`CARGO_ENCODED_RUSTFLAGS` doesn't include flags for builtin Cargo options

[madsmtm]

In particular, it doesn't include LTO flags when using Cargo's [profile.*] lto = "..." option.

This prevents us from correctly passing -flto=..., which affects Rust's bootstrap, and which I hit recently in rust-lang/rust#148925.

I'm unsure if this is something that Cargo is aware of, though see e.g. rust-lang/cargo#12437 for a similar issue. CC @weihanglo, how do you think Cargo would best expose to build scripts like cc-rs that it's doing LTO?

[weihanglo]

It is more similar to these:

• Allow build scripts to detect debug_assertions setting cargo#15760
• How can debuginfo be detected from a build.rs ? cargo#5891

Unfortunately CARGO_ENCODED_RUSTFLAGS doesn't not expose flags controlled by Cargo. It represents extra flags that user passes directly.

Feel free to create a Cargo issue and we can discuss there!