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Implement From<TryReserveError> for io::Error #96505

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Implement From<TryReserveError> for io::Error #96505

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a191d1c
Implement `From<TryReserveError>` for `io::Error`
nvzqz Apr 28, 2022
ecb94ba
Add error messages
nvzqz Apr 28, 2022
c35eb0a
Switch to `io::Error::new`
nvzqz Apr 28, 2022
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14 changes: 14 additions & 0 deletions library/std/src/io/error.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@ mod repr_unpacked;
#[cfg(not(target_pointer_width = "64"))]
use repr_unpacked::Repr;

use crate::collections::{TryReserveError, TryReserveErrorKind};
use crate::convert::From;
use crate::error;
use crate::fmt;
Expand Down Expand Up @@ -465,6 +466,19 @@ impl From<ErrorKind> for Error {
}
}

#[stable(feature = "io_error_from_try_reserve", since = "1.62.0")]
impl From<TryReserveError> for Error {
#[inline]
fn from(error: TryReserveError) -> Error {
let kind = match error.kind() {
TryReserveErrorKind::CapacityOverflow => ErrorKind::InvalidInput,
TryReserveErrorKind::AllocError { .. } => ErrorKind::OutOfMemory,
};

Error::new(kind, error)
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Note that this involves a double allocation, effectively a Box<Box<dyn io::Error>>.

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I didn't realize io::Error::new was a double-allocation! I wonder if thin dyn box would solve that.

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Even worse: if you pass it a string literal ( Error::new(kind, "...")) you get a triple allocation (and a strcpy/memcpy) as the inner box will contain a String.

ThinBox can save one allocation here. I think that'd be a simple change to make, but there might be some non-obvious pitfall.

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See #83352

}
}

impl Error {
/// Creates a new I/O error from a known kind of error as well as an
/// arbitrary error payload.
Expand Down