linux.git/rust/kernel/num/casts.rs, branch v7.3-rc2 Linux kernel source tree rust: add functions and traits for lossless integer conversions 2026-08-13T11:41:28+00:00 Alexandre Courbot acourbot@nvidia.com 2026-08-06T07:35:53+00:00 47f27155f17498fccb1f222f79089642337498a9 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 of supporting larger-than-64bit architectures in the future. However, the kernel supports a narrower set of architectures, with a considerable amount of code that is architecture-specific. This makes it helpful and desirable to provide more infallible conversions, lest we rely on the `as` keyword and carry the risk of silently losing data. Thus, introduce a new module `num::casts` 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. Some conversions are architecture-specific: for instance, converting a `u64` to a `usize` is only lossless on 64-bit platforms. These conversions are made available via a dedicated `arch` sub-module. Suggested-by: Danilo Krummrich <dakr@kernel.org> Link: https://lore.kernel.org/rust-for-linux/DDK4KADWJHMG.1FUPL3SDR26XF@kernel.org/ Signed-off-by: Alexandre Courbot <acourbot@nvidia.com> Reviewed-by: Danilo Krummrich <dakr@kernel.org> Link: https://patch.msgid.link/20260806-as_casts-v2-1-cb76a4d3a6ef@nvidia.com [ Added a few more intra-doc links. - Miguel ] Signed-off-by: Miguel Ojeda <ojeda@kernel.org> 
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 of supporting larger-than-64bit architectures in the future.

However, the kernel supports a narrower set of architectures, with a
considerable amount of code that is architecture-specific. This makes it
helpful and desirable to provide more infallible conversions, lest we
rely on the `as` keyword and carry the risk of silently losing data.

Thus, introduce a new module `num::casts` 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.

Some conversions are architecture-specific: for instance, converting a
`u64` to a `usize` is only lossless on 64-bit platforms. These
conversions are made available via a dedicated `arch` sub-module.

Suggested-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/rust-for-linux/DDK4KADWJHMG.1FUPL3SDR26XF@kernel.org/
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260806-as_casts-v2-1-cb76a4d3a6ef@nvidia.com
[ Added a few more intra-doc links. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>