musli_binary_common/
fixed_bytes.rs

1//! A container which can store up to a fixed number of uninitialized bytes on
2//! the stack and read into and from it.
3
4use core::fmt;
5use core::marker;
6use core::mem::MaybeUninit;
7use core::ptr;
8
9use musli::error::Error;
10
11use crate::writer::Writer;
12
13/// A fixed-size bytes storage which keeps track of how much has been initialized.
14pub struct FixedBytes<const N: usize, E = FixedBytesWriterError> {
15    /// Data storage.
16    data: [MaybeUninit<u8>; N],
17    /// How many bytes have been initialized.
18    init: usize,
19    /// Error type to raise when this is used as a `Writer` implementation.
20    _marker: marker::PhantomData<E>,
21}
22
23impl<const N: usize, E> FixedBytes<N, E> {
24    /// Construct a new fixed bytes array storage.
25    pub const fn new() -> Self {
26        Self {
27            // SAFETY: MaybeUnint::uninit_array is not stable.
28            data: unsafe { MaybeUninit::<[MaybeUninit<u8>; N]>::uninit().assume_init() },
29            init: 0,
30            _marker: marker::PhantomData,
31        }
32    }
33
34    /// Get the length of the collection.
35    pub const fn len(&self) -> usize {
36        self.init
37    }
38
39    /// Coerce into the underlying bytes if all of them have been initialized.
40    pub fn into_bytes(self) -> Option<[u8; N]> {
41        if self.init == N {
42            // SAFETY: All of the bytes in the sequence have been initialized
43            // and can be safety transmuted.
44            //
45            // Method of transmuting comes from the implementation of
46            // `MaybeUninit::array_assume_init` which is not yet stable.
47            unsafe { Some((&self.data as *const _ as *const [u8; N]).read()) }
48        } else {
49            None
50        }
51    }
52
53    /// Coerce into the slice of initialized memory which is present.
54    pub fn as_bytes(&self) -> &[u8] {
55        if self.init == 0 {
56            return &[];
57        }
58
59        // SAFETY: We've asserted that `initialized` accounts for the number of
60        // bytes that have been initialized.
61        unsafe { core::slice::from_raw_parts(self.data.as_ptr() as *const u8, self.init) }
62    }
63}
64
65decl_message_repr!(FixedBytesWriterErrorRepr, "failed to write to fixed bytes");
66
67/// An error raised while decoding a slice.
68#[derive(Debug)]
69pub struct FixedBytesWriterError(FixedBytesWriterErrorRepr);
70
71impl fmt::Display for FixedBytesWriterError {
72    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
73        self.0.fmt(f)
74    }
75}
76
77impl Error for FixedBytesWriterError {
78    #[inline]
79    fn custom<T>(message: T) -> Self
80    where
81        T: 'static + Send + Sync + fmt::Display + fmt::Debug,
82    {
83        Self(FixedBytesWriterErrorRepr::collect(message))
84    }
85
86    #[inline]
87    fn message<T>(message: T) -> Self
88    where
89        T: fmt::Display,
90    {
91        Self(FixedBytesWriterErrorRepr::collect(message))
92    }
93}
94
95#[cfg(feature = "std")]
96impl std::error::Error for FixedBytesWriterError {}
97
98impl<const N: usize, E> Writer for FixedBytes<N, E>
99where
100    E: Error,
101{
102    type Error = E;
103
104    #[inline]
105    fn write_bytes(&mut self, bytes: &[u8]) -> Result<(), Self::Error> {
106        if bytes.len() > N.saturating_sub(self.init) {
107            return Err(E::message(BufferOverflow {
108                at: self.init,
109                additional: bytes.len(),
110                capacity: N,
111            }));
112        }
113
114        unsafe {
115            let dst = (self.data.as_mut_ptr() as *mut u8).add(self.init);
116            ptr::copy_nonoverlapping(bytes.as_ptr(), dst, bytes.len());
117        }
118
119        self.init += bytes.len();
120        Ok(())
121    }
122}
123
124struct BufferOverflow {
125    at: usize,
126    additional: usize,
127    capacity: usize,
128}
129
130impl fmt::Display for BufferOverflow {
131    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
132        let Self {
133            at,
134            additional,
135            capacity,
136        } = *self;
137
138        write!(
139            f,
140            "Overflow when writing {additional} bytes at {at} with capacity {capacity}"
141        )
142    }
143}
musli_binary_common/fixed_bytes.rs

musli_binary_common/
fixed_bytes.rs
