I might be misinterpreting this issue, but whenever I put the #[cxx::bridge] anywhere but the crate root I still get

cxxbridge: no #[cxx::bridge] module found

Reproduce

1. Open project cxx/demo
2. Add pub mod bridge; to main.rs, along with corresponding file src/bridge.rs
3. Move #[cxx::bridge(namespace = "org::blobstore")] mod ffi { … } from main.rs to bridge.rs
4. Mark mod ffi as pub mod ffi.
5. cargo build

Module structure

crate demo
└── pub mod bridge
    │   #[cxx::bridge(…)]
    └── pub mod ffi

Error

❯ cargo build
   Compiling demo v0.0.0 (/cxx/demo)
error: failed to run custom build command for `demo v0.0.0 (/cxx/demo)`

Caused by:
  process didn't exit successfully: `/cxx/target/debug/build/demo-02f344be574bf590/build-script-build` (exit code: 1)
  --- stdout
  cargo:CXXBRIDGE_PREFIX=demo
  cargo:CXXBRIDGE_DIR0=/.../cxx/target/debug/build/demo-9b77e4bc641295bf/out/cxxbridge/include
  cargo:CXXBRIDGE_DIR1=/.../cxx/target/debug/build/demo-9b77e4bc641295bf/out/cxxbridge/crate

  --- stderr
  cxxbridge: no #[cxx::bridge] module found

Probably your cxx_build::bridge call in build.rs is pointed to the wrong file.

Oh, so it needs to be within the file passed to cxx_build::bridge?
My understanding was that with #508 it now could be in any sub-module of the root module.

Makes sense. So the correct call would be cxx_build::bridge("src/bridge.rs") then.
Works. 👍

It can be within any submodule in the given file -- like in the comment at the top of this issue the bridge is the submodule self::details::ffi inside lib.rs.