`PRUSTI_CHECK_PANICS=false` causes a compiler panic for a called pure arithmetic function

[lynae99]

Prusti 0.2.2 reports a normal verification error with its default configuration,
but panics when panic checking is disabled.

Reproducer

Save the following as PureCallCheckPanics.rs:

use prusti_contracts::*;

#[pure]
fn negate(value: i32) -> i32 {
    -value
}

fn main() {
    negate(0);
}

Reproduction commands

PRUSTI=/path/to/prusti-rustc
PRUSTI_HOME=/path/to/prusti
DEFAULT_OUT=$(mktemp -d)
TARGET_OUT=$(mktemp -d)

RUSTUP_TOOLCHAIN=nightly-2023-09-15 \
  "$PRUSTI" PureCallCheckPanics.rs --crate-name pure_call_check_panics \
  --edition=2021 -L "$PRUSTI_HOME" -L "$PRUSTI_HOME/deps" \
  --out-dir "$DEFAULT_OUT"

PRUSTI_CHECK_PANICS=false RUSTUP_TOOLCHAIN=nightly-2023-09-15 \
  "$PRUSTI" PureCallCheckPanics.rs --crate-name pure_call_check_panics \
  --edition=2021 -L "$PRUSTI_HOME" -L "$PRUSTI_HOME/deps" \
  --out-dir "$TARGET_OUT"

Actual result

The default configuration reports a normal verification error:

assertion might fail with "attempt to negate with overflow"

With PRUSTI_CHECK_PANICS=false, the compiler exits with status 101:

thread 'rustc' panicked at prusti/src/verifier.rs:76:17
assertion failed: env.diagnostic.has_errors() || ...

The result was reproduced in three independent runs.

Expected result

Disabling panic checking should not cause the compiler to panic. Prusti should
finish normally or report a regular diagnostic.

Version

Prusti 0.2.2, commit 0d4a8d4
rustc 1.74.0-nightly