Enable LTO for profile.release and profile.bench in new packages

[foxtran]

Hi!

This PR enables fat Link-Time Optimization (LTO) by default for new packages which are started via cargo init command. To speed-up development cycle, LTO is enabled only for performance-related build types (release, bench). For library targets, only bench profile gets enabled LTO.

The main advantage is that new projects will no longer need to enable LTO manually. This means users will benefit from binaries that are a bit more optimized for both size and speed just from the start of their projects. Since projects are generally small in the early stages, this change should not significantly impact compilation time or memory usage — these issues usually arise only in the compilation with LTO of really large projects.

This PR does not affect existing packages. I've test these changes via cargo test command or more precisely:

cargo test --test testsuite -- cargo_init::
cargo test --test testsuite -- cargo_new::

because only these tests were affected by my changes.

Hope these changes will be merged and all Rust community gets faster packages from scratch :)

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @weihanglo (or someone else) some time within the next two weeks.

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[Rustin170506]

Thanks for your PR. But before you submit your PR I would suggest opening an issue to discuss your request and changes. See more https://doc.crates.io/contrib/process/working-on-cargo.html#before-hacking-on-cargo.

[epage]

Agreed with @Rustin170506. Thank you for your interest in improving the ecosystem. To add to why we prefer Issues is that there are many angles to look at (what settings, set in init or default profile, etc) and PRs aren't ideal for this type of setup.

#11298 might be a good place for discussing this. Closing this until there is resolution.