Different behavior for --cfg debug_assertions versus -C debug_assertions

[WesleyRosenblum]

The following two commands have different behavior:

1. RUSTFLAGS="--cfg debug_assertions" cargo run --release
2. RUSTFLAGS="-C debug_assertions" cargo run --release

1 does not enable integer overflow checks (and alignment checks), while 2 does. The rustc Codegen docs that mention "overflow checks are enabled if debug-assertions are enabled, disabled otherwise", do not apply to the --cfg debug_assertions version.

This can be tested using this small program:

fn subtract(input: u64) -> u64 {
    0 - input
}

pub fn main() {
    let result = subtract(5);
    debug_assert_ne!(result, 18446744073709551611);
}

$ RUSTFLAGS="--cfg debug_assertions" cargo run --release 
thread 'main' panicked at 'assertion failed: `(left != right)`
  left: `18446744073709551611`,
 right: `18446744073709551611`', src/main.rs:7:5

$ RUSTFLAGS="-C debug_assertions" cargo run --release 
thread 'main' panicked at 'attempt to subtract with overflow', src/main.rs:2:5

I'm not sure if this just needs a documentation update, or something more substantial like prohibiting usage of --cfg debug_assertions without also setting -C debug_assertions (à la rust-lang/compiler-team#610). Either way, it seems like a sharp edge that could trip people up.