Add some documentation

src/librustc_trans/trans/base.rs

@@ -1393,6 +1393,16 @@ fn build_cfg(tcx: &ty::ctxt, id: ast::NodeId) -> (ast::NodeId, Option<cfg::CFG>)
     (blk.id, Some(cfg::CFG::new(tcx, &**blk)))
 }
 
+// Checks for the presence of "nested returns" in a function.
+// Nested returns are when the inner expression of a return expression
+// (the 'expr' in 'return expr') contains a return expression. Only cases
+// where the outer return is actually reachable are considered. Implicit
+// returns from the end of blocks are considered as well.
+//
+// This check is needed to handle the case where the inner expression is
+// part of a larger expression that may have already partially-filled the
+// return slot alloca. This can cause errors related to clean-up due to
+// the clobbering of the existing value in the return slot.
 fn has_nested_returns(tcx: &ty::ctxt, cfg: &cfg::CFG, blk_id: ast::NodeId) -> bool {
     for n in cfg.graph.depth_traverse(cfg.entry) {
         match tcx.map.find(n.id) {

src/librustc_trans/trans/controlflow.rs

@@ -112,8 +112,17 @@ pub fn trans_block<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
 
     if dest != expr::Ignore {
         let block_ty = node_id_type(bcx, b.id);
+
         if b.expr.is_none() || type_is_zero_size(bcx.ccx(), block_ty) {
             dest = expr::Ignore;
+        } else if b.expr.is_some() {
+            // If the block has an expression, but that expression isn't reachable,
+            // don't save into the destination given, ignore it.
+            if let Some(ref cfg) = bcx.fcx.cfg {
+                if !cfg.node_is_reachable(b.expr.as_ref().unwrap().id) {
+                    dest = expr::Ignore;
+                }
+            }
         }
     }
 

src/librustc_trans/trans/expr.rs

@@ -928,7 +928,25 @@ fn trans_rvalue_stmt_unadjusted<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
             controlflow::trans_cont(bcx, expr.id, label_opt)
         }
         ast::ExprRet(ref ex) => {
-            controlflow::trans_ret(bcx, ex.as_ref().map(|e| &**e))
+            // Check to see if the return expression itself is reachable.
+            // This can occur when the inner expression contains a return
+            let reachable = if let Some(ref cfg) = bcx.fcx.cfg {
+                cfg.node_is_reachable(expr.id)
+            } else {
+                true
+            };
+
+            if reachable {
+                controlflow::trans_ret(bcx, ex.as_ref().map(|e| &**e))
+            } else {
+                // If it's not reachable, just translate the inner expression
+                // directly. This avoids having to manage a return slot when
+                // it won't actually be used anyway.
+                if let &Some(ref x) = ex {
+                    bcx = trans_into(bcx, &**x, Ignore);
+                } 
+                bcx
+            }
         }
         ast::ExprWhile(ref cond, ref body, _) => {
             controlflow::trans_while(bcx, expr.id, &**cond, &**body)