rustdoc doesn't pass through `-Cdebug_assertions=<bool>` to doctest (consequently `cargo doc` doesn't compile doctests in release mode if `--release` is passed)

[WildCryptoFox]

doc-tests do not compile in release mode with --release

#[cfg(debug_assertions)]
compile_error!("use --release mode!");

   Compiling playground v0.0.1 (/playground)
    Finished release [optimized] target(s) in 0.58s
     Running target/release/deps/playground-c4b7af53795dc52c
   Doc-tests playground
error: use --release mode!
 --> /playground/src/lib.rs:2:1
  |
2 | compile_error!("use --release mode!");
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Standard Output

running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

[ehuss]

This is both a cargo issue and a rustdoc issue. There are more details at rust-lang/cargo#6570 (comment). Will need to hear from t-rustdoc to know what to do next, but I think it would be nice to unify the codegen option parsing between rustc and rustdoc.

[steveklabnik]

Triage: not aware of any changes here.

[fmease]

For posterity, here's a rustdoc reproducer:

rustdoc --test -Cdebug_assertions=no file.rs:

//! ```
//! #[cfg(debug_assertions)]
//! compile_error!("use --release mode!");
//! ```

This used to fail (as reported in the issue description) but since rustdoc now forwards "-C* flags" to doctests this compiles fine now.

I guess we can close this? Obviously, --release (well, the default Cargo profile release) entails not just -Cdebug_assertions but also -Copt-level. I need to double check if -O also gets forwarded nowadays, I mean it very likely will because it's just an alias for -Copt-level=3.

I'll just close this in favor of rust-lang/cargo#6570. Please ping if we should reopen.