Optional features are part of the crate — the project should say so

zackw:

What would be so bad about moving everything, and I really do mean everything , from .cargo/config.toml to Cargo.toml, right now, so that we can deprecate the existence of .cargo/config.toml in packages and workspaces as soon as possible?

I'd recommend actually looking at the config before making such broad statements

doc.rust-lang.org

Configuration format - Configuration - The Cargo Book

This document explains how Cargo’s configuration system works, as well as available keys or configuration. For configuration of a package through its manifest, see the manifest format.

• alias: there is no manifest lookup for these today
• build: half of these are caller, machine, or user specific
• credential-alias: user specific
• doc: user or machine specific
• env: unenumerable use case
• future-incompat-report: user specific
• cache: user or machine specific
• cargo-new: user as the point is that you aren't in a repo
• http: user or machine specific
• install: caller, user, or machine specific
• net: ditto
• patch: already in manifest
• profile: ditto
• resolver: controlled by workspace.resolver. Maybe more can go in but design work is needed
• registries: unsure
• source: ditto
• target links: overriding build scripts should likely not be in a static file
• term: caller, user, or machine specific

Another problem is RUSTFLAGS is a raw escape hatch that users can shoot themselves in the foot with and so it needs a "here be dragons" rather than having a paved path in the manifest,