[PATCH v8 03/16] gpu: nova-core: num: add functions to safely convert a const value to a smaller type

[Alexandre Courbot <acourbot@nvidia.com>]

There are times where we need to store a constant value defined as a
larger type (e.g. through a binding) into a smaller type, knowing
that the value will fit. Rust, unfortunately, only provides us with the
`as` operator for that purpose, the use of which is discouraged as it
silently strips data.

Extend the `num` module with functions allowing to perform the
conversion infallibly, at compile time.

Example:

    const FOO_VALUE: u32 = 1;

    // `FOO_VALUE` fits into a `u8`, so the conversion is valid.
    let foo = num::u32_to_u8::<{ FOO_VALUE }>();

We are going to use this feature extensively in Nova.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 drivers/gpu/nova-core/num.rs | 51 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/drivers/gpu/nova-core/num.rs b/drivers/gpu/nova-core/num.rs
index 92a91b9e30de..f3740ab6cb9d 100644
--- a/drivers/gpu/nova-core/num.rs
+++ b/drivers/gpu/nova-core/num.rs
@@ -163,3 +163,54 @@ fn into_safe_cast(self) -> T {
         T::from_safe_cast(self)
     }
 }
+
+macro_rules! impl_const_into {
+    ($from:ty => { $($into:ty),* }) => {
+        $(
+        paste! {
+            #[doc = ::core::concat!(
+                "Performs a build-time safe conversion of a [`",
+                ::core::stringify!($from),
+                "`] constant value into a [`",
+                ::core::stringify!($into),
+                "`].")]
+            ///
+            /// This checks at compile-time that the conversion is lossless, and triggers a build
+            /// error if it isn't.
+            ///
+            /// # Examples
+            ///
+            /// ```
+            /// use kernel::num;
+            ///
+            /// // Succeeds because the value of the source fits into the destination's type.
+            #[doc = ::core::concat!(
+                "assert_eq!(num::",
+                ::core::stringify!($from),
+                "_into_",
+                ::core::stringify!($into),
+                "(1",
+                ::core::stringify!($from),
+                "), 1",
+                ::core::stringify!($into),
+                ");")]
+            /// ```
+            #[allow(unused)]
+            pub(crate) const fn [<$from _into_ $into>]<const N: $from>() -> $into {
+                // Make sure that the target type is smaller than the source one.
+                static_assert!($from::BITS >= $into::BITS);
+                // CAST: we statically enforced above that `$from` is larger than `$into`, so the
+                // `as` conversion will be lossless.
+                build_assert!(N >= $into::MIN as $from && N <= $into::MAX as $from);
+
+                N as $into
+            }
+        }
+        )*
+    };
+}
+
+impl_const_into!(usize => { u8, u16, u32 });
+impl_const_into!(u64 => { u8, u16, u32 });
+impl_const_into!(u32 => { u8, u16 });
+impl_const_into!(u16 => { u8 });

-- 
2.51.2