In certain circumstances, true != !false is considered true.

Steps to reproduce

1. Create a new project with cargo new negation --bin
2. Change the file src/main.rs so that the contents is the following:

fn g() -> bool {
    false
}

fn main() {
    let a = !g();
    if a != !g() {
        println!("wrong");
    } else {
        println!("correct");
    }
}

3.Run with cargo run

Expected and observed behavior

When run with cargo run, the compiled program prints wrong.
I expect correct to be printed because after the assignment,
a should still equal the value it was assigned to (note that g() always
returns the same value).

Note that cargo run --release results in printing correct, so even if it's me who is being confused, the output should still be the same in debug mode and in release mode.

Meta

When I do a normal rustc main.rs and witch cargo run --release the resulting
program behaves as I expect.
The unexpected behaviour only occurs when using cargo run (or running target/debug/negation).

rustc --version --verbose:

rustc 1.12.0 (3191fbae9 2016-09-23)
binary: rustc
commit-hash: 3191fbae9da539442351f883bdabcad0d72efcb6
commit-date: 2016-09-23
host: x86_64-unknown-linux-gnu
release: 1.12.0

cargo --version --verbose

cargo 0.13.0-nightly (109cb7c 2016-08-19)

Edit: added short description.