Synthesize calls to box_free language item

This gets rid of Drop(Free, _) MIR construct by synthesizing a call to language item which
takes care of dropping instead.

Author: nagisa

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;

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;

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

src/librustc/middle/lang_items.rs

@@ -36,9 +36,6 @@ 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 {
@@ -69,8 +66,8 @@ 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 {
@@ -334,6 +331,7 @@ 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;

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);

src/librustc/mir/repr.rs

@@ -490,23 +490,15 @@ 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
+    Drop(Lvalue<'tcx>),
 }
 
 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),
+            Drop(ref lv) => write!(fmt, "drop {:?}", lv),
         }
     }
 }

src/librustc/mir/visit.rs

@@ -124,7 +124,7 @@ macro_rules! make_mir_visitor {
                                           ref $($mutability)* rvalue) => {
                         self.visit_assign(block, lvalue, rvalue);
                     }
-                    StatementKind::Drop(_, ref $($mutability)* lvalue) => {
+                    StatementKind::Drop(ref $($mutability)* lvalue) => {
                         self.visit_lvalue(lvalue, LvalueContext::Drop);
                     }
                 }

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);

src/librustc_mir/build/cfg.rs

@@ -32,16 +32,21 @@ 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>) {
+    pub fn push_drop(&mut self, block: BasicBlock, span: Span, lvalue: &Lvalue<'tcx>) {
         self.push(block, Statement {
             span: span,
-            kind: StatementKind::Drop(kind, lvalue.clone())
+            kind: StatementKind::Drop(lvalue.clone())
         });
     }
 

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));