I'm not 100% sure the README example works as expected?

[brandonros]

README example:

FROM lukemathwalker/cargo-chef:latest-rust-1 AS chef
WORKDIR /app

FROM chef AS planner
COPY . .
RUN cargo chef prepare --recipe-path recipe.json

FROM chef AS builder 
COPY --from=planner /app/recipe.json recipe.json
# Build dependencies - this is the caching Docker layer!
RUN cargo chef cook --release --recipe-path recipe.json
# Build application
COPY . .
RUN cargo build --release --bin app

# We do not need the Rust toolchain to run the binary!
FROM debian:bookworm-slim AS runtime
WORKDIR /app
COPY --from=builder /app/target/release/app /usr/local/bin
ENTRYPOINT ["/usr/local/bin/app"]

It could be a mistake on my end but I have a question about this part:

FROM chef AS planner
COPY . .
RUN cargo chef prepare --recipe-path recipe.json

Shouldn't this be

FROM chef AS planner
COPY Cargo.toml .
COPY Cargo.lock .
# let cargo chef mock a src/main.rs or src/lib.rs for dummy project
RUN cargo chef prepare --recipe-path recipe.json

so that the way Docker-image caching works, the recipe is only rebuilt + re-ran + re-calculated if dependencies change?

I've been using cargo-chef and unless I'm using it incorrectly or misunderstanding something, if I make a 1 whitespace change in the Rust code (not in Cargo.lock/Cargo.toml), it recompiles all of the dependencies again no matter what instead of benefitting from image layer caching?

[LukeMathWalker]

No, the example is correct.
The COPY statement only invalidates the planner stage of the build. The building stage gets invalidated if and only if recipe.json changes.

[brandonros]

Is it out of scope for this tool to "cache" the compilation of the 3rd party dependencies (Cargo.toml) and not need to re-compile them every time there is a src/ code change?

[LukeMathWalker]

That's precisely what the tool does 👀
If that's not happening on a specific project of yours, let's open a bug report to see what's wrong.
If this is a theoretical question, the launch blog post has a longer explanation as to why this works as expected.