UB: matching on const value with mir-opt-level=3

[matthiaskrgr]

I tried this code:

#[derive(PartialEq, Eq, Clone)]
struct Foo {
    field: i64
}

const FOO: Foo = Foo {
    field: 5,
};

fn main() {
    match FOO {
        FOO => unreachable!(),
        _ => unreachable!(),
    }
}

When I run miri on this with mir-opt-level=3, miri reports UB, maybe some of the mir-opts is a bit broken?

MIRIFLAGS="-Zmir-opt-level=3 -Zmiri-backtrace=full" cargo miri run

error: Undefined Behavior: constructing invalid value: encountered uninitialized memory, but expected an integer
  --> src/main.rs:11:11
   |
11 |     match FOO {
   |           ^^^ constructing invalid value: encountered uninitialized memory, but expected an integer
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
   = note: BACKTRACE:
   = note: inside `main` at src/main.rs:11:11: 11:14
   = note: inside `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5: 250:71
   = note: inside `std::sys_common::backtrace::__rust_begin_short_backtrace::<fn(), ()>` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys_common/backtrace.rs:121:18: 121:21
   = note: inside closure at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:166:18: 166:82
   = note: inside `std::ops::function::impls::<impl std::ops::FnOnce<()> for &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>::call_once` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:287:13: 287:31
   = note: inside `std::panicking::r#try::do_call::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:483:40: 483:43
   = note: inside `std::panicking::r#try::<i32, &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:447:19: 447:81
   = note: inside `std::panic::catch_unwind::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:140:14: 140:33
   = note: inside closure at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:148:48: 148:73
   = note: inside `std::panicking::r#try::do_call::<[closure@std::rt::lang_start_internal::{closure#2}], isize>` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:483:40: 483:43
   = note: inside `std::panicking::r#try::<isize, [closure@std::rt::lang_start_internal::{closure#2}]>` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:447:19: 447:81
   = note: inside `std::panic::catch_unwind::<[closure@std::rt::lang_start_internal::{closure#2}], isize>` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:140:14: 140:33
   = note: inside `std::rt::lang_start_internal` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:148:20: 148:98
   = note: inside `std::rt::lang_start::<()>` at /home/matthias/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:165:17: 170:6

error: aborting due to previous error

[matthiaskrgr]

zulip thread https://rust-lang.zulipchat.com/#narrow/stream/269128-miri/topic/miri.20reports.20ub.20when.20matching.20on.20matching.20on.20enum

[matthiaskrgr]

another one:

fn test<F>(f: F) -> usize where F: FnOnce(usize) -> usize {
    return f(22);
}

pub fn main() {
    let y = test(|x| 4 * x);
    assert_eq!(y, 88);
}

error: Undefined Behavior: constructing invalid value: encountered uninitialized memory, but expected an integer
 --> src/main.rs:2:12
  |
2 |     return f(22);
  |            ^^^^^ constructing invalid value: encountered uninitialized memory, but expected an integer
  |
  = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
  = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
  = note: BACKTRACE:
  = note: inside `main` error: Undefined Behavior: constructing invalid value: encountered uninitialized memory, but expected an integer