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Simplify s![] #1195

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Simplify s![] #1195

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05b781a
Simplify `s![]`
CAD97 Aug 13, 2022
7aecb92
Move macro binder evaluation out of unsafe context
CAD97 Aug 13, 2022
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8 changes: 2 additions & 6 deletions src/slice.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -842,15 +842,11 @@ macro_rules! s(
}
};
// empty call, i.e. `s![]`
(@parse ::core::marker::PhantomData::<$crate::Ix0>, ::core::marker::PhantomData::<$crate::Ix0>, []) => {
(@parse $in_dim:expr, $out_dim:expr, []) => {
{
#[allow(unsafe_code)]
unsafe {
$crate::SliceInfo::new_unchecked(
[],
::core::marker::PhantomData::<$crate::Ix0>,
::core::marker::PhantomData::<$crate::Ix0>,
)
$crate::SliceInfo::new_unchecked([], $in_dim, $out_dim)
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@bluss bluss Aug 13, 2022

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what's the impact on the unsafe code block here?

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@CAD97 CAD97 Aug 13, 2022 •
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It's still necessary, as SliceInfo::new_unchecked is unsafe.

Technically, this means that someone using the macro could write

s! { @parse
    {
        // statements in an unsafe context
        ::core::marker::PhantomData::<::ndarray::Ix0>
    },
    ::core::marker::PhantomData<::ndaray::Ix0>,
    []
}

to get an unsafe context without writing unsafe, but this is an internal rule, so that's not too worrisome. However, it is reasonable to evaluate the arguments outside of the unsafe block just for thoroughness.

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@jturner314 jturner314 Aug 13, 2022

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Thanks for bringing this up. Some of the other @parse arms had safety issues if called directly. I've created #1196 to fix that. With that change, we can change this PR to modify the "empty case" arm to the following:

    // empty call, i.e. `s![]`
    (@parse $in_dim:expr, $out_dim:expr, []) => {
        {
            debug_assert_eq!($in_dim, ::core::marker::PhantomData::<$crate::Ix0>);
            debug_assert_eq!($out_dim, ::core::marker::PhantomData::<$crate::Ix0>);
            (
                [],
                ::core::marker::PhantomData::<$crate::Ix0>,
                ::core::marker::PhantomData::<$crate::Ix0>,
            )
        }
    };

This is safe, since it always expands to valid values for the SliceInfo::new_unchecked call (which is in the ($($t:tt)*) arm with #1196). It has debug assertions to ensure that $in_dim and $out_dim are the expected values. (They'll always be the expected values, unless the user calls s![@parse ...] directly.

}
}
};
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