Trait zerovec::ule::VarULE

source ·
pub unsafe trait VarULE: 'static {
    // Required methods
    fn validate_byte_slice(_bytes: &[u8]) -> Result<(), ZeroVecError>;
    unsafe fn from_byte_slice_unchecked(bytes: &[u8]) -> &Self;

    // Provided methods
    fn parse_byte_slice(bytes: &[u8]) -> Result<&Self, ZeroVecError> { ... }
    fn as_byte_slice(&self) -> &[u8]  { ... }
    fn to_boxed(&self) -> Box<Self> { ... }
Expand description

Variable-width, byte-aligned data that can be cast to and from a little-endian byte slice.

If you need to implement this trait, consider using #[make_varule] or #[derive(VarULE)] instead.

This trait is mostly for unsized types like str and [T]. It can be implemented on sized types; however, it is much more preferable to use ULE for that purpose. The custom module contains additional documentation on how this type can be implemented on custom types.

If deserialization with VarZeroVec is desired is recommended to implement Deserialize for Box<T> (serde does not do this automatically for unsized T).

For convenience it is typically desired to implement EncodeAsVarULE and ZeroFrom on some stack type to convert to and from the ULE type efficiently when necessary.


Safety checklist for VarULE:

  1. The type must not include any uninitialized or padding bytes.
  2. The type must have an alignment of 1 byte.
  3. The impl of VarULE::validate_byte_slice() must return an error if the given byte slice would not represent a valid slice of this type.
  4. The impl of VarULE::validate_byte_slice() must return an error if the given byte slice cannot be used in its entirety.
  5. The impl of VarULE::from_byte_slice_unchecked() must produce a reference to the same underlying data assuming that the given bytes previously passed validation.
  6. All other methods must be left with their default impl, or else implemented according to their respective safety guidelines.
  7. Acknowledge the following note about the equality invariant.

If the ULE type is a struct only containing other ULE/VarULE types (or other types which satisfy invariants 1 and 2, like [u8; N]), invariants 1 and 2 can be achieved via #[repr(packed)] or #[repr(transparent)].

Equality invariant

A non-safety invariant is that if Self implements PartialEq, the it must be logically equivalent to byte equality on Self::as_byte_slice().

It may be necessary to introduce a “canonical form” of the ULE if logical equality does not equal byte equality. In such a case, Self::validate_byte_slice() should return an error for any values that are not in canonical form. For example, the decimal strings “1.23e4” and “12.3e3” are logically equal, but not byte-for-byte equal, so we could define a canonical form where only a single digit is allowed before ..

There may also be cases where a VarULE has muiltiple canonical forms, such as a faster version and a smaller version. The cleanest way to handle this case would be separate types. However, if this is not feasible, then the application should ensure that the data it is deserializing is in the expected form. For example, if the data is being loaded from an external source, then requests could carry information about the expected form of the data.

Failure to follow this invariant will cause surprising behavior in PartialEq, which may result in unpredictable operations on ZeroVec, VarZeroVec, and ZeroMap.

Required Methods§


fn validate_byte_slice(_bytes: &[u8]) -> Result<(), ZeroVecError>

Validates a byte slice, &[u8].

If Self is not well-defined for all possible bit values, the bytes should be validated. If the bytes can be transmuted, in their entirety, to a valid &Self, then Ok should be returned; otherwise, Self::Error should be returned.


unsafe fn from_byte_slice_unchecked(bytes: &[u8]) -> &Self

Takes a byte slice, &[u8], and return it as &Self with the same lifetime, assuming that this byte slice has previously been run through Self::parse_byte_slice() with success.


Callers of this method must take care to ensure that bytes was previously passed through Self::validate_byte_slice() with success (and was not changed since then).


Implementations of this method may call unsafe functions to cast the pointer to the correct type, assuming the “Callers” invariant above.

Safety checklist:

  1. This method must return the same result as Self::parse_byte_slice().
  2. This method must return a slice to the same region of memory as the argument.

Provided Methods§


fn parse_byte_slice(bytes: &[u8]) -> Result<&Self, ZeroVecError>

Parses a byte slice, &[u8], and return it as &Self with the same lifetime.

If Self is not well-defined for all possible bit values, the bytes should be validated, and an error should be returned in the same cases as Self::validate_byte_slice().

The default implementation executes Self::validate_byte_slice() followed by Self::from_byte_slice_unchecked.

Note: The following equality should hold: size_of_val(result) == size_of_val(bytes), where result is the successful return value of the method. This means that the return value spans the entire byte slice.


fn as_byte_slice(&self) -> &[u8]

Given &Self, returns a &[u8] with the same lifetime.

The default implementation performs a pointer cast to the same region of memory.


Implementations of this method should call potentially unsafe functions to cast the pointer to the correct type.


fn to_boxed(&self) -> Box<Self>

Allocate on the heap as a Box<T>

Object Safety§

This trait is not object safe.

Implementations on Foreign Types§


impl VarULE for str


unsafe fn from_byte_slice_unchecked(bytes: &[u8]) -> &Self

Invariant: must be safe to call when called on a slice that previously succeeded with parse_byte_slice


fn validate_byte_slice(bytes: &[u8]) -> Result<(), ZeroVecError>


fn parse_byte_slice(bytes: &[u8]) -> Result<&Self, ZeroVecError>


impl<T> VarULE for [T]where T: ULE,

Note: VarULE is well-defined for all [T] where T: ULE, but ZeroSlice is more ergonomic when T is a low-level ULE type. For example:

// OK: [u8] is a useful type
let _: VarZeroVec<[u8]> = unimplemented!();

// Technically works, but [u32::ULE] is not very useful
let _: VarZeroVec<[<u32 as AsULE>::ULE]> = unimplemented!();

// Better: ZeroSlice<u32>
let _: VarZeroVec<ZeroSlice<u32>> = unimplemented!();