add -C overflow-checks option

In addition to defining and handling the new option, we also add a
method on librustc::Session for determining the necessity of overflow
checks.  This method provides a single point to sort out the three (!)
different ways for turning on overflow checks: -C debug-assertions, -C
overflow-checks, and -Z force-overflow-checks.

Fixes rust-lang#33134.

src/librustc/session/config.rs

@@ -804,6 +804,8 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
         "save all temporary output files during compilation"),
     rpath: bool = (false, parse_bool, [UNTRACKED],
         "set rpath values in libs/exes"),
+    overflow_checks: Option<bool> = (None, parse_opt_bool, [TRACKED],
+        "use overflow checks for integer arithmetic"),
     no_prepopulate_passes: bool = (false, parse_bool, [TRACKED],
         "don't pre-populate the pass manager with a list of passes"),
     no_vectorize_loops: bool = (false, parse_bool, [TRACKED],
@@ -2345,6 +2347,10 @@ mod tests {
         opts.cg.llvm_args = vec![String::from("1"), String::from("2")];
         assert!(reference.dep_tracking_hash() != opts.dep_tracking_hash());
 
+        opts = reference.clone();
+        opts.cg.overflow_checks = Some(true);
+        assert!(reference.dep_tracking_hash() != opts.dep_tracking_hash());
+
         opts = reference.clone();
         opts.cg.no_prepopulate_passes = true;
         assert!(reference.dep_tracking_hash() != opts.dep_tracking_hash());

src/librustc/session/mod.rs

@@ -372,6 +372,11 @@ impl Session {
     pub fn nonzeroing_move_hints(&self) -> bool {
         self.opts.debugging_opts.enable_nonzeroing_move_hints
     }
+    pub fn overflow_checks(&self) -> bool {
+        self.opts.cg.overflow_checks
+            .or(self.opts.debugging_opts.force_overflow_checks)
+            .unwrap_or(self.opts.debug_assertions)
+    }
 
     pub fn must_not_eliminate_frame_pointers(&self) -> bool {
         self.opts.debuginfo != DebugInfoLevel::NoDebugInfo ||

src/librustc_incremental/calculate_svh/svh_visitor.rs

@@ -63,8 +63,7 @@ impl<'a, 'hash, 'tcx> StrictVersionHashVisitor<'a, 'hash, 'tcx> {
                hash_spans: bool,
                hash_bodies: bool)
                -> Self {
-        let check_overflow = tcx.sess.opts.debugging_opts.force_overflow_checks
-            .unwrap_or(tcx.sess.opts.debug_assertions);
+        let check_overflow = tcx.sess.overflow_checks();
 
         StrictVersionHashVisitor {
             st: st,

src/librustc_mir/hair/cx/mod.rs

@@ -59,13 +59,8 @@ impl<'a, 'gcx, 'tcx> Cx<'a, 'gcx, 'tcx> {
         let mut check_overflow = attrs.iter()
             .any(|item| item.check_name("rustc_inherit_overflow_checks"));
 
-        // Respect -Z force-overflow-checks=on and -C debug-assertions.
-        check_overflow |= infcx.tcx
-            .sess
-            .opts
-            .debugging_opts
-            .force_overflow_checks
-            .unwrap_or(infcx.tcx.sess.opts.debug_assertions);
+        // Respect -C overflow-checks.
+        check_overflow |= infcx.tcx.sess.overflow_checks();
 
         // Constants and const fn's always need overflow checks.
         check_overflow |= constness == hir::Constness::Const;

src/librustc_trans/base.rs

@@ -1132,11 +1132,7 @@ pub fn trans_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     let ty::CrateAnalysis { export_map, reachable, name, .. } = analysis;
     let exported_symbols = find_exported_symbols(tcx, reachable);
 
-    let check_overflow = if let Some(v) = tcx.sess.opts.debugging_opts.force_overflow_checks {
-        v
-    } else {
-        tcx.sess.opts.debug_assertions
-    };
+    let check_overflow = tcx.sess.overflow_checks();
 
     let link_meta = link::build_link_meta(incremental_hashes_map, &name);
 

src/test/run-pass/iter-sum-overflow-overflow-checks.rs

@@ -0,0 +1,35 @@
+// 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: -C overflow-checks
+
+use std::panic;
+
+fn main() {
+    let r = panic::catch_unwind(|| {
+        [1, i32::max_value()].iter().sum::<i32>();
+    });
+    assert!(r.is_err());
+
+    let r = panic::catch_unwind(|| {
+        [2, i32::max_value()].iter().product::<i32>();
+    });
+    assert!(r.is_err());
+
+    let r = panic::catch_unwind(|| {
+        [1, i32::max_value()].iter().cloned().sum::<i32>();
+    });
+    assert!(r.is_err());
+
+    let r = panic::catch_unwind(|| {
+        [2, i32::max_value()].iter().cloned().product::<i32>();
+    });
+    assert!(r.is_err());
+}