Auto merge of #36372 - sfackler:sum-prod-overflow, r=alexcrichton

Inherit overflow checks for sum and product

We have previously documented the fact that these will panic on overflow, but I think this behavior is what people actually want/expect. `#[rustc_inherit_overflow_checks]` didn't exist when we discussed these for stabilization.

r? @alexcrichton

Closes #35807

2 files changed, 10 insertions, 14 deletions

diff --git a/src/libcore/iter/iterator.rs b/src/libcore/iter/iterator.rs
index 6b616f80181..0e74bbe9c26 100644
--- a/src/libcore/iter/iterator.rs
+++ b/src/libcore/iter/iterator.rs
@@ -1867,7 +1867,8 @@ pub trait Iterator {
     /// # Panics
     ///
     /// When calling `sum` and a primitive integer type is being returned, this
-    /// method will panic if the computation overflows.
+    /// method will panic if the computation overflows and debug assertions are
+    /// enabled.
     ///
     /// # Examples
     ///
@@ -1894,7 +1895,8 @@ pub trait Iterator {
     /// # Panics
     ///
     /// When calling `product` and a primitive integer type is being returned,
-    /// this method will panic if the computation overflows.
+    /// method will panic if the computation overflows and debug assertions are
+    /// enabled.
     ///
     /// # Examples
     ///
diff --git a/src/libcore/iter/traits.rs b/src/libcore/iter/traits.rs
index 59e23c4d960..b55d6f96af9 100644
--- a/src/libcore/iter/traits.rs
+++ b/src/libcore/iter/traits.rs
@@ -7,6 +7,7 @@
 // <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.
+use ops::{Mul, Add};
 
 /// Conversion from an `Iterator`.
 ///
@@ -581,41 +582,34 @@ pub trait Product<A = Self>: Sized {
     fn product<I: Iterator<Item=A>>(iter: I) -> Self;
 }
 
+// NB: explicitly use Add and Mul here to inherit overflow checks
 macro_rules! integer_sum_product {
     ($($a:ident)*) => ($(
         #[stable(feature = "iter_arith_traits", since = "1.12.0")]
         impl Sum for $a {
             fn sum<I: Iterator<Item=$a>>(iter: I) -> $a {
-                iter.fold(0, |a, b| {
-                    a.checked_add(b).expect("overflow in sum")
-                })
+                iter.fold(0, Add::add)
             }
         }
 
         #[stable(feature = "iter_arith_traits", since = "1.12.0")]
         impl Product for $a {
             fn product<I: Iterator<Item=$a>>(iter: I) -> $a {
-                iter.fold(1, |a, b| {
-                    a.checked_mul(b).expect("overflow in product")
-                })
+                iter.fold(1, Mul::mul)
             }
         }
 
         #[stable(feature = "iter_arith_traits", since = "1.12.0")]
         impl<'a> Sum<&'a $a> for $a {
             fn sum<I: Iterator<Item=&'a $a>>(iter: I) -> $a {
-                iter.fold(0, |a, b| {
-                    a.checked_add(*b).expect("overflow in sum")
-                })
+                iter.cloned().fold(0, Add::add)
             }
         }
 
         #[stable(feature = "iter_arith_traits", since = "1.12.0")]
         impl<'a> Product<&'a $a> for $a {
             fn product<I: Iterator<Item=&'a $a>>(iter: I) -> $a {
-                iter.fold(1, |a, b| {
-                    a.checked_mul(*b).expect("overflow in product")
-                })
+                iter.cloned().fold(1, Mul::mul)
             }
         }
     )*)