Out-of-bound array access not reported when checking the crate

[scrabsha]

It seems that rustc is able to check for invalid array indexing at compile-time under some circumstances.

For instance, let's consider a crate, with the following code in it.

fn main() {
    let a = [42];
    // Incorrect indexing
    println!("{}", a[1]);
}

cargo run and cargo build both report the following error:

error: this operation will panic at runtime
 --> src/main.rs:4:20
  |
4 |     println!("{}", a[1]);
  |                    ^^^^ index out of bounds: the length is 1 but the index is 1
  |
  = note: `#[deny(unconditional_panic)]` on by default

However, cargo check reports nothing.

Running cargo clean before each command does not change anything.

I have set up a repository, which may help people to test it by themselves.

Meta

While I discovered this on the stable channel, nightly and beta are also affected.

rustc --version --verbose:

rustc 1.49.0 (e1884a8e3 2020-12-29)
binary: rustc
commit-hash: e1884a8e3c3e813aada8254edfa120e85bf5ffca
commit-date: 2020-12-29
host: x86_64-unknown-linux-gnu
release: 1.49.0

rustc +nightly --version --verbose :

rustc 1.51.0-nightly (c5a96fb79 2021-01-19)
binary: rustc
commit-hash: c5a96fb7973649807a7943e7395456db158dcab6
commit-date: 2021-01-19
host: x86_64-unknown-linux-gnu
release: 1.51.0-nightly
LLVM version: 11.0.1

rustc +beta --version --verbose:

rustc 1.50.0-beta.6 (ea20aa255 2021-01-14)
binary: rustc
commit-hash: ea20aa255b54708fb6a86f3146244eb20d079513
commit-date: 2021-01-14
host: x86_64-unknown-linux-gnu
release: 1.50.0-beta.6

[jonas-schievink]

I believe this is expected behavior, since these warnings are reported by the constant-propagation pass, which might not always run

[oli-obk]

yea, in order to make cargo check as fast as possible, we skip the entire MIR optimization pipeline, which also includes const prop. This is obviously not ideal, but so far we haven't been able to come up with anything that won't either duplicate the const prop pass or make cargo check much more expensive to run in some cases.

[Enselic]

Triage: Closing as wont fix as discussed above.