proc-bitfield

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

Typical usage

bitfield! {
    #[derive(Clone, Copy, PartialEq, Eq)]
    pub struct Example(pub u16): Debug {
        // A single field spanning the entire bitfield, using an unbounded range
        pub raw: u16 @ ..,

        // Single-bit flags
        pub vblank: bool [read_only] @ 0,
        pub hblank: bool [write_only] @ 1,
        pub vcount_match: bool @ 2,

        // Multi-bit field, specified using an inclusive range
        pub irq_mask: u8 @ 3..=5,

        // Bit 6 is ignored

        // Single-bit field, specified using an exclusive range
        pub vcount_compare_high: u8 @ 7..8,

        // 8-bit field specified using its start bit and length
        pub vcount_compare_low: u8 @ 8; 8,
    }
}

Automatic Debug implementation

A fmt::Debug implementation can be implemented automatically for a given bitfield struct by adding : Debug after the tuple struct-like storage type declaration; the generated fmt function will output the type's raw value as well as all of its fields' values.

nightly feature

Optionally, the nightly feature can be enabled to use const Trait functionality: this makes the BitRange and Bit traits be implemented using const fns for all integer types, and enables the option to use const fns for field accessors.

With the feature enabled, const fn accessors can be enabled globally for a struct by replacing struct with const struct (i.e. const struct Example(pub u16)), or on a field-by-field basis by prepending const to its type (i.e. raw: const u16 @ ..).

Field declarations

Fields can be declared by using the form:

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

Attributes 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 and write_only attributes.

Field type conversions

Fields' "raw" types as specified after the colon are restricted by BitRange implementations on the bitfield's contained type; however, accessors can perform conversions specified through optional attributes. These can be:

• Infallible conversions, using the From and Into traits, the relevant attributes being:
  • get Type, specifying the type that the raw value will be converted into using From<T> for reads
  • set Type, specifying the type that will be converted into the raw value using Into<T> for writes
  • Type, as shorthand for get Type and set Type
• Fallible conversions, using the TryFrom and TryInto traits, the relevant attributes being:
  • try_get Type, specifying the type that the raw value will be fallibly converted into using TryFrom<T> for reads
  • try_set Type, specifying the type that will be fallibly converted into the raw value into using TryInto<T> for writes
  • try Type, as shorthand for try_get Type and try_set Type
• Unsafe (for reads) conversions, using the UnsafeFrom and Into traits, the relevant attributes being:
  • unsafe_get Type, specifying the type that the raw value will be unsafely converted into using UnsafeFrom<T> for reads
  • unsafe Type, as shorthand for unsafe_get Type and set Type