Add add/sub methods that only panic with debug assertions to rustc

This mitigates the perf impact of enabling overflow checks on rustc.
The change to use overflow checks will be done in a later PR.

compiler/rustc_serialize/src/int_overflow.rs

@@ -0,0 +1,65 @@
+// This would belong to `rustc_data_structures`, but `rustc_serialize` needs it too.
+
+/// Addition, but only overflow checked when `cfg(debug_assertions)` is set
+/// instead of respecting `-Coverflow-checks`.
+///
+/// This exists for performance reasons, as we ship rustc with overflow checks.
+/// While overflow checks are perf neutral in almost all of the compiler, there
+/// are a few particularly hot areas where we don't want overflow checks in our
+/// dist builds. Overflow is still a bug there, so we want overflow check for
+/// builds with debug assertions.
+///
+/// That's a long way to say that this should be used in areas where overflow
+/// is a bug but overflow checking is too slow.
+pub trait DebugStrictAdd {
+    /// See [`DebugStrictAdd`].
+    fn debug_strict_add(self, other: Self) -> Self;
+}
+
+macro_rules! impl_debug_strict_add {
+    ($( $ty:ty )*) => {
+        $(
+            impl DebugStrictAdd for $ty {
+                fn debug_strict_add(self, other: Self) -> Self {
+                    if cfg!(debug_assertions) {
+                        self + other
+                    } else {
+                        self.wrapping_add(other)
+                    }
+                }
+            }
+        )*
+    };
+}
+
+/// See [`DebugStrictAdd`].
+pub trait DebugStrictSub {
+    /// See [`DebugStrictAdd`].
+    fn debug_strict_sub(self, other: Self) -> Self;
+}
+
+macro_rules! impl_debug_strict_sub {
+    ($( $ty:ty )*) => {
+        $(
+            impl DebugStrictSub for $ty {
+                fn debug_strict_sub(self, other: Self) -> Self {
+                    if cfg!(debug_assertions) {
+                        self - other
+                    } else {
+                        self.wrapping_sub(other)
+                    }
+                }
+            }
+        )*
+    };
+}
+
+impl_debug_strict_add! {
+    u8 u16 u32 u64 u128 usize
+    i8 i16 i32 i64 i128 isize
+}
+
+impl_debug_strict_sub! {
+    u8 u16 u32 u64 u128 usize
+    i8 i16 i32 i64 i128 isize
+}