converging { ref destination, .. } |
-            CallKind::ConvergingCleanup { ref destination, .. } => Some(destination),
-            CallKind::Diverging |
-            CallKind::DivergingCleanup(_) => None
-        }
-    }
-
-    pub fn destination_mut(&mut self) -> Option<&mut Lvalue<'tcx>> {
-        match *self {
-            CallKind::Converging { ref mut destination, .. } |
-            CallKind::ConvergingCleanup { ref mut destination, .. } => Some(destination),
-            CallKind::Diverging |
-            CallKind::DivergingCleanup(_) => None
-        }
-    }
-}
-
 impl<'tcx> Terminator<'tcx> {
-    pub fn successors(&self) -> &[BasicBlock] {
+    pub fn successors(&self) -> Cow<[BasicBlock]> {
         use self::Terminator::*;
         match *self {
-            Goto { target: ref b } => slice::ref_slice(b),
-            If { targets: ref b, .. } => b.as_slice(),
-            Switch { targets: ref b, .. } => b,
-            SwitchInt { targets: ref b, .. } => b,
-            Resume => &[],
-            Return => &[],
-            Call { ref kind, .. } => kind.successors(),
+            Goto { target: ref b } => slice::ref_slice(b).into_cow(),
+            If { targets: (b1, b2), .. } => vec![b1, b2].into_cow(),
+            Switch { targets: ref b, .. } => b[..].into_cow(),
+            SwitchInt { targets: ref b, .. } => b[..].into_cow(),
+            Resume => (&[]).into_cow(),
+            Return => (&[]).into_cow(),
+            Call { destination: Some((_, t)), cleanup: Some(c), .. } => vec![t, c].into_cow(),
+            Call { destination: Some((_, ref t)), cleanup: None, .. } =>
+                slice::ref_slice(t).into_cow(),
+            Call { destination: None, cleanup: Some(ref c), .. } => slice::ref_slice(c).into_cow(),
+            Call { destination: None, cleanup: None, .. } => (&[]).into_cow(),
+            Drop { target, unwind: Some(unwind), .. } => vec![target, unwind].into_cow(),
+            Drop { ref target, .. } => slice::ref_slice(target).into_cow(),
         }
     }
 
-    pub fn successors_mut(&mut self) -> &mut [BasicBlock] {
+    // FIXME: no mootable cow. I’m honestly not sure what a “cow” between `&mut [BasicBlock]` and
+    // `Vec<&mut BasicBlock>` would look like in the first place.
+    pub fn successors_mut(&mut self) -> Vec<&mut BasicBlock> {
         use self::Terminator::*;
         match *self {
-            Goto { target: ref mut b } => slice::mut_ref_slice(b),
-            If { targets: ref mut b, .. } => b.as_mut_slice(),
-            Switch { targets: ref mut b, .. } => b,
-            SwitchInt { targets: ref mut b, .. } => b,
-            Resume => &mut [],
-            Return => &mut [],
-            Call { ref mut kind, .. } => kind.successors_mut(),
+            Goto { target: ref mut b } => vec![b],
+            If { targets: (ref mut b1, ref mut b2), .. } => vec![b1, b2],
+            Switch { targets: ref mut b, .. } => b.iter_mut().collect(),
+            SwitchInt { targets: ref mut b, .. } => b.iter_mut().collect(),
+            Resume => Vec::new(),
+            Return => Vec::new(),
+            Call { destination: Some((_, ref mut t)), cleanup: Some(ref mut c), .. } => vec![t, c],
+            Call { destination: Some((_, ref mut t)), cleanup: None, .. } => vec![t],
+            Call { destination: None, cleanup: Some(ref mut c), .. } => vec![c],
+            Call { destination: None, cleanup: None, .. } => vec![],
+            Drop { ref mut target, unwind: Some(ref mut unwind), .. } => vec![target, unwind],
+            Drop { ref mut target, .. } => vec![target]
         }
     }
 }
@@ -424,8 +385,9 @@ impl<'tcx> Terminator<'tcx> {
             SwitchInt { discr: ref lv, .. } => write!(fmt, "switchInt({:?})", lv),
             Return => write!(fmt, "return"),
             Resume => write!(fmt, "resume"),
-            Call { ref kind, ref func, ref args } => {
-                if let Some(destination) = kind.destination() {
+            Drop { ref value, .. } => write!(fmt, "drop({:?})", value),
+            Call { ref func, ref args, ref destination, .. } => {
+                if let Some((ref destination, _)) = *destination {
                     try!(write!(fmt, "{:?} = ", destination));
                 }
                 try!(write!(fmt, "{:?}(", func));
@@ -445,12 +407,12 @@ impl<'tcx> Terminator<'tcx> {
         use self::Terminator::*;
         match *self {
             Return | Resume => vec![],
-            Goto { .. } => vec!["".into_cow()],
-            If { .. } => vec!["true".into_cow(), "false".into_cow()],
+            Goto { .. } => vec!["".into()],
+            If { .. } => vec!["true".into(), "false".into()],
             Switch { ref adt_def, .. } => {
                 adt_def.variants
                        .iter()
-                       .map(|variant| variant.name.to_string().into_cow())
+                       .map(|variant| variant.name.to_string().into())
                        .collect()
             }
             SwitchInt { ref values, .. } => {
@@ -458,21 +420,18 @@ impl<'tcx> Terminator<'tcx> {
                       .map(|const_val| {
                           let mut buf = String::new();
                           fmt_const_val(&mut buf, const_val).unwrap();
-                          buf.into_cow()
+                          buf.into()
                       })
-                      .chain(iter::once(String::from("otherwise").into_cow()))
+                      .chain(iter::once(String::from("otherwise").into()))
                       .collect()
             }
-            Call { ref kind, .. } => match *kind {
-                CallKind::Diverging =>
-                    vec![],
-                CallKind::DivergingCleanup(..) =>
-                    vec!["unwind".into_cow()],
-                CallKind::Converging { .. } =>
-                    vec!["return".into_cow()],
-                CallKind::ConvergingCleanup { .. } =>
-                    vec!["return".into_cow(), "unwind".into_cow()],
-            },
+            Call { destination: Some(_), cleanup: Some(_), .. } =>
+                vec!["return".into_cow(), "unwind".into_cow()],
+            Call { destination: Some(_), cleanup: None, .. } => vec!["return".into_cow()],
+            Call { destination: None, cleanup: Some(_), .. } => vec!["unwind".into_cow()],
+            Call { destination: None, cleanup: None, .. } => vec![],
+            Drop { unwind: None, .. } => vec!["return".into_cow()],
+            Drop { .. } => vec!["return".into_cow(), "unwind".into_cow()],
         }
     }
 }
@@ -490,23 +449,13 @@ pub struct Statement<'tcx> {
 #[derive(Clone, Debug, RustcEncodable, RustcDecodable)]
 pub enum StatementKind<'tcx> {
     Assign(Lvalue<'tcx>, Rvalue<'tcx>),
-    Drop(DropKind, Lvalue<'tcx>),
-}
-
-#[derive(Copy, Clone, Debug, PartialEq, Eq, RustcEncodable, RustcDecodable)]
-pub enum DropKind {
-    /// free a partially constructed box, should go away eventually
-    Free,
-    Deep
 }
 
 impl<'tcx> Debug for Statement<'tcx> {
     fn fmt(&self, fmt: &mut Formatter) -> fmt::Result {
         use self::StatementKind::*;
         match self.kind {
-            Assign(ref lv, ref rv) => write!(fmt, "{:?} = {:?}", lv, rv),
-            Drop(DropKind::Free, ref lv) => write!(fmt, "free {:?}", lv),
-            Drop(DropKind::Deep, ref lv) => write!(fmt, "drop {:?}", lv),
+            Assign(ref lv, ref rv) => write!(fmt, "{:?} = {:?}", lv, rv)
         }
     }
 }
diff --git a/src/librustc/mir/visit.rs b/src/librustc/mir/visit.rs
index a38ef078c6f..fb4e0e97054 100644
--- a/src/librustc/mir/visit.rs
+++ b/src/librustc/mir/visit.rs
@@ -124,9 +124,6 @@ macro_rules! make_mir_visitor {
                                           ref $($mutability)* rvalue) => {
                         self.visit_assign(block, lvalue, rvalue);
                     }
-                    StatementKind::Drop(_, ref $($mutability)* lvalue) => {
-                        self.visit_lvalue(lvalue, LvalueContext::Drop);
-                    }
                 }
             }
 
@@ -177,30 +174,25 @@ macro_rules! make_mir_visitor {
                     Terminator::Return => {
                     }
 
+                    Terminator::Drop { ref $($mutability)* value, target, unwind } => {
+                        self.visit_lvalue(value, LvalueContext::Drop);
+                        self.visit_branch(block, target);
+                        unwind.map(|t| self.visit_branch(block, t));
+                    }
+
                     Terminator::Call { ref $($mutability)* func,
                                        ref $($mutability)* args,
-                                       ref $($mutability)* kind } => {
+                                       ref $($mutability)* destination,
+                                       cleanup } => {
                         self.visit_operand(func);
                         for arg in args {
                             self.visit_operand(arg);
                         }
-                        match *kind {
-                            CallKind::Converging {
-                                ref $($mutability)* destination,
-                                ..
-                            }        |
-                            CallKind::ConvergingCleanup {
-                                ref $($mutability)* destination,
-                                ..
-                            } => {
-                                self.visit_lvalue(destination, LvalueContext::Store);
-                            }
-                            CallKind::Diverging           |
-                            CallKind::DivergingCleanup(_) => {}
-                        }
-                        for &target in kind.successors() {
+                        if let Some((ref $($mutability)* destination, target)) = *destination {
+                            self.visit_lvalue(destination, LvalueContext::Store);
                             self.visit_branch(block, target);
                         }
+                        cleanup.map(|t| self.visit_branch(block, t));
                     }
                 }
             }
diff --git a/src/librustc_metadata/encoder.rs b/src/librustc_metadata/encoder.rs
index 1e50868f664..8df72016078 100644
--- a/src/librustc_metadata/encoder.rs
+++ b/src/librustc_metadata/encoder.rs
@@ -1649,7 +1649,7 @@ fn encode_crate_deps(rbml_w: &mut Encoder, cstore: &cstore::CStore) {
 fn encode_lang_items(ecx: &EncodeContext, rbml_w: &mut Encoder) {
     rbml_w.start_tag(tag_lang_items);
 
-    for (i, &opt_def_id) in ecx.tcx.lang_items.items() {
+    for (i, &opt_def_id) in ecx.tcx.lang_items.items().iter().enumerate() {
         if let Some(def_id) = opt_def_id {
             if def_id.is_local() {
                 rbml_w.start_tag(tag_lang_items_item);
diff --git a/src/librustc_mir/build/cfg.rs b/src/librustc_mir/build/cfg.rs
index 523ac85cdc5..c7147d111aa 100644
--- a/src/librustc_mir/build/cfg.rs
+++ b/src/librustc_mir/build/cfg.rs
@@ -32,19 +32,17 @@ impl<'tcx> CFG<'tcx> {
         BasicBlock::new(node_index)
     }
 
+    pub fn start_new_cleanup_block(&mut self) -> BasicBlock {
+        let bb = self.start_new_block();
+        self.block_data_mut(bb).is_cleanup = true;
+        bb
+    }
+
     pub fn push(&mut self, block: BasicBlock, statement: Statement<'tcx>) {
         debug!("push({:?}, {:?})", block, statement);
         self.block_data_mut(block).statements.push(statement);
     }
 
-    pub fn push_drop(&mut self, block: BasicBlock, span: Span,
-                     kind: DropKind, lvalue: &Lvalue<'tcx>) {
-        self.push(block, Statement {
-            span: span,
-            kind: StatementKind::Drop(kind, lvalue.clone())
-        });
-    }
-
     pub fn push_assign(&mut self,
                        block: BasicBlock,
                        span: Span,
@@ -81,4 +79,3 @@ impl<'tcx> CFG<'tcx> {
         self.block_data_mut(block).terminator = Some(terminator);
     }
 }
-
diff --git a/src/librustc_mir/build/expr/as_rvalue.rs b/src/librustc_mir/build/expr/as_rvalue.rs
index e556d707a2d..dea2d750b98 100644
--- a/src/librustc_mir/build/expr/as_rvalue.rs
+++ b/src/librustc_mir/build/expr/as_rvalue.rs
@@ -59,25 +59,18 @@ impl<'a,'tcx> Builder<'a,'tcx> {
                 let arg = unpack!(block = this.as_operand(block, arg));
                 block.and(Rvalue::UnaryOp(op, arg))
             }
-            ExprKind::Box { value } => {
+            ExprKind::Box { value, value_extents } => {
                 let value = this.hir.mirror(value);
                 let result = this.temp(expr.ty);
-
                 // to start, malloc some memory of suitable type (thus far, uninitialized):
-                let rvalue = Rvalue::Box(value.ty);
-                this.cfg.push_assign(block, expr_span, &result, rvalue);
-
-                // schedule a shallow free of that memory, lest we unwind:
-                let extent = this.extent_of_innermost_scope();
-                this.schedule_drop(expr_span, extent, DropKind::Free, &result, value.ty);
-
-                // initialize the box contents:
-                let contents = result.clone().deref();
-                unpack!(block = this.into(&contents, block, value));
-
-                // now that the result is fully initialized, cancel the drop
-                // by "using" the result (which is linear):
-                block.and(Rvalue::Use(Operand::Consume(result)))
+                this.cfg.push_assign(block, expr_span, &result, Rvalue::Box(value.ty));
+                this.in_scope(value_extents, block, |this| {
+                    // schedule a shallow free of that memory, lest we unwind:
+                    this.schedule_box_free(expr_span, value_extents, &result, value.ty);
+                    // initialize the box contents:
+                    unpack!(block = this.into(&result.clone().deref(), block, value));
+                    block.and(Rvalue::Use(Operand::Consume(result)))
+                })
             }
             ExprKind::Cast { source } => {
                 let source = unpack!(block = this.as_operand(block, source));
diff --git a/src/librustc_mir/build/expr/as_temp.rs b/src/librustc_mir/build/expr/as_temp.rs
index 53f8090ad0f..27c374e1ac2 100644
--- a/src/librustc_mir/build/expr/as_temp.rs
+++ b/src/librustc_mir/build/expr/as_temp.rs
@@ -41,7 +41,7 @@ impl<'a,'tcx> Builder<'a,'tcx> {
                 this.hir.span_bug(expr.span, "no temp_lifetime for expr");
             }
         };
-        this.schedule_drop(expr.span, temp_lifetime, DropKind::Deep, &temp, expr_ty);
+        this.schedule_drop(expr.span, temp_lifetime, &temp, expr_ty);
 
         // Careful here not to cause an infinite cycle. If we always
         // called `into`, then for lvalues like `x.f`, it would
diff --git a/src/librustc_mir/build/expr/into.rs b/src/librustc_mir/build/expr/into.rs
index 4261efe8215..f50e5689df0 100644
--- a/src/librustc_mir/build/expr/into.rs
+++ b/src/librustc_mir/build/expr/into.rs
@@ -188,7 +188,7 @@ impl<'a,'tcx> Builder<'a,'tcx> {
                 // operators like x[j] = x[i].
                 let rhs = unpack!(block = this.as_operand(block, rhs));
                 let lhs = unpack!(block = this.as_lvalue(block, lhs));
-                this.cfg.push_drop(block, expr_span, DropKind::Deep, &lhs);
+                unpack!(block = this.build_drop(block, lhs.clone()));
                 this.cfg.push_assign(block, expr_span, &lhs, Rvalue::Use(rhs));
                 block.unit()
             }
@@ -253,17 +253,11 @@ impl<'a,'tcx> Builder<'a,'tcx> {
                 this.cfg.terminate(block, Terminator::Call {
                     func: fun,
                     args: args,
-                    kind: match (cleanup, diverges) {
-                        (None, true) => CallKind::Diverging,
-                        (Some(c), true) => CallKind::DivergingCleanup(c),
-                        (None, false) => CallKind::Converging {
-                            destination: destination.clone(),
-                            target: success
-                        },
-                        (Some(c), false) => CallKind::ConvergingCleanup {
-                            destination: destination.clone(),
-                            targets: (success, c)
-                        }
+                    cleanup: cleanup,
+                    destination: if diverges {
+                        None
+                    } else {
+                        Some ((destination.clone(), success))
                     }
                 });
                 success.unit()
diff --git a/src/librustc_mir/build/matches/mod.rs b/src/librustc_mir/build/matches/mod.rs
index c2c87fcbd20..e6430b7d634 100644
--- a/src/librustc_mir/build/matches/mod.rs
+++ b/src/librustc_mir/build/matches/mod.rs
@@ -601,7 +601,7 @@ impl<'a,'tcx> Builder<'a,'tcx> {
             ty: var_ty.clone(),
         });
         let index = index as u32;
-        self.schedule_drop(span, var_extent, DropKind::Deep, &Lvalue::Var(index), var_ty);
+        self.schedule_drop(span, var_extent, &Lvalue::Var(index), var_ty);
         self.var_indices.insert(var_id, index);
 
         debug!("declare_binding: index={:?}", index);
diff --git a/src/librustc_mir/build/scope.rs b/src/librustc_mir/build/scope.rs
index dae525c84d0..27654127a6c 100644
--- a/src/librustc_mir/build/scope.rs
+++ b/src/librustc_mir/build/scope.rs
@@ -86,10 +86,10 @@ should go to.
 
 */
 
-use build::{BlockAnd, BlockAndExtension, Builder};
+use build::{BlockAnd, BlockAndExtension, Builder, CFG};
 use rustc::middle::region::CodeExtent;
 use rustc::middle::lang_items;
-use rustc::middle::subst::Substs;
+use rustc::middle::subst::{Substs, Subst, VecPerParamSpace};
 use rustc::middle::ty::{self, Ty};
 use rustc::mir::repr::*;
 use syntax::codemap::{Span, DUMMY_SP};
@@ -97,8 +97,37 @@ use syntax::parse::token::intern_and_get_ident;
 
 pub struct Scope<'tcx> {
     extent: CodeExtent,
-    drops: Vec<(DropKind, Span, Lvalue<'tcx>)>,
-    cached_block: Option<BasicBlock>,
+    drops: Vec<DropData<'tcx>>,
+    // A scope may only have one associated free, because:
+    // 1. We require a `free` to only be scheduled in the scope of `EXPR` in `box EXPR`;
+    // 2. It only makes sense to have it translated into the diverge-path.
+    //
+    // This kind of drop will be run *after* all the regular drops scheduled onto this scope,
+    // because drops may have dependencies on the allocated memory.
+    //
+    // This is expected to go away once `box EXPR` becomes a sugar for placement protocol and gets
+    // desugared in some earlier stage.
+    free: Option<FreeData<'tcx>>,
+}
+
+struct DropData<'tcx> {
+    value: Lvalue<'tcx>,
+    // NB: per-drop “cache” is necessary for the build_scope_drops function below.
+    /// The cached block for the cleanups-on-diverge path. This block contains code to run the
+    /// current drop and all the preceding drops (i.e. those having lower index in Drop’s
+    /// Scope drop array)
+    cached_block: Option<BasicBlock>
+}
+
+struct FreeData<'tcx> {
+    span: Span,
+    /// Lvalue containing the allocated box.
+    value: Lvalue<'tcx>,
+    /// type of item for which the box was allocated for (i.e. the T in Box<T>).
+    item_ty: Ty<'tcx>,
+    /// The cached block containing code to run the free. The block will also execute all the drops
+    /// in the scope.
+    cached_block: Option<BasicBlock>
 }
 
 #[derive(Clone, Debug)]
@@ -114,7 +143,38 @@ pub struct LoopScope {
     pub might_break: bool
 }
 
+impl<'tcx> Scope<'tcx> {
+    /// Invalidate all the cached blocks in the scope.
+    ///
+    /// Should always be run for all inner scopes when a drop is pushed into some scope enclosing a
+    /// larger extent of code.
+    fn invalidate_cache(&mut self) {
+        for dropdata in &mut self.drops {
+            dropdata.cached_block = None;
+        }
+        if let Some(ref mut freedata) = self.free {
+            freedata.cached_block = None;
+        }
+    }
+
+    /// Returns the cached block for this scope.
+    ///
+    /// Precondition: the caches must be fully filled (i.e. diverge_cleanup is called) in order for
+    /// this method to work correctly.
+    fn cached_block(&self) -> Option<BasicBlock> {
+        if let Some(data) = self.drops.last() {
+            Some(data.cached_block.expect("drop cache is not filled"))
+        } else if let Some(ref data) = self.free {
+            Some(data.cached_block.expect("free cache is not filled"))
+        } else {
+            None
+        }
+    }
+}
+
 impl<'a,'tcx> Builder<'a,'tcx> {
+    // Adding and removing scopes
+    // ==========================
     /// Start a loop scope, which tracks where `continue` and `break`
     /// should branch to. See module comment for more details.
     ///
@@ -146,9 +206,9 @@ impl<'a,'tcx> Builder<'a,'tcx> {
         where F: FnOnce(&mut Builder<'a, 'tcx>) -> BlockAnd<R>
     {
         debug!("in_scope(extent={:?}, block={:?})", extent, block);
-        self.push_scope(extent, block);
+        self.push_scope(extent);
         let rv = unpack!(block = f(self));
-        self.pop_scope(extent, block);
+        unpack!(block = self.pop_scope(extent, block));
         debug!("in_scope: exiting extent={:?} block={:?}", extent, block);
         block.and(rv)
     }
@@ -157,35 +217,51 @@ impl<'a,'tcx> Builder<'a,'tcx> {
     /// scope and call `pop_scope` afterwards. Note that these two
     /// calls must be paired; using `in_scope` as a convenience
     /// wrapper maybe preferable.
-    pub fn push_scope(&mut self, extent: CodeExtent, block: BasicBlock) {
-        debug!("push_scope({:?}, {:?})", extent, block);
-
-        // push scope, execute `f`, then pop scope again
+    pub fn push_scope(&mut self, extent: CodeExtent) {
+        debug!("push_scope({:?})", extent);
         self.scopes.push(Scope {
             extent: extent.clone(),
             drops: vec![],
-            cached_block: None,
+            free: None
         });
     }
 
     /// Pops a scope, which should have extent `extent`, adding any
     /// drops onto the end of `block` that are needed.  This must
     /// match 1-to-1 with `push_scope`.
-    pub fn pop_scope(&mut self, extent: CodeExtent, block: BasicBlock) {
+    pub fn pop_scope(&mut self, extent: CodeExtent, block: BasicBlock) -> BlockAnd<()> {
         debug!("pop_scope({:?}, {:?})", extent, block);
+        // We need to have `cached_block`s available for all the drops, so we call diverge_cleanup
+        // to make sure all the `cached_block`s are filled in.
+        self.diverge_cleanup();
         let scope = self.scopes.pop().unwrap();
-
         assert_eq!(scope.extent, extent);
+        build_scope_drops(block, &scope, &self.scopes[..], &mut self.cfg)
+    }
+
+
+    /// Branch out of `block` to `target`, exiting all scopes up to
+    /// and including `extent`.  This will insert whatever drops are
+    /// needed, as well as tracking this exit for the SEME region. See
+    /// module comment for details.
+    pub fn exit_scope(&mut self,
+                      span: Span,
+                      extent: CodeExtent,
+                      mut block: BasicBlock,
+                      target: BasicBlock) {
+        let scope_count = 1 + self.scopes.iter().rev().position(|scope| scope.extent == extent)
+                                                      .unwrap_or_else(||{
+            self.hir.span_bug(span, &format!("extent {:?} does not enclose", extent))
+        });
 
-        // add in any drops needed on the fallthrough path (any other
-        // exiting paths, such as those that arise from `break`, will
-        // have drops already)
-        for (kind, span, lvalue) in scope.drops {
-            self.cfg.push_drop(block, span, kind, &lvalue);
+        for (idx, ref scope) in self.scopes.iter().enumerate().rev().take(scope_count) {
+            unpack!(block = build_scope_drops(block, scope, &self.scopes[..idx], &mut self.cfg));
         }
+        self.cfg.terminate(block, Terminator::Goto { target: target });
     }
 
-
+    // Finding scopes
+    // ==============
     /// Finds the loop scope for a given label. This is used for
     /// resolving `break` and `continue`.
     pub fn find_loop_scope(&mut self,
@@ -208,30 +284,81 @@ impl<'a,'tcx> Builder<'a,'tcx> {
         }.unwrap_or_else(|| hir.span_bug(span, "no enclosing loop scope found?"))
     }
 
-    /// Branch out of `block` to `target`, exiting all scopes up to
-    /// and including `extent`.  This will insert whatever drops are
-    /// needed, as well as tracking this exit for the SEME region. See
-    /// module comment for details.
-    pub fn exit_scope(&mut self,
-                      span: Span,
-                      extent: CodeExtent,
-                      block: BasicBlock,
-                      target: BasicBlock) {
-        let Builder { ref mut scopes, ref mut cfg, ref mut hir, .. } = *self;
+    pub fn extent_of_innermost_scope(&self) -> CodeExtent {
+        self.scopes.last().map(|scope| scope.extent).unwrap()
+    }
 
-        let scope_count = 1 + scopes.iter().rev().position(|scope| scope.extent == extent)
-                                                 .unwrap_or_else(||{
-            hir.span_bug(span, &format!("extent {:?} does not enclose", extent))
-        });
+    pub fn extent_of_outermost_scope(&self) -> CodeExtent {
+        self.scopes.first().map(|scope| scope.extent).unwrap()
+    }
+
+    // Scheduling drops
+    // ================
+    /// Indicates that `lvalue` should be dropped on exit from
+    /// `extent`.
+    pub fn schedule_drop(&mut self,
+                         span: Span,
+                         extent: CodeExtent,
+                         lvalue: &Lvalue<'tcx>,
+                         lvalue_ty: Ty<'tcx>) {
+        if !self.hir.needs_drop(lvalue_ty) {
+            return
+        }
+        for scope in self.scopes.iter_mut().rev() {
+            if scope.extent == extent {
+                // No need to invalidate any caches here. The just-scheduled drop will branch into
+                // the drop that comes before it in the vector.
+                scope.drops.push(DropData {
+                    value: lvalue.clone(),
+                    cached_block: None
+                });
+                return;
+            } else {
+                // We must invalidate all the cached_blocks leading up to the scope we’re
+                // looking for, because all of the blocks in the chain will become incorrect.
+                scope.invalidate_cache()
+            }
+        }
+        self.hir.span_bug(span,
+                          &format!("extent {:?} not in scope to drop {:?}", extent, lvalue));
+    }
 
-        for scope in scopes.iter_mut().rev().take(scope_count) {
-            for &(kind, drop_span, ref lvalue) in &scope.drops {
-                cfg.push_drop(block, drop_span, kind, lvalue);
+    /// Schedule dropping of a not-yet-fully-initialised box.
+    ///
+    /// This cleanup will only be translated into unwind branch.
+    /// The extent should be for the `EXPR` inside `box EXPR`.
+    /// There may only be one “free” scheduled in any given scope.
+    pub fn schedule_box_free(&mut self,
+                             span: Span,
+                             extent: CodeExtent,
+                             value: &Lvalue<'tcx>,
+                             item_ty: Ty<'tcx>) {
+        for scope in self.scopes.iter_mut().rev() {
+            if scope.extent == extent {
+                assert!(scope.free.is_none(), "scope already has a scheduled free!");
+                // We also must invalidate the caches in the scope for which the free is scheduled
+                // because the drops must branch into the free we schedule here.
+                scope.invalidate_cache();
+                scope.free = Some(FreeData {
+                    span: span,
+                    value: value.clone(),
+                    item_ty: item_ty,
+                    cached_block: None
+                });
+                return;
+            } else {
+                // We must invalidate all the cached_blocks leading up to the scope we’re looking
+                // for, because otherwise some/most of the blocks in the chain will become
+                // incorrect.
+                scope.invalidate_cache();
             }
         }
-        cfg.terminate(block, Terminator::Goto { target: target });
+        self.hir.span_bug(span,
+                          &format!("extent {:?} not in scope to free {:?}", extent, value));
     }
 
+    // Other
+    // =====
     /// Creates a path that performs all required cleanup for unwinding.
     ///
     /// This path terminates in Resume. Returns the start of the path.
@@ -241,64 +368,40 @@ impl<'a,'tcx> Builder<'a,'tcx> {
         if self.scopes.is_empty() {
             return None;
         }
+        let unit_temp = self.get_unit_temp();
+        let Builder { ref mut hir, ref mut cfg, ref mut scopes, .. } = *self;
+        let mut next_block = None;
 
-        let mut terminator = Terminator::Resume;
         // Given an array of scopes, we generate these from the outermost scope to the innermost
         // one. Thus for array [S0, S1, S2] with corresponding cleanup blocks [B0, B1, B2], we will
-        // generate B0 <- B1 <- B2 in left-to-right order. The outermost scope (B0) will always
-        // terminate with a Resume terminator.
-        for scope in self.scopes.iter_mut().filter(|s| !s.drops.is_empty()) {
-            if let Some(b) = scope.cached_block {
-                terminator = Terminator::Goto { target: b };
-                continue;
-            } else {
-                let new_block = self.cfg.start_new_block();
-                self.cfg.block_data_mut(new_block).is_cleanup = true;
-                self.cfg.terminate(new_block, terminator);
-                terminator = Terminator::Goto { target: new_block };
-                for &(kind, span, ref lvalue) in scope.drops.iter().rev() {
-                    self.cfg.push_drop(new_block, span, kind, lvalue);
-                }
-                scope.cached_block = Some(new_block);
-            }
+        // generate B0 <- B1 <- B2 in left-to-right order. Control flow of the generated blocks
+        // always ends up at a block with the Resume terminator.
+        for scope in scopes.iter_mut().filter(|s| !s.drops.is_empty() || s.free.is_some()) {
+            next_block = Some(build_diverge_scope(hir.tcx(),
+                                                  cfg,
+                                                  unit_temp.clone(),
+                                                  scope,
+                                                  next_block));
         }
-        // Return the innermost cached block, most likely the one we just generated.
-        // Note that if there are no cleanups in scope we return None.
-        self.scopes.iter().rev().flat_map(|b| b.cached_block).next()
+        scopes.iter().rev().flat_map(|x| x.cached_block()).next()
     }
 
-    /// Indicates that `lvalue` should be dropped on exit from
-    /// `extent`.
-    pub fn schedule_drop(&mut self,
-                         span: Span,
-                         extent: CodeExtent,
-                         kind: DropKind,
-                         lvalue: &Lvalue<'tcx>,
-                         lvalue_ty: Ty<'tcx>) {
-        if self.hir.needs_drop(lvalue_ty) {
-            for scope in self.scopes.iter_mut().rev() {
-                // We must invalidate all the cached_blocks leading up to the scope we’re looking
-                // for, because otherwise some/most of the blocks in the chain might become
-                // incorrect (i.e. they still are pointing at old cached_block).
-                scope.cached_block = None;
-                if scope.extent == extent {
-                    scope.drops.push((kind, span, lvalue.clone()));
-                    return;
-                }
-            }
-            self.hir.span_bug(span,
-                              &format!("extent {:?} not in scope to drop {:?}", extent, lvalue));
-        }
-    }
-
-    pub fn extent_of_innermost_scope(&self) -> CodeExtent {
-        self.scopes.last().map(|scope| scope.extent).unwrap()
+    /// Utility function for *non*-scope code to build their own drops
+    pub fn build_drop(&mut self, block: BasicBlock, value: Lvalue<'tcx>) -> BlockAnd<()> {
+        let next_target = self.cfg.start_new_block();
+        let diverge_target = self.diverge_cleanup();
+        self.cfg.terminate(block, Terminator::Drop {
+            value: value,
+            target: next_target,
+            unwind: diverge_target,
+        });
+        next_target.unit()
     }
 
-    pub fn extent_of_outermost_scope(&self) -> CodeExtent {
-        self.scopes.first().map(|scope| scope.extent).unwrap()
-    }
 
+    // Panicking
+    // =========
+    // FIXME: should be moved into their own module
     pub fn panic_bounds_check(&mut self,
                              block: BasicBlock,
                              index: Operand<'tcx>,
@@ -328,10 +431,8 @@ impl<'a,'tcx> Builder<'a,'tcx> {
         self.cfg.terminate(block, Terminator::Call {
             func: Operand::Constant(func),
             args: vec![Operand::Consume(tuple_ref), index, len],
-            kind: match cleanup {
-                None => CallKind::Diverging,
-                Some(c) => CallKind::DivergingCleanup(c)
-            }
+            destination: None,
+            cleanup: cleanup,
         });
     }
 
@@ -368,10 +469,8 @@ impl<'a,'tcx> Builder<'a,'tcx> {
         self.cfg.terminate(block, Terminator::Call {
             func: Operand::Constant(func),
             args: vec![Operand::Consume(tuple_ref)],
-            kind: match cleanup {
-                None => CallKind::Diverging,
-                Some(c) => CallKind::DivergingCleanup(c)
-            }
+            cleanup: cleanup,
+            destination: None,
         });
     }
 
@@ -405,4 +504,135 @@ impl<'a,'tcx> Builder<'a,'tcx> {
             literal: self.hir.usize_literal(span_lines.line)
         })
     }
+
+}
+
+/// Builds drops for pop_scope and exit_scope.
+fn build_scope_drops<'tcx>(mut block: BasicBlock,
+                           scope: &Scope<'tcx>,
+                           earlier_scopes: &[Scope<'tcx>],
+                           cfg: &mut CFG<'tcx>)
+                           -> BlockAnd<()> {
+    let mut iter = scope.drops.iter().rev().peekable();
+    while let Some(drop_data) = iter.next() {
+        // Try to find the next block with its cached block for us to diverge into in case the
+        // drop panics.
+        let on_diverge = iter.peek().iter().flat_map(|dd| dd.cached_block.into_iter()).next();
+        // If there’s no `cached_block`s within current scope, we must look for one in the
+        // enclosing scope.
+        let on_diverge = on_diverge.or_else(||{
+            earlier_scopes.iter().rev().flat_map(|s| s.cached_block()).next()
+        });
+        let next = cfg.start_new_block();
+        cfg.terminate(block, Terminator::Drop {
+            value: drop_data.value.clone(),
+            target: next,
+            unwind: on_diverge
+        });
+        block = next;
+    }
+    block.unit()
+}
+
+fn build_diverge_scope<'tcx>(tcx: &ty::ctxt<'tcx>,
+                             cfg: &mut CFG<'tcx>,
+                             unit_temp: Lvalue<'tcx>,
+                             scope: &mut Scope<'tcx>,
+                             target: Option<BasicBlock>)
+                             -> BasicBlock {
+    debug_assert!(!scope.drops.is_empty() || scope.free.is_some());
+
+    // First, we build the drops, iterating the drops array in reverse. We do that so that as soon
+    // as we find a `cached_block`, we know that we’re finished and don’t need to do anything else.
+    let mut previous = None;
+    let mut last_drop_block = None;
+    for drop_data in scope.drops.iter_mut().rev() {
+        if let Some(cached_block) = drop_data.cached_block {
+            if let Some((previous_block, previous_value)) = previous {
+                cfg.terminate(previous_block, Terminator::Drop {
+                    value: previous_value,
+                    target: cached_block,
+                    unwind: None
+                });
+                return last_drop_block.unwrap();
+            } else {
+                return cached_block;
+            }
+        } else {
+            let block = cfg.start_new_cleanup_block();
+            drop_data.cached_block = Some(block);
+            if let Some((previous_block, previous_value)) = previous {
+                cfg.terminate(previous_block, Terminator::Drop {
+                    value: previous_value,
+                    target: block,
+                    unwind: None
+                });
+            } else {
+                last_drop_block = Some(block);
+            }
+            previous = Some((block, drop_data.value.clone()));
+        }
+    }
+
+    // Prepare the end target for this chain.
+    let mut target = target.unwrap_or_else(||{
+        let b = cfg.start_new_cleanup_block();
+        cfg.terminate(b, Terminator::Resume);
+        b
+    });
+
+    // Then, build the free branching into the prepared target.
+    if let Some(ref mut free_data) = scope.free {
+        target = if let Some(cached_block) = free_data.cached_block {
+            cached_block
+        } else {
+            let t = build_free(tcx, cfg, unit_temp, free_data, target);
+            free_data.cached_block = Some(t);
+            t
+        }
+    };
+
+    if let Some((previous_block, previous_value)) = previous {
+        // Finally, branch into that just-built `target` from the `previous_block`.
+        cfg.terminate(previous_block, Terminator::Drop {
+            value: previous_value,
+            target: target,
+            unwind: None
+        });
+        last_drop_block.unwrap()
+    } else {
+        // If `previous.is_none()`, there were no drops in this scope – we return the
+        // target.
+        target
+    }
+}
+
+fn build_free<'tcx>(tcx: &ty::ctxt<'tcx>,
+                    cfg: &mut CFG<'tcx>,
+                    unit_temp: Lvalue<'tcx>,
+                    data: &FreeData<'tcx>,
+                    target: BasicBlock)
+                    -> BasicBlock {
+    let free_func = tcx.lang_items.box_free_fn()
+                       .expect("box_free language item is missing");
+    let substs = tcx.mk_substs(Substs::new(
+        VecPerParamSpace::new(vec![], vec![], vec![data.item_ty]),
+        VecPerParamSpace::new(vec![], vec![], vec![])
+    ));
+    let block = cfg.start_new_cleanup_block();
+    cfg.terminate(block, Terminator::Call {
+        func: Operand::Constant(Constant {
+            span: data.span,
+            ty: tcx.lookup_item_type(free_func).ty.subst(tcx, substs),
+            literal: Literal::Item {
+                def_id: free_func,
+                kind: ItemKind::Function,
+                substs: substs
+            }
+        }),
+        args: vec![Operand::Consume(data.value.clone())],
+        destination: Some((unit_temp, target)),
+        cleanup: None
+    });
+    block
 }
diff --git a/src/librustc_mir/build/stmt.rs b/src/librustc_mir/build/stmt.rs
index c70b22893ae..6c0f1c7081b 100644
--- a/src/librustc_mir/build/stmt.rs
+++ b/src/librustc_mir/build/stmt.rs
@@ -42,16 +42,16 @@ impl<'a,'tcx> Builder<'a,'tcx> {
                 None => {
                     let (extent, _) = stmt_lists.pop().unwrap();
                     if let Some(extent) = extent {
-                        this.pop_scope(extent, block);
+                        unpack!(block = this.pop_scope(extent, block));
                     }
                     continue
                 }
             };
 
-            let Stmt { span, kind } = this.hir.mirror(stmt);
+            let Stmt { span: _, kind } = this.hir.mirror(stmt);
             match kind {
                 StmtKind::Let { remainder_scope, init_scope, pattern, initializer, stmts } => {
-                    this.push_scope(remainder_scope, block);
+                    this.push_scope(remainder_scope);
                     stmt_lists.push((Some(remainder_scope), stmts.into_iter()));
                     unpack!(block = this.in_scope(init_scope, block, move |this| {
                         // FIXME #30046                              ^~~~
@@ -72,7 +72,7 @@ impl<'a,'tcx> Builder<'a,'tcx> {
                         let expr = this.hir.mirror(expr);
                         let temp = this.temp(expr.ty.clone());
                         unpack!(block = this.into(&temp, block, expr));
-                        this.cfg.push_drop(block, span, DropKind::Deep, &temp);
+                        unpack!(block = this.build_drop(block, temp));
                         block.unit()
                     }));
                 }
diff --git a/src/librustc_mir/hair/cx/expr.rs b/src/librustc_mir/hair/cx/expr.rs
index d8a1930fd5c..f9a9c99f63d 100644
--- a/src/librustc_mir/hair/cx/expr.rs
+++ b/src/librustc_mir/hair/cx/expr.rs
@@ -375,7 +375,10 @@ impl<'tcx> Mirror<'tcx> for &'tcx hir::Expr {
             hir::ExprType(ref source, _) =>
                 return source.make_mirror(cx),
             hir::ExprBox(ref value) =>
-                ExprKind::Box { value: value.to_ref() },
+                ExprKind::Box {
+                    value: value.to_ref(),
+                    value_extents: cx.tcx.region_maps.node_extent(value.id)
+                },
             hir::ExprVec(ref fields) =>
                 ExprKind::Vec { fields: fields.to_ref() },
             hir::ExprTup(ref fields) =>
diff --git a/src/librustc_mir/hair/mod.rs b/src/librustc_mir/hair/mod.rs
index b356e3886d1..0891c2845b0 100644
--- a/src/librustc_mir/hair/mod.rs
+++ b/src/librustc_mir/hair/mod.rs
@@ -123,6 +123,7 @@ pub enum ExprKind<'tcx> {
     },
     Box {
         value: ExprRef<'tcx>,
+        value_extents: CodeExtent,
     },
     Call {
         ty: ty::Ty<'tcx>,
diff --git a/src/librustc_mir/transform/erase_regions.rs b/src/librustc_mir/transform/erase_regions.rs
index 66604786d46..d0dbdeb033d 100644
--- a/src/librustc_mir/transform/erase_regions.rs
+++ b/src/librustc_mir/transform/erase_regions.rs
@@ -69,9 +69,6 @@ impl<'a, 'tcx> EraseRegions<'a, 'tcx> {
                 self.erase_regions_lvalue(lvalue);
                 self.erase_regions_rvalue(rvalue);
             }
-            StatementKind::Drop(_, ref mut lvalue) => {
-                self.erase_regions_lvalue(lvalue);
-            }
         }
     }
 
@@ -93,8 +90,11 @@ impl<'a, 'tcx> EraseRegions<'a, 'tcx> {
                 self.erase_regions_lvalue(discr);
                 *switch_ty = self.tcx.erase_regions(switch_ty);
             },
-            Terminator::Call { ref mut func, ref mut args, ref mut kind } => {
-                if let Some(destination) = kind.destination_mut() {
+            Terminator::Drop { ref mut value, .. } => {
+                self.erase_regions_lvalue(value);
+            }
+            Terminator::Call { ref mut func, ref mut args, ref mut destination, .. } => {
+                if let Some((ref mut destination, _)) = *destination {
                     self.erase_regions_lvalue(destination);
                 }
                 self.erase_regions_operand(func);
diff --git a/src/librustc_mir/transform/simplify_cfg.rs b/src/librustc_mir/transform/simplify_cfg.rs
index 7a5a00a8d56..9d4c73b90f8 100644
--- a/src/librustc_mir/transform/simplify_cfg.rs
+++ b/src/librustc_mir/transform/simplify_cfg.rs
@@ -29,7 +29,7 @@ impl SimplifyCfg {
 
         let mut worklist = vec![START_BLOCK];
         while let Some(bb) = worklist.pop() {
-            for succ in mir.basic_block_data(bb).terminator().successors() {
+            for succ in mir.basic_block_data(bb).terminator().successors().iter() {
                 if !seen[succ.index()] {
                     seen[succ.index()] = true;
                     worklist.push(*succ);
diff --git a/src/librustc_trans/trans/mir/block.rs b/src/librustc_trans/trans/mir/block.rs
index 5651710aa80..5be585c4189 100644
--- a/src/librustc_trans/trans/mir/block.rs
+++ b/src/librustc_trans/trans/mir/block.rs
@@ -18,10 +18,11 @@ use trans::base;
 use trans::build;
 use trans::common::{self, Block, LandingPad};
 use trans::debuginfo::DebugLoc;
+use trans::Disr;
 use trans::foreign;
+use trans::glue;
 use trans::type_of;
 use trans::type_::Type;
-use trans::Disr;
 
 use super::MirContext;
 use super::operand::OperandValue::{FatPtr, Immediate, Ref};
@@ -94,7 +95,39 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
                 base::build_return_block(bcx.fcx, bcx, return_ty, DebugLoc::None);
             }
 
-            mir::Terminator::Call { ref func, ref args, ref kind } => {
+            mir::Terminator::Drop { ref value, target, unwind } => {
+                let lvalue = self.trans_lvalue(bcx, value);
+                let ty = lvalue.ty.to_ty(bcx.tcx());
+                // Double check for necessity to drop
+                if !glue::type_needs_drop(bcx.tcx(), ty) {
+                    build::Br(bcx, self.llblock(target), DebugLoc::None);
+                    return;
+                }
+                let drop_fn = glue::get_drop_glue(bcx.ccx(), ty);
+                let drop_ty = glue::get_drop_glue_type(bcx.ccx(), ty);
+                let llvalue = if drop_ty != ty {
+                    build::PointerCast(bcx, lvalue.llval,
+                                       type_of::type_of(bcx.ccx(), drop_ty).ptr_to())
+                } else {
+                    lvalue.llval
+                };
+                if let Some(unwind) = unwind {
+                    let uwbcx = self.bcx(unwind);
+                    let unwind = self.make_landing_pad(uwbcx);
+                    build::Invoke(bcx,
+                                  drop_fn,
+                                  &[llvalue],
+                                  self.llblock(target),
+                                  unwind.llbb,
+                                  None,
+                                  DebugLoc::None);
+                } else {
+                    build::Call(bcx, drop_fn, &[llvalue], None, DebugLoc::None);
+                    build::Br(bcx, self.llblock(target), DebugLoc::None);
+                }
+            }
+
+            mir::Terminator::Call { ref func, ref args, ref destination, ref cleanup } => {
                 // Create the callee. This will always be a fn ptr and hence a kind of scalar.
                 let callee = self.trans_operand(bcx, func);
                 let attrs = attributes::from_fn_type(bcx.ccx(), callee.ty);
@@ -115,7 +148,7 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
                 };
 
                 // Prepare the return value destination
-                let (ret_dest_ty, must_copy_dest) = if let Some(d) = kind.destination() {
+                let (ret_dest_ty, must_copy_dest) = if let Some((ref d, _)) = *destination {
                     let dest = self.trans_lvalue(bcx, d);
                     let ret_ty = dest.ty.to_ty(bcx.tcx());
                     if !is_foreign && type_of::return_uses_outptr(bcx.ccx(), ret_ty) {
@@ -144,9 +177,9 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
                 }
 
                 // Many different ways to call a function handled here
-                match (is_foreign, base::avoid_invoke(bcx), kind) {
+                match (is_foreign, base::avoid_invoke(bcx), cleanup, destination) {
                     // The two cases below are the only ones to use LLVM’s `invoke`.
-                    (false, false, &mir::CallKind::DivergingCleanup(cleanup)) => {
+                    (false, false, &Some(cleanup), &None) => {
                         let cleanup = self.bcx(cleanup);
                         let landingpad = self.make_landing_pad(cleanup);
                         let unreachable_blk = self.unreachable_block();
@@ -158,14 +191,13 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
                                       Some(attrs),
                                       debugloc);
                     },
-                    (false, false, &mir::CallKind::ConvergingCleanup { ref targets, .. }) => {
-                        let cleanup = self.bcx(targets.1);
+                    (false, false, &Some(cleanup), &Some((_, success))) => {
+                        let cleanup = self.bcx(cleanup);
                         let landingpad = self.make_landing_pad(cleanup);
                         let (target, postinvoke) = if must_copy_dest {
-                            (bcx.fcx.new_block("", None),
-                             Some(self.bcx(targets.0)))
+                            (bcx.fcx.new_block("", None), Some(self.bcx(success)))
                         } else {
-                            (self.bcx(targets.0), None)
+                            (self.bcx(success), None)
                         };
                         let invokeret = build::Invoke(bcx,
                                                       callee.immediate(),
@@ -205,19 +237,11 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
                             build::Br(target, postinvoketarget.llbb, debugloc);
                         }
                     },
-                    (false, _, &mir::CallKind::DivergingCleanup(_)) |
-                    (false, _, &mir::CallKind::Diverging) => {
+                    (false, _, _, &None) => {
                         build::Call(bcx, callee.immediate(), &llargs[..], Some(attrs), debugloc);
                         build::Unreachable(bcx);
                     }
-                    (false, _, k@&mir::CallKind::ConvergingCleanup { .. }) |
-                    (false, _, k@&mir::CallKind::Converging { .. }) => {
-                        // FIXME: Bug #20046
-                        let target = match *k {
-                            mir::CallKind::ConvergingCleanup { targets, .. } => targets.0,
-                            mir::CallKind::Converging { target, .. } => target,
-                            _ => unreachable!()
-                        };
+                    (false, _, _, &Some((_, target))) => {
                         let llret = build::Call(bcx,
                                                 callee.immediate(),
                                                 &llargs[..],
@@ -231,7 +255,7 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
                         build::Br(bcx, self.llblock(target), debugloc);
                     }
                     // Foreign functions
-                    (true, _, k) => {
+                    (true, _, _, destination) => {
                         let (dest, _) = ret_dest_ty
                             .expect("return destination is not set");
                         bcx = foreign::trans_native_call(bcx,
@@ -241,13 +265,9 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
                                                    &llargs[..],
                                                    arg_tys,
                                                    debugloc);
-                        match *k {
-                            mir::CallKind::ConvergingCleanup { targets, .. } =>
-                                build::Br(bcx, self.llblock(targets.0), debugloc),
-                            mir::CallKind::Converging { target, .. } =>
-                                build::Br(bcx, self.llblock(target), debugloc),
-                            _ => ()
-                        };
+                        if let Some((_, target)) = *destination {
+                            build::Br(bcx, self.llblock(target), debugloc);
+                        }
                     },
                 }
             }
diff --git a/src/librustc_trans/trans/mir/constant.rs b/src/librustc_trans/trans/mir/constant.rs
index 84cc87e9b13..3b763599f77 100644
--- a/src/librustc_trans/trans/mir/constant.rs
+++ b/src/librustc_trans/trans/mir/constant.rs
@@ -82,13 +82,13 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
                           constant: &mir::Constant<'tcx>)
                           -> OperandRef<'tcx>
     {
-        let ty = bcx.monomorphize(&constant.ty);
         match constant.literal {
             mir::Literal::Item { def_id, kind, substs } => {
                 let substs = bcx.tcx().mk_substs(bcx.monomorphize(&substs));
-                self.trans_item_ref(bcx, ty, kind, substs, def_id)
+                self.trans_item_ref(bcx, constant.ty, kind, substs, def_id)
             }
             mir::Literal::Value { ref value } => {
+                let ty = bcx.monomorphize(&constant.ty);
                 self.trans_constval(bcx, value, ty)
             }
         }
diff --git a/src/librustc_trans/trans/mir/lvalue.rs b/src/librustc_trans/trans/mir/lvalue.rs
index a6ba069742d..d994f1ea7b0 100644
--- a/src/librustc_trans/trans/mir/lvalue.rs
+++ b/src/librustc_trans/trans/mir/lvalue.rs
@@ -65,7 +65,7 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
                 assert!(lvalue.llextra != ptr::null_mut());
                 lvalue.llextra
             }
-            _ => bcx.sess().bug("unexpected type in get_base_and_len"),
+            _ => bcx.sess().bug("unexpected type in lvalue_len"),
         }
     }
 
diff --git a/src/librustc_trans/trans/mir/mod.rs b/src/librustc_trans/trans/mir/mod.rs
index cfe48af35ca..b19ecc45a4e 100644
--- a/src/librustc_trans/trans/mir/mod.rs
+++ b/src/librustc_trans/trans/mir/mod.rs
@@ -195,8 +195,8 @@ fn arg_value_refs<'bcx, 'tcx>(bcx: Block<'bcx, 'tcx>,
 mod analyze;
 mod block;
 mod constant;
+mod did;
 mod lvalue;
-mod rvalue;
 mod operand;
+mod rvalue;
 mod statement;
-mod did;
diff --git a/src/librustc_trans/trans/mir/statement.rs b/src/librustc_trans/trans/mir/statement.rs
index dae0d3b55c0..fc888564737 100644
--- a/src/librustc_trans/trans/mir/statement.rs
+++ b/src/librustc_trans/trans/mir/statement.rs
@@ -8,11 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use rustc::middle::ty::LvaluePreference;
 use rustc::mir::repr as mir;
 use trans::common::Block;
-use trans::debuginfo::DebugLoc;
-use trans::glue;
 
 use super::MirContext;
 use super::TempRef;
@@ -51,20 +48,6 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
                     }
                 }
             }
-
-            mir::StatementKind::Drop(mir::DropKind::Deep, ref lvalue) => {
-                let tr_lvalue = self.trans_lvalue(bcx, lvalue);
-                let ty = tr_lvalue.ty.to_ty(bcx.tcx());
-                glue::drop_ty(bcx, tr_lvalue.llval, ty, DebugLoc::None)
-            }
-
-            mir::StatementKind::Drop(mir::DropKind::Free, ref lvalue) => {
-                let tr_lvalue = self.trans_lvalue(bcx, lvalue);
-                let ty = tr_lvalue.ty.to_ty(bcx.tcx());
-                let content_ty = ty.builtin_deref(true, LvaluePreference::NoPreference);
-                let content_ty = content_ty.unwrap().ty;
-                glue::trans_exchange_free_ty(bcx, tr_lvalue.llval, content_ty, DebugLoc::None)
-            }
         }
     }
 }
diff --git a/src/test/run-fail/mir_drop_panics.rs b/src/test/run-fail/mir_drop_panics.rs
new file mode 100644
index 00000000000..df20700e016
--- /dev/null
+++ b/src/test/run-fail/mir_drop_panics.rs
@@ -0,0 +1,36 @@
+// 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(rustc_attrs)]
+
+// ignore-msvc: FIXME(#30941)
+// error-pattern:panic 1
+// error-pattern:drop 2
+use std::io::{self, Write};
+
+struct Droppable(u32);
+impl Drop for Droppable {
+    fn drop(&mut self) {
+        if self.0 == 1 {
+            panic!("panic 1");
+        } else {
+            write!(io::stderr(), "drop {}", self.0);
+        }
+    }
+}
+
+#[rustc_mir]
+fn mir() {
+    let x = Droppable(2);
+    let y = Droppable(1);
+}
+
+fn main() {
+    mir();
+}