Document that `-C opt-level=0` implies `-C debug-assertions` more prominently

[mystor]

All I was able to find documenting this is an ambiguous mention in the rust reference:
https://github.com/rust-lang/rust/blob/17cae033787aca26b00a8c820c1599926a1fd94d/src/doc/reference.md#conditional-compilation

It would be nice if, for example, the documentation for std::debug_assert! was updated to mention:

Unlike assert!, debug_assert! statements are only enabled in non optimized builds by default. An optimized build will omit all debug_assert! statements unless -C debug-assertions or -C opt-level=0 is passed to the compiler. This makes debug_assert! useful for checks that are too expensive to be present in a release build but may be helpful during development.

[nagisa]

“An optimized build” seems to very clearly imply that opt-level ≠ 0 to me.

[mystor]

That's true - I missed that on the first reading.

[daxpedda]

Was currently trying to work with the cargo api and was struggling to find the issue until I found out about this.

In the new documentation this is mentioned nowhere:
https://doc.rust-lang.org/nightly/rustc/codegen-options/index.html#opt-level