char::encode_utf8 crash on nightly

[davidhewitt]

On nightly 2021-03-05, with RUSTFLAGS="-Ccodegen-units=1 -Cinline-threshold=0 -Clink-dead-code", I am experiencing undefined behaviour which seems to be based around char::encode_utf8.

I ran this code:

fn make_string(ch: char) -> String {
    let mut bytes = [0u8; 4];
    ch.encode_utf8(&mut bytes).into()
}

fn main() {
    let ch = '😃';
    dbg!(ch);
    let string = make_string(ch);
    dbg!(string);
}

I expected to see the following output:

[src/bin/string_crash.rs:8] ch = '😃'
[src/bin/string_crash.rs:10] string = "😃"

I get the above output on stable 1.50.0, or on the same nightly version if I remove at least one of the three RUSTFLAGS listed above.

With all three flags present, on nightly 2021-03-05 I see the following output:

[src/bin/string_crash.rs:8] ch = '😃'
memory allocation of 140730017967032 bytes failed
Aborted

The exact bytes count varies, so this looks like UB to me.

Meta

rustc +nightly --version --verbose:

rustc 1.52.0-nightly (caca2121f 2021-03-05)
binary: rustc
commit-hash: caca2121ffe4cb47d8ea2d9469c493995f57e0b5
commit-date: 2021-03-05
host: x86_64-unknown-linux-gnu
release: 1.52.0-nightly
LLVM version: 12.0.0

[JohnTitor]

Regressed since rustc 1.52.0-nightly (45b3c28 2021-03-04). At a glance, #81451 might be related.

[LeSeulArtichaut]

Compiling with nightly-2021-03-05 on x86_64-apple-darwin:

rustc 1.52.0-nightly (caca2121f 2021-03-05)
binary: rustc
commit-hash: caca2121ffe4cb47d8ea2d9469c493995f57e0b5
commit-date: 2021-03-05
host: x86_64-apple-darwin
release: 1.52.0-nightly
LLVM version: 12.0.0

I am not able to reproduce the crash, though the result is still incorrect:

$ RUSTFLAGS="-Ccodegen-units=1 -Cinline-threshold=0 -Clink-dead-code" cargo +nightly run
   Compiling issue_82833 v0.1.0 (/Users/tous/Documents/Dev/rustc_issues/issue_82833)
    Finished dev [unoptimized + debuginfo] target(s) in 46.25s
     Running `target/debug/issue_82833`
[src/main.rs:8] ch = '😃'
[src/main.rs:10] string = "\u{0}\u{0}\u{0}\u{0}"

[LeSeulArtichaut]

#81451 is indeed the culprit:

$ RUSTFLAGS="-Ccodegen-units=1 -Cinline-threshold=0 -Clink-dead-code" cargo +ec7f258d543e1ac7d0b94435972331e85da8c509 run
   Compiling issue_82833 v0.1.0 (/Users/tous/Documents/Dev/rustc_issues/issue_82833)
    Finished dev [unoptimized + debuginfo] target(s) in 5.15s
     Running `target/debug/issue_82833`
[src/main.rs:8] ch = '😃'
[src/main.rs:10] string = "😃"

$ RUSTFLAGS="-Ccodegen-units=1 -Cinline-threshold=0 -Clink-dead-code" cargo +409920873cf8a95739a55dc5fe5adb05e1b4758e run
   Compiling issue_82833 v0.1.0 (/Users/tous/Documents/Dev/rustc_issues/issue_82833)
    Finished dev [unoptimized + debuginfo] target(s) in 16.55s
     Running `target/debug/issue_82833`
[src/main.rs:8] ch = '😃'
[src/main.rs:10] string = "\u{0}\u{0}\u{0}\u{0}"

[LeSeulArtichaut]

@rustbot ping llvm