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smaller type
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.
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
[acourbot@nvidia.com: fix mistake in example pointed out by Mikko.]
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Message-ID: <20251110-gsp_boot-v9-3-8ae4058e3c0e@nvidia.com>
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Use the newly-introduced `num` module to replace the use of `as`
wherever it is safe to do. This ensures that a given conversion cannot
lose data if its source or destination type ever changes.
Acked-by: Danilo Krummrich <dakr@kernel.org>
[acourbot@nvidia.com: fix merge conflicts after rebase.]
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Message-ID: <20251029-nova-as-v3-5-6a30c7333ad9@nvidia.com>
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The core library's `From` implementations do not cover conversions
that are not portable or future-proof. For instance, even though it is
safe today, `From<usize>` is not implemented for `u64` because of the
possibility to support larger-than-64bit architectures in the future.
However, the kernel supports a narrower set of architectures, and in the
case of Nova we only support 64-bit. This makes it helpful and desirable
to provide more infallible conversions, lest we need to rely on the `as`
keyword and carry the risk of silently losing data.
Thus, introduce a new module `num` that provides safe const functions
performing more conversions allowed by the build target, as well as
`FromSafeCast` and `IntoSafeCast` traits that are just extensions of
`From` and `Into` to conversions that are known to be lossless.
Suggested-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/rust-for-linux/DDK4KADWJHMG.1FUPL3SDR26XF@kernel.org/
Acked-by: Danilo Krummrich <dakr@kernel.org>
[acourbot@nvidia.com: fix merge conflicts after rebase.]
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Message-ID: <20251029-nova-as-v3-4-6a30c7333ad9@nvidia.com>
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