char::from_u32_unchecked panics at runtime in 1.78.0 but works in 1.77.1

[bvanjoi]

Code

I tried this code:

fn main() {
    let ch = unsafe {
        std::char::from_u32_unchecked(55357)
    };
    dbg!(ch);
}

I expected to see this happen: run success.

Instead, this happened: panic at runtime and report "invalid value for char"

Version it worked on

It most recently worked on: 1.77.1

Version with regression

rustc --version --verbose:

rustc 1.78.0 (9b00956e5 2024-04-29)
binary: rustc
commit-hash: 9b00956e56009bab2aa15d7bff10916599e3d6d6
commit-date: 2024-04-29
host: aarch64-apple-darwin
release: 1.78.0
LLVM version: 18.1.2

Backtrace

Backtrace

thread 'main' panicked at library/core/src/panicking.rs:156:5:
unsafe precondition(s) violated: invalid value for `char`
stack backtrace:
   0: rust_begin_unwind
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/std/src/panicking.rs:645:5
   1: core::panicking::panic_nounwind_fmt::runtime
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/panicking.rs:110:18
   2: core::panicking::panic_nounwind_fmt
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/panicking.rs:123:9
   3: core::panicking::panic_nounwind
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/panicking.rs:156:5
   4: core::char::convert::from_u32_unchecked::precondition_check
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/intrinsics.rs:2799:21
   5: core::char::convert::from_u32_unchecked
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/char/convert.rs:28:9
   6: core::char::from_u32_unchecked
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/char/mod.rs:131:14
   7: rust_local_test::main
             at ./src/main.rs:29:9
   8: core::ops::function::FnOnce::call_once
             at /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/ops/function.rs:250:5

[Urgau]

I think this is expected, it was in the compatibility notes of Rust 1.78:

Compatibility Notes

• Many unsafe precondition checks now run for user code with debug assertions enabled This change helps users catch undefined behavior in their code, though the details of how much is checked are generally not stable.

[bvanjoi]

@Urgau Thanks for the quick reply! I think we can close this issue.