rustc: Don't generate code for unreachable expressions

The way we deal with unreachable expressions in trans is pretty ad hoc,
but this at least doesn't make it worse, and eliminates the LLVM
assertion failure reported in #5741.

Author: catamorphism

src/librustc/middle/trans/expr.rs

@@ -499,6 +499,10 @@ fn trans_rvalue_stmt_unadjusted(bcx: block, expr: @ast::expr) -> block {
     let mut bcx = bcx;
     let _icx = bcx.insn_ctxt("trans_rvalue_stmt");
 
+    if bcx.unreachable {
+        return bcx;
+    }
+
     trace_span!(bcx, expr.span, @shorten(bcx.expr_to_str(expr)));
 
     match expr.node {

src/test/run-pass/issue-5741.rs

@@ -0,0 +1,14 @@
+// Copyright 2013 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.
+
+fn main() {
+    return;
+    while io::stdin().read_line() != ~"quit" { };
+}