Crate proc_bitfield

proc-bitfield

A Rust crate to expressively declare bitfield-like structs, automatically ensuring their correctness at compile time and declaring accessors.

nightly feature

Optionally, the nightly feature can be enabled to use experimental features exclusive to nightly Rust. This currently enables the UnwrapBitRange derive.

The bitfield! macro

Automatic trait implementations

After the struct’s name and its storage type declaration, a list of automatic trait implementations can be optionally added. For example, the following declaration will result in all automatic implementations being applied:

bitfield! {
    pub struct Example(pub u8): Debug, FromRaw, IntoRaw, DerefRaw { /* ... */ }
}

Currently, the allowed automatic implementations are Debug, FromRaw, IntoRaw and DerefRaw.

Debug

If specified, core::fmt::Debug will be implemented automatically for the current bitfield struct; the generated fmt function will output the type’s raw value as well as all of its readable fields’ values.

FromRaw

If specified, core::convert::From<$storage_ty> will be implemented automatically for the current bitfield struct; the generated from function will construct an instance of the bitfield struct from the provided value directly, with no additional checks, analogously to $bitfield_ty(raw) in a context where the bitfield struct’s raw value field is accessible. This does not check or change the previously declared visibility of the bitfield struct’s raw value field (bitfield.0), or any other such manually-declared fields, so care must be taken to maintain consistency.

IntoRaw

If specified, core::convert::From<$bitfield_ty> will be implemented automatically for the current bitfield struct’s storage type (and consequently, core::convert::Into<$storage_ty> for the bitfield type); the generated from function will read the bitfield’s raw value, with no additional changes, analogously to bitfield.0 in a context where the bitfield struct’s raw value field is accessible. Analogously to FromRaw, care must be taken to maintain consistency with the visibility of the bitfield struct’s raw value outside this implementation.

DerefRaw

If specified, core::ops::Deref will be implemented automatically for the current bitfield struct; the generated deref function will read the bitfield’s raw value directly, analogously to &bitfield.0 in a context where the bitfield struct’s raw value field is accessible. Analogously to FromRaw, care must be taken to maintain consistency with the visibility of the bitfield struct’s raw value outside this implementation.

Field declarations

Fields can be declared by using the form:

Visibility IDENTIFIER : Type ([(Option ,)^* Option])^? @ FieldRange

where FieldRange corresponds to any of (where L is an alias for LiteralExpression):

• .., to use every bit
• L..=L, to use the bits specified by an inclusive range
• L..L, to use the bits specified by an exclusive range
• L; L, to use bits specified by a (start, length) pair
• L, to use a single bit; unlike all other specifications, this is only valid for bool fields, and will use the Bit trait instead of BitRange

Options can be optionally specified in brackets, matching any of the ones defined below.

Access restrictions

Fields are both readable and writable by default, but can be declared read-only or write-only using respectively the read_only/ro and write_only/wo options.

Field type conversions

Fields’ “raw” types as specified after the colon are restricted by BitRange<T> implementations on the bitfield’s contained type; however, accessors can perform conversions specified through optional options. These can be:

• Infallible conversions, using the From<T> and Into<T> traits, the relevant options being:
  • get Type, specifying the type that the raw value will be converted into on reads, using From<T>
  • set Type, specifying the type that will be converted into the raw value on writes, using Into<T>
  • Type, as a shorthand for get Type and set Type
• Fallible conversions, using the TryFrom<T> and TryInto<T> traits, the relevant options being:
  • try_get Type, specifying the type that the raw value will be fallibly converted into on reads, using TryFrom<T>
  • try_set Type, specifying the type that will be fallibly converted into the raw value on writes, using TryInto<T>
  • try_both Type, as a shorthand for try_get Type and try_set Type
  • try Type, as a shorthand for try_get Type and set Type
• Unwrapping conversions, using the TryFrom<T> and TryInto<T> traits and unwrapping the conversion results, the relevant options being:
  • unwrap_get Type, specifying the type that the raw value will be fallibly converted into and unwrapped on reads, using TryFrom<T> and then Result::unwrap
  • unwrap_set Type, specifying the type that will be fallibly converted into the raw value and unwrapped on writes, using TryInto<T> and then Result::unwrap
  • unwrap_both Type, as a shorthand for unwrap_get Type and unwrap_set Type
  • try Type, as a shorthand for unwrap_get Type and set Type
• Unsafe conversions, using the UnsafeFrom<T> and UnsafeInto<T> traits, the relevant options being:
  • unsafe_get Type, specifying the type that the raw value will be unsafely converted into on reads, using UnsafeFrom<T>
  • unsafe_set Type, specifying the type that will be unsafely converted into the raw value on writes, using UnsafeInto<T>
  • unsafe_both Type, as shorthand for unsafe_get Type and unsafe_set Type
  • unsafe Type, as shorthand for unsafe_get Type and set Type

The bits!, with_bits! and set_bits! macros

These macros provide simplified bitfield functionality without the need to declare a bitfield struct: the value serving as an anonymous bitfield is provided as their first argument, followed by the FieldRange to access analogously to the fields declarations in the bitfield! macro, i.e. bits!(0x1234_u16, 0..=15).

For the with_bits! and set_bits! macros, a new value for the field is provided by appending = value to the field bit range specification, i.e. with_bits!(0x1234, 0..4 = 0xF), set_bits!(b, 15 = true).

Specifying bitfield and field types

In cases where type inference fails, the accessed field’s type T can be specified by prepending T @ to the FieldRange specification, i.e. bits!(0x1234_u16, u8 @ 0..=7). The macros can also detect simple as casts in the provided expressions (for the bitfield’s value in all cases, and for the field’s new value for with_bits! and set_bits!) and treat them as explicit type specifications.

Due to implementation limitations, specifying the bitfield’s storage type through a cast is required when the field’s bit range is .., i.e. bits!(0x1234 as u16, ..).

An explicit field type mustn’t be specified when accessing a single bit as a boolean (using the single LiteralExpression form of FieldRange), as analogously to bitfield! fields it’s always fixed to bool.

Formal syntax

The general formal syntax for macro calls is:

bits!:

Expression , (Type@)^? FieldRange

with_bits! and set_bits!:

Expression , (Type@)^? FieldRange = Expression

Other derive macros

The crate provides other supporting derive macros associated with bitfield functionality.

ConvRaw

ConvRaw is a derive macro to implement any applicable conversion traits between a non-empty fieldless enum and the builtin integer types corresponding to variant discriminants.

It will implement TryFrom<T> for Enum for all builtin integer types T, and From<Enum> for T for all types that can fit all the enum discriminants.

UnwrapBitRange

UnwrapBitRange is a derive macro to implement BitRange<T> for U for a type T and all builtin integer types U used as bitfield storage types.

For each integer type U, it requires T: TryFrom<U> + Into<U>, and unwraps the result of <T as TryFrom<U>>::try_from to convert the field’s raw integer value to T on reads.

This derive macro is currently gated behind the nightly feature, as it requires #![feature(trivial_bounds)] to be enabled in the crate using it.

Modules

• example
  Sample bitfields to showcase the crate’s features

Macros

• bitfield
  The main focus of the crate. Defines a bitfield struct.
• bits
  Reads a single field from an anonymous bitfield, without creating a bitfield struct.
• set_bits
  Modifies a single field in an anonymous bitfield, without creating a bitfield struct.
• with_bits
  Returns an anonymous bitfield with a single field modified, without creating a bitfield struct.

Traits

• Bit
  A trait to read or modify a single bit inside a value.
• BitRange
  A trait to read or modify a range of bits inside a value.
• UnsafeFrom
  Unsafe equivalent of From.
• UnsafeInto
  Unsafe equivalent of Into.

Derive Macros

• ConvRaw
  A derive macro to implement any applicable conversion traits between an enum and the builtin integer types corresponding to variant discriminants.
• UnwrapBitRangenightly
  A derive macro to implement BitRange<T> for U for a type T and all integer bitfield storage types U, by unwrapping the conversion results.