fix: make Serde (de)serialization no_std compatible #337
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`multihash` v0.19 introduced a regression. It's no longer possible to compile with `no_std` and `serde` feature enabled. This commit fixes that regression. Due to generic_const_exprs not being a thing yet, we need to use unsafe code to provide this functionality. Fixes #336.
Stebalien
approved these changes
Aug 25, 2023
src/serde.rs
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| // Make sure that the struct allocated continuous memory, as we exploit that fact with the | ||
| // `as_slice` and `as_mut_slice()` methods. | ||
| let start_first = ptr::addr_of!(buffer.first) as *const u8; | ||
| let start_second = ptr::addr_of!(buffer.second) as *const u8; | ||
| unsafe { | ||
| // This should never happen, hence it's OK to panic. | ||
| if start_second.offset_from(start_first) != SIZE_FIRST as isize { | ||
| panic!("alignment of the struct is wrong") | ||
| } | ||
| } |
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This shouldn't be necessary as packed guarantees this.
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But you could consider writing a test to assert this (but I'd avoid doing this at runtime).
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Actually, you can static_assertions to assert that mem::size_of::<Self>() == SIZE_FIRST + SIZE_SECOND at compile time.
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You can also assert tat Self and [u8] have the same alignment.
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I've tried
const_assert!(mem::size_of::<Self>() == SIZE_FIRST + SIZE_SECOND);but sadly it cannot be compiled:
> cargo build --no-default-features --features serde
error[E0401]: can't use generic parameters from outer function
--> src/serde.rs:38:38
|
29 | impl<const SIZE_FIRST: usize, const SIZE_SECOND: usize> Buffer<SIZE_FIRST, SIZE_SECOND> {
| ---- `Self` type implicitly declared here, by this `impl`
...
38 | const_assert!(mem::size_of::<Self>() == SIZE_FIRST + SIZE_SECOND);
| ^^^^
| |
| use of generic parameter from outer function
| use a type here instead
error[E0401]: can't use generic parameters from outer function
--> src/serde.rs:38:49
|
29 | impl<const SIZE_FIRST: usize, const SIZE_SECOND: usize> Buffer<SIZE_FIRST, SIZE_SECOND> {
| ---------- const parameter from outer function
...
38 | const_assert!(mem::size_of::<Self>() == SIZE_FIRST + SIZE_SECOND);
| ^^^^^^^^^^ use of generic parameter from outer function
error[E0401]: can't use generic parameters from outer function
--> src/serde.rs:38:62
|
29 | impl<const SIZE_FIRST: usize, const SIZE_SECOND: usize> Buffer<SIZE_FIRST, SIZE_SECOND> {
| ----------- const parameter from outer function
...
38 | const_assert!(mem::size_of::<Self>() == SIZE_FIRST + SIZE_SECOND);
| ^^^^^^^^^^^ use of generic parameter from outer function
For more information about this error, try `rustc --explain E0401`.
error: could not compile `multihash` (lib) due to 3 previous errorsSo would you prefer if I remove that run-time check and write a test instead?
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I've moved it into a test. Though if anyone has an idea on how to assert it at compile-time, that would even be better.
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Ah... yeah, generics. I... don't. But I think a test is probably enough.
Instead move the alignment check into a test. Also remove the `len()` method and use `mem::size_of()` instead.
Stebalien
approved these changes
Sep 1, 2023
thomaseizinger
approved these changes
Sep 4, 2023
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multihashv0.19 introduced a regression. It's no longer possible to compile withno_stdandserdefeature enabled. This commit fixes that regression.Due to generic_const_exprs not being a thing yet, we need to use unsafe code to provide this functionality.
Fixes #336.