Auto merge of rust-lang#35300 - eddyb:mir-neg-overflow, r=arielb1

rustc_trans: don't Assert(Overflow(Neg)) when overflow checks are off.

Generic functions using `Neg` on primitive types would panic even in release mode, with MIR trans.
The solution is a bit hacky, as I'm checking the message, since there's no dedicated `CheckedUnOp`.

Blocks Servo rustup ([failure #1](http://build.servo.org/builders/linux-rel/builds/2477/steps/test_3/logs/stdio), [failure #2](http://build.servo.org/builders/mac-rel-css/builds/2364/steps/test/logs/stdio)) - this should be the last hurdle, it affects only one test.

src/librustc_trans/mir/block.rs

@@ -261,7 +261,23 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
 
             mir::TerminatorKind::Assert { ref cond, expected, ref msg, target, cleanup } => {
                 let cond = self.trans_operand(&bcx, cond).immediate();
-                let const_cond = common::const_to_opt_uint(cond).map(|c| c == 1);
+                let mut const_cond = common::const_to_opt_uint(cond).map(|c| c == 1);
+
+                // This case can currently arise only from functions marked
+                // with #[rustc_inherit_overflow_checks] and inlined from
+                // another crate (mostly core::num generic/#[inline] fns),
+                // while the current crate doesn't use overflow checks.
+                // NOTE: Unlike binops, negation doesn't have its own
+                // checked operation, just a comparison with the minimum
+                // value, so we have to check for the assert message.
+                if !bcx.ccx().check_overflow() {
+                    use rustc_const_math::ConstMathErr::Overflow;
+                    use rustc_const_math::Op::Neg;
+
+                    if let mir::AssertMessage::Math(Overflow(Neg)) = *msg {
+                        const_cond = Some(expected);
+                    }
+                }
 
                 // Don't translate the panic block if success if known.
                 if const_cond == Some(expected) {

src/test/run-pass/mir_overflow_off.rs

@@ -0,0 +1,26 @@
+// 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.
+
+// compile-flags: -Z force-overflow-checks=off -Z orbit
+
+// Test that with MIR trans, overflow checks can be
+// turned off, even when they're from core::ops::*.
+
+use std::ops::*;
+
+fn main() {
+    assert_eq!(i8::neg(-0x80), -0x80);
+
+    assert_eq!(u8::add(0xff, 1), 0_u8);
+    assert_eq!(u8::sub(0, 1), 0xff_u8);
+    assert_eq!(u8::mul(0xff, 2), 0xfe_u8);
+    assert_eq!(u8::shl(1, 9), 2_u8);
+    assert_eq!(u8::shr(2, 9), 1_u8);
+}