Generating entire crates during build.rs

Describe the problem you are trying to solve
When having entire autogenerated crates (eg https://github.com/dropbox/pb-jelly) - there isn't a good way to generate the crate at build time via build.rs.

Eg when building https://github.com/dropbox/pb-jelly/blob/a0c0550/examples/Cargo.toml, we get

nipunn-mbp:examples nipunn$ cargo build
error: failed to read `/Users/nipunn/src/pb-jelly/pb-test/gen/pb-jelly/proto_pbtest/Cargo.toml`

Caused by:
  No such file or directory (os error 2)

This is because proto_pbtest is generated in the build phase.

Current workarounds include

• commenting out the proto_pbtest dependency, building, uncommenting, building again
• Creating a dummy empty proto_pbtest Cargo.toml to be overwritten
• Splitting the build phase out into a separate crate

Describe the solution you'd like

Idea 1

• Parse the Cargo.toml - tolerating missing path dependencies
• Build the build.rs - only looking at build dependencies - ensuring they're all available
• Run build.rs
• Re-evaluate cargo task graph to search for the missing dependencies
• Run rest of the build

Idea 2

• Provide a command (eg cargo build --prebuild-only) to only run the build.rs build phase tolerating missing path dependencies in the other phases
• Developer can run cargo build --prebuild-only followed by cargo build for projects like this
  One idea, may be to

Idea 3 [hacky workaround]

• Provide a mode to tolerate missing path dependencies cargo build --tolerate-missing-path-deps
• Developer can run cargo build ; cargo build

Notes

Reproduce the failure from the pb-jelly examples/Cargo.toml and inspect how its build.rs generates pb-test/gen/pb-jelly/proto_pbtest/Cargo.toml. Compare the three proposed approaches and determine which Cargo behavior should support generated path dependencies; done means the generated crate builds without the listed manual workarounds.