Bitfields in Rust.
Regent provides the attribute macro bitwise which generates methods for accessing struct fields of arbitrary bit widths.
Add the regent crate from crates.io to your Cargo.toml:
toml
[dependencies]
regent = "0.1"
This struct:
```rust
struct Timestamp { // 1900..=2411 year: u9, // 1..=16 month: u4, // 1..=32 day: u5, } ```
...roughly expands to:
```rust
struct Timestamp(u32);
// (Function and trait implementations are omitted for brevity.)
impl Timestamp { fn new(year: u16, month: u8, day: u8) -> Self {/* ... */}
fn year(&self) -> u16 {/* ... */}
fn month(&self) -> u8 {/* ... */}
fn day(&self) -> u8 {/* ... */}
fn set_year(&mut self, field: u16) {/* ... */}
fn set_month(&mut self, field: u8) {/* ... */}
fn set_day(&mut self, field: u8) {/* ... */}
}
impl From
To serialize a Timestamp into a u32, you might write:
rust
let se = u32::from(Timestamp::new(year, month, day));
To deserialize a Timestamp from a u32, you might write:
```rust
let de = Timestamp::from(se); let year = de.year(); let month = de.month(); let day = de.day(); ```
The first field in a struct annotated with #[regent::bitwise] is taken to be 'least-significant', and the last field is 'most-significant'.
The bit width of a field is inferred by the type, which can be bool, an arbitrary-width unsigned integer of the form uwidth, or a tuple or array of these types, with the exception of zero-sized types such as the 0-width integer u0, the unit (), and the 0-element array [_; 0], which are explicitly disallowed. Fields, as well as tuple and array elements, are tightly packed and are not aligned in any way.
| Type | Width (bits) |
|----------------|------------------------------------|
| bool | 1 |
| uwidth | width |
| (A, ...) | sum of widths of constituent types |
| [T; len] | product of len and width of T |
bitwise structs are transparently represented by an unsigned integer of the minimum available width large enough to contain all fields. Because Rust offers a limited set of unsigned integer primitives, a struct in which the total width of its fields does not correspond to an existing primitive is 'rounded up' to the next-smallest representation.
Take the Timestamp struct from the Examples section:
```rust
struct Timestamp { year: u9, month: u4, day: u5, } ```
A 9-bit year, 4-bit month, and 5-bit day makes an 18-bit timestamp, which is rounded up to a u32. Hence, Regent generates this definition:
```rust
struct Timestamp(u32); ```
Internally, Timestamp is represented in binary as:
txt
(unused) day month year
00000000000000 ddddd mmmm yyyyyyyyy
^ bit 31 ^ bit 15 ^ bit 0
This definition of Timestamp does not permit field access via dot-notation like timestamp.year. Instead, Regent generates the methods Timestamp::year and Timestamp::set_year to read from and write to the year field.
Similarly to how struct representations are 'rounded up' to the next-smallest unsigned integer primitive, the return type of Timestamp::year and argument type to Timestamp::set_year are 'rounded up' from the imaginary u9 type to the u16 primitive. Hence, Regent generates these method signatures:
rust
impl Timestamp {
fn year(&self) -> u16 {/* ... */}
fn set_year(&mut self, field: u16) {/* ... */}
}
Timestamp::year zero-extends the year field, and Timestamp::set_year ignores the most-significant 5 bits of field.
Regent respects the visibility of both structs and struct fields. Visibility of the struct item propagates to the new method, and per-field visibility propagates to getter and setter methods.
Regent has basic support for tuple structs annotated with bitwise. This definition:
```rust
struct TupleStruct(u7, bool); ```
...roughly expands to:
```rust
struct TupleStruct(u8);
impl TupleStruct { fn 0(&self) -> u8 {/* .. */} fn set0(&mut self, field: u8) {/* ... */}
fn _1(&self) -> bool {/* ... */}
fn set_1(&mut self, field: bool) {/* ... */}
}
// ... ```
Regent places some restrictions on how the bitwise macro may be used. These include:
bitwise cannot be zero-sized, nor can the type of any single field within a struct be zero-sized.Generic parameters are not supported on structs, e.g., you cannot have:
```rust
struct A
This may be made a feature in the future.
8 * 4 are not supported for technical reasons.Bug reports, feature requests, and other enhancements are greatly appreciated. Kindly submit them to the GitHub repository.
Regent was originally developed as part of the PlayStation emulator Noctane to model CPU and MMIO registers, but it has since been factored out into its own GitHub repository.
Regent is inspired by the modular-bitfield crate, created by Robin Freyler (@Robbepop) and itself inspired by David Tolnay's (@dtolnay) proc macro workshop. (Go check it out!)
In particular, I liked the ergonomics of field access via Struct::field and Struct::set_field methods but found the Struct::new and Struct::with_field mechanism made it too easy to accidentally leave fields uninitialized, and I did not like how many items are brought into scope with use modular_bitfield::prelude::*, which defines analogue types to Regent's uwidth pseudo-types (among other things). modular-bitfield is a superb crate overall, but I wanted something a little different for Noctane.