Auto merge of #59148 - lcnr:unchecked_maths, r=eddyb

add support for unchecked math

add compiler support for
```rust
/// Returns the result of an unchecked addition, resulting in
/// undefined behavior when `x + y > T::max_value()` or `x + y < T::min_value()`.
pub fn unchecked_add<T>(x: T, y: T) -> T;

/// Returns the result of an unchecked substraction, resulting in
/// undefined behavior when `x - y > T::max_value()` or `x - y < T::min_value()`.
pub fn unchecked_sub<T>(x: T, y: T) -> T;

/// Returns the result of an unchecked multiplication, resulting in
/// undefined behavior when `x * y > T::max_value()` or `x * y < T::min_value()`.
pub fn unchecked_mul<T>(x: T, y: T) -> T;
```

cc https://github.com/rust-lang/rfcs/issues/2508

1 files changed, 15 insertions, 0 deletions

diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs
index 782a7ba4559..31a4e380a3d 100644
--- a/src/libcore/intrinsics.rs
+++ b/src/libcore/intrinsics.rs
@@ -1240,6 +1240,21 @@ extern "rust-intrinsic" {
     /// y < 0 or y >= N, where N is the width of T in bits.
     pub fn unchecked_shr<T>(x: T, y: T) -> T;
 
+    /// Returns the result of an unchecked addition, resulting in
+    /// undefined behavior when `x + y > T::max_value()` or `x + y < T::min_value()`.
+    #[cfg(not(stage0))]
+    pub fn unchecked_add<T>(x: T, y: T) -> T;
+
+    /// Returns the result of an unchecked substraction, resulting in
+    /// undefined behavior when `x - y > T::max_value()` or `x - y < T::min_value()`.
+    #[cfg(not(stage0))]
+    pub fn unchecked_sub<T>(x: T, y: T) -> T;
+
+    /// Returns the result of an unchecked multiplication, resulting in
+    /// undefined behavior when `x * y > T::max_value()` or `x * y < T::min_value()`.
+    #[cfg(not(stage0))]
+    pub fn unchecked_mul<T>(x: T, y: T) -> T;
+
     /// Performs rotate left.
     /// The stabilized versions of this intrinsic are available on the integer
     /// primitives via the `rotate_left` method. For example,