Put info about Rust toolchain into recipe and use it in `cargo chef cook`

[haaawk]

Problem

1. Currently cargo chef cook uses default Rust toolchain.
2. The project can define a Rust toolchain it wants to be built with using rust-toolchain.toml.
3. This may lead to dependencies being built using different toolchain than the actual project. As a result, cargo build will compile the dependencies again using the toolchain defined in rust-toolchain.toml.

The solution would be to include info about the toolchain into the recipe and use that info in cargo chef cook to make sure it compiles dependencies using the correct toolchain.

Problem reproducer:

1. cargo new --lib dummy
2. cd dummy
3. Create rust-toolchain.toml with following content:

   [toolchain]
   profile = "default"
   channel = "1.70.0"

4. Add tokio as a dependency in Cargo.toml to be able to observe it being compiled twice:

   
   [package]
   name = "dummy"
   version = "0.1.0"
   edition = "2021"

[dependencies] tokio = { version = "1", features = ["full"] }

5. Create a `Dockerfile`:

FROM rust:slim-bullseye AS chef RUN cargo install cargo-chef

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

FROM chef AS builder COPY --from=planner /recipe.json recipe.json RUN cargo chef cook --recipe-path recipe.json COPY . . RUN cargo build

6. Run `docker build --progress=plain --target builder --no-cache . 2>&1 | tee build-wrong.log`
7. Observe dependencies being compiled both by `cargo chef cook` and `cargo build`:

...

10 [builder 2/4] RUN cargo chef cook --recipe-path recipe.json

10 0.334 Updating crates.io index

10 0.699 Downloading crates ...

10 1.053 Downloaded parking_lot_core v0.9.8

10 1.058 Downloaded bytes v1.4.0

10 1.064 Downloaded signal-hook-registry v1.4.1

10 1.065 Downloaded num_cpus v1.16.0

10 1.072 Downloaded lock_api v0.4.10

10 1.076 Downloaded tokio-macros v2.1.0

10 1.080 Downloaded scopeguard v1.2.0

10 1.083 Downloaded pin-project-lite v0.2.10

10 1.095 Downloaded autocfg v1.1.0

10 1.097 Downloaded socket2 v0.4.9

10 1.100 Downloaded parking_lot v0.12.1

10 1.103 Downloaded smallvec v1.11.0

10 1.106 Downloaded mio v0.8.8

10 1.128 Downloaded tokio v1.29.1

10 1.189 Compiling libc v0.2.147

10 1.189 Compiling autocfg v1.1.0

10 1.189 Compiling proc-macro2 v1.0.66

10 1.192 Compiling unicode-ident v1.0.11

10 1.192 Compiling parking_lot_core v0.9.8

10 1.192 Compiling scopeguard v1.2.0

10 1.194 Compiling smallvec v1.11.0

10 1.194 Compiling cfg-if v1.0.0

10 1.230 Compiling pin-project-lite v0.2.10

10 1.245 Compiling bytes v1.4.0

10 1.430 Compiling lock_api v0.4.10

10 1.430 Compiling tokio v1.29.1

10 1.850 Compiling quote v1.0.32

10 1.972 Compiling num_cpus v1.16.0

10 1.972 Compiling mio v0.8.8

10 1.974 Compiling socket2 v0.4.9

10 1.974 Compiling signal-hook-registry v1.4.1

10 1.980 Compiling syn v2.0.27

10 2.121 Compiling parking_lot v0.12.1

10 4.373 Compiling tokio-macros v2.1.0

10 7.876 Compiling dummy v0.0.1 (/)

10 9.263 Finished dev [unoptimized + debuginfo] target(s) in 8.96s

10 DONE 9.4s

...

12 [builder 4/4] RUN cargo build

12 0.232 info: syncing channel updates for '1.70.0-x86_64-unknown-linux-gnu'

12 0.472 info: latest update on 2023-06-01, rust version 1.70.0 (90c541806 2023-05-31)

12 0.472 info: downloading component 'cargo'

12 0.705 info: downloading component 'clippy'

12 0.790 info: downloading component 'rust-docs'

12 1.259 info: downloading component 'rust-std'

12 2.162 info: downloading component 'rustc'

12 4.139 info: downloading component 'rustfmt'

12 4.230 info: installing component 'cargo'

12 4.788 info: installing component 'clippy'

12 5.059 info: installing component 'rust-docs'

12 7.217 info: installing component 'rust-std'

12 9.429 info: installing component 'rustc'

12 14.03 info: installing component 'rustfmt'

12 14.53 Compiling libc v0.2.147

12 14.53 Compiling autocfg v1.1.0

12 14.53 Compiling proc-macro2 v1.0.66

12 14.53 Compiling unicode-ident v1.0.11

12 14.53 Compiling parking_lot_core v0.9.8

12 14.53 Compiling cfg-if v1.0.0

12 14.53 Compiling smallvec v1.11.0

12 14.53 Compiling scopeguard v1.2.0

12 14.61 Compiling pin-project-lite v0.2.10

12 14.64 Compiling bytes v1.4.0

12 14.85 Compiling lock_api v0.4.10

12 14.85 Compiling tokio v1.29.1

12 15.34 Compiling quote v1.0.32

12 15.49 Compiling mio v0.8.8

12 15.49 Compiling socket2 v0.4.9

12 15.49 Compiling signal-hook-registry v1.4.1

12 15.49 Compiling num_cpus v1.16.0

12 15.55 Compiling syn v2.0.27

12 15.69 Compiling parking_lot v0.12.1

12 18.01 Compiling tokio-macros v2.1.0

12 21.67 Compiling dummy v0.1.0 (/)

12 23.05 Finished dev [unoptimized + debuginfo] target(s) in 8.70s

12 DONE 23.2s

...

# Workaround
If we copy `rust-toolchain.toml` into `chef` image, it will make `cargo chef cook` to use the same Rust toolchain as the project being compiled:

FROM rust:slim-bullseye AS chef RUN cargo install cargo-chef COPY rust-toolchain.toml rust-toolchain.toml # <=== THIS IS ADDED

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

FROM chef AS builder COPY --from=planner /recipe.json recipe.json RUN cargo chef cook --recipe-path recipe.json COPY . . RUN cargo build

Now, when we build it with `docker build --progress=plain --target builder --no-cache . 2>&1 | tee build-correct.log` we will see only `cargo chef cook` compiling dependencies and `cargo build` not doing that.

Both logs attached
[build-correct.log](https://github.com/LukeMathWalker/cargo-chef/files/12165328/build-correct.log)
[build-wrong.log](https://github.com/LukeMathWalker/cargo-chef/files/12165329/build-wrong.log)

[LukeMathWalker]

I agree that this would be an improvement.

The only caveat is that you need to be careful in how you define your layers, otherwise you'll end up re-installing the toolchain at each stage. But at least it'll be the right one.

[LukeMathWalker]

This has been solved by #254.

[LtdSauce]

Hi,

could it be that this has not been fully solved or that there is a regression making this broken again? #271 looks like it is this issue.