Use offset instead of add #5
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This ensures compatibility with older Rust releases. Fixes marshallpierce/rust-base64#112.
| @@ -44,7 +44,7 @@ pub fn copy_over<T: Copy>(slice: &mut [T], src_idx: usize, dest_idx: usize, len: | |||
| let ptr = slice.as_mut_ptr(); | |||
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| unsafe { | |||
| ptr::copy(ptr.add(src_idx), ptr.add(dest_idx), len); | |||
| ptr::copy(ptr.offset(src_idx as isize), ptr.offset(dest_idx as isize), len); | |||
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Should we check that this cast doesn't overflow? We already bounds-check these indices, and while the docs say that overflowing isize is a non-issue for allocations issued by the stdlib, and impossible on current x86_64 systems, they also point out that it could happen with manually allocated memory on systems with PAE.
I'm also aware that .add() also just naively casts to isize but I'm wondering if we want to be overly paranoid here and check all preconditions.
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This isn't an issue. Documentation for <[_]>::from_raw_parts outright says "The total size of the slice must be no larger than isize::MAX bytes in memory." Due to this, it's not possible to create a valid non-ZST slice that takes more than isize::MAX bytes in memory. Violating this will lead to UB, so it's not an issue for this crate.
Another way of getting a slice bigger than isize::MAX is by using CStr::from_ptr. This method doesn't have this requirement, but I believe that to be a bug in standard library (I will be proposing a request to change the documentation here soon).
For ZST types, it is possible to create a slice where .len() > isize::MAX. That said, while the cast to isize can make the value negative, because .offset is essentially a no-op for ZST, providing a wrong value to .offset is non-issue, after all, -50 * 0 == 0, so the pointer gets moved by 0 bytes, which is still in-bounds for ZST object.
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Reported the documentation issue for CStr::from_ptr in rust-lang/rust#63590.
This ensures compatibility with older Rust releases.
Fixes marshallpierce/rust-base64#112.