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

[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.