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// SPDX-License-Identifier: GPL-2.0
//! String representations.
use core::fmt;
/// Allows formatting of [`fmt::Arguments`] into a raw buffer.
///
/// It does not fail if callers write past the end of the buffer so that they can calculate the
/// size required to fit everything.
///
/// # Invariants
///
/// The memory region between `pos` (inclusive) and `end` (exclusive) is valid for writes if `pos`
/// is less than `end`.
pub(crate) struct RawFormatter {
// Use `usize` to use `saturating_*` functions.
#[allow(dead_code)]
beg: usize,
pos: usize,
end: usize,
}
impl RawFormatter {
/// Creates a new instance of [`RawFormatter`] with the given buffer pointers.
///
/// # Safety
///
/// If `pos` is less than `end`, then the region between `pos` (inclusive) and `end`
/// (exclusive) must be valid for writes for the lifetime of the returned [`RawFormatter`].
pub(crate) unsafe fn from_ptrs(pos: *mut u8, end: *mut u8) -> Self {
// INVARIANT: The safety requirements guarantee the type invariants.
Self {
beg: pos as _,
pos: pos as _,
end: end as _,
}
}
/// Returns the current insert position.
///
/// N.B. It may point to invalid memory.
pub(crate) fn pos(&self) -> *mut u8 {
self.pos as _
}
}
impl fmt::Write for RawFormatter {
fn write_str(&mut self, s: &str) -> fmt::Result {
// `pos` value after writing `len` bytes. This does not have to be bounded by `end`, but we
// don't want it to wrap around to 0.
let pos_new = self.pos.saturating_add(s.len());
// Amount that we can copy. `saturating_sub` ensures we get 0 if `pos` goes past `end`.
let len_to_copy = core::cmp::min(pos_new, self.end).saturating_sub(self.pos);
if len_to_copy > 0 {
// SAFETY: If `len_to_copy` is non-zero, then we know `pos` has not gone past `end`
// yet, so it is valid for write per the type invariants.
unsafe {
core::ptr::copy_nonoverlapping(
s.as_bytes().as_ptr(),
self.pos as *mut u8,
len_to_copy,
)
};
}
self.pos = pos_new;
Ok(())
}
}
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