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fix: replace transmute UB in error constructors with safe alternatives #58

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Merged
nlordell merged 3 commits into from
Apr 20, 2026
Merged

fix: replace transmute UB in error constructors with safe alternatives #58

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480d584
fix: replace transmute UB in error constructors with safe alternatives
LuciferYang Apr 18, 2026
a91605d
address review: restore const fn, use unreachable!(), remove Zero branch
LuciferYang Apr 19, 2026
f031067
remove parse_int_error_zero test that only tests stdlib behavior
LuciferYang Apr 19, 2026
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38 changes: 27 additions & 11 deletions src/error.rs
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Original file line number Diff line number Diff line change
@@ -1,19 +1,35 @@
//! Module with hacks for creating error variants for standard library errors
//! without public interfaces.
//! Module with safe helpers for creating error variants for standard library
//! errors without public constructors.

use core::{
mem,
num::{IntErrorKind, ParseIntError, TryFromIntError},
};
use core::num::{IntErrorKind, ParseIntError, TryFromIntError};

/// Returns a `ParseIntError` from an `IntErrorKind`.
pub const fn pie(kind: IntErrorKind) -> ParseIntError {
unsafe { mem::transmute(kind) }
/// Returns a `ParseIntError` with the specified `IntErrorKind`.
pub fn pie(kind: IntErrorKind) -> ParseIntError {
match kind {
IntErrorKind::Empty => u8_parse_error(""),
IntErrorKind::InvalidDigit => u8_parse_error("?"),
IntErrorKind::PosOverflow => u8_parse_error("256"),
IntErrorKind::NegOverflow => i8_parse_error("-129"),
IntErrorKind::Zero => zero_parse_error(),

@nlordell nlordell Apr 19, 2026 •
edited
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The Zero is only used in the test below, so it can be removed.

@LuciferYang LuciferYang Apr 19, 2026

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Removed. Also dropped the NonZeroU32 import that was only there for the Zero test.

_ => u8_parse_error("?"), // fallback for future variants

@nlordell nlordell Apr 19, 2026

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This should be unreachable!() IMO - the input kind is limited to the uses of this function internal to the library, so we know that this branch will never be reached.

@LuciferYang LuciferYang Apr 19, 2026

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Done, switched to unreachable\!().

}
}

fn u8_parse_error(s: &str) -> ParseIntError {
s.parse::<u8>().unwrap_err()
}

@nlordell nlordell Apr 19, 2026

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If we use https://doc.rust-lang.org/stable/std/primitive.u8.html#method.from_str_radix, then the function can stay const:

const fn u8_parse_error(s: &str) -> std::num::ParseIntError {
    let Err(err) = u8::from_str_radix(s, 10) else {
        panic!("not a parse error!");
    };
    err
}

fn main() {
    let x = u8_parse_error("-1");
    println!("{x:?}!");
}

Same for the i8_parse_error function, (allowing pie to stay const).

@LuciferYang LuciferYang Apr 19, 2026

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Nice, from_str_radix works perfectly here. pie() and the helpers are const fn again. tfie() stays non-const since TryFrom is not const-stable yet, but no call site needs it in const context anyway.


fn i8_parse_error(s: &str) -> ParseIntError {
s.parse::<i8>().unwrap_err()
}

fn zero_parse_error() -> ParseIntError {
"0".parse::<core::num::NonZeroU8>().unwrap_err()
}

/// Returns a `TryFromIntError`.
pub const fn tfie() -> TryFromIntError {
unsafe { mem::transmute(()) }
pub fn tfie() -> TryFromIntError {
u8::try_from(-1i8).unwrap_err()
}

#[cfg(test)]
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