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 <[email protected]>
---
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
