为什么货物构建缓存无效？

Question

I have a barebones workspace project:

.
├── build-debug.sh
├── Cargo.lock
├── Cargo.toml
├── common
│   ├── Cargo.toml
│   └── src
│       └── lib.rs
├── rs-test.iml
├── server
│   ├── Cargo.toml
│   └── src
│       └── main.rs
└── wui
    ├── Cargo.toml
    └── src
        └── lib.rs

The rs files either empty or just an empty main function.

The server and the wui depends on common: common = { path = "../common" } .

The common project has one crates.io dependency with I suppose build script or proc macro dependency.

The build script:

cargo build -p wui --target wasm32-unknown-unknown
cargo build -p server

The problem:

When I rebuild the unchanged project, some wui dependencies are getting invalidated/rebuilt, then the same for server.

Either:

• remove the wasm32 target flag
• replace the dependency with a simple crate without build time compiled dependencies

It does not rebuild the subprojects anymore.

Is this a cargo bug? What can I do?

Answer 1

It's probably not a cargo bug. What is likely happening here is that your crates.io dependency (you don't mention what it is, which might have been useful) has different dependencies or features depending on the target architecture. Thus, as you alternate between building the WASM target and your host target, stuff is being rebuilt.

Perhaps it would be better in this case to stop using the Cargo workspace and build the server and wui separately; this way you'll have separate target directories for the server and wui, which takes some extra disk space and takes longer for non-incremental compilation, but will prevent you from having to rebuild that stuff all the time as you build both.