Invalid E0432 when using Docker build cache layers

[jennydaman]

Problem

For convenience, a repository reproducing this bug can be found here: https://github.com/jennydaman/rust_in_docker_bug

A common pattern with using Dockerfile and Rust is to leverage layer caching like this:

FROM rust:1.71.0-slim-bullseye

WORKDIR /app
COPY Cargo.lock Cargo.toml ./
RUN mkdir src && touch src/lib.rs  # dummy project
RUN cargo build
COPY src src
RUN cargo build

In past projects, this worked for me if src does not contain a lib.rs. However, I am currently working on a project with both main.rs and lib.rs. Using the Dockerfile pattern above causes cargo to emit an invalid E0432 error.

main.rs

use owo_colors::OwoColorize;
use rust_in_docker_bug::add_one;

fn main() {
    println!("Number is {}", add_one(12344).cyan());
}

lib.rs

pub fn add_one(x: u32) -> u32 {
    x + 1
}

Build

podman build .                     
STEP 1/7: FROM rust:1.71.0-slim-bullseye
STEP 2/7: WORKDIR /app
--> 958baf24a72b
STEP 3/7: COPY Cargo.lock Cargo.toml ./
--> f038ed695fa8
STEP 4/7: RUN mkdir src && touch src/lib.rs
--> 1fc0beaa5a68
STEP 5/7: RUN cargo build
    Updating crates.io index
 Downloading crates ...
  Downloaded owo-colors v3.5.0
   Compiling owo-colors v3.5.0
   Compiling rust-in-docker-bug v0.1.0 (/app)
    Finished dev [unoptimized + debuginfo] target(s) in 0.58s
--> d9535e0aa0fa
STEP 6/7: COPY src src
--> 8530d262ea49
STEP 7/7: RUN cargo build
   Compiling rust-in-docker-bug v0.1.0 (/app)
error[E0432]: unresolved import `rust_in_docker_bug::add_one`
 --> src/main.rs:2:5
  |
2 | use rust_in_docker_bug::add_one;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ no `add_one` in the root

warning: unused import: `owo_colors::OwoColorize`
 --> src/main.rs:1:5
  |
1 | use owo_colors::OwoColorize;
  |     ^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

For more information about this error, try `rustc --explain E0432`.
warning: `rust-in-docker-bug` (bin "rust-in-docker-bug") generated 1 warning
error: could not compile `rust-in-docker-bug` (bin "rust-in-docker-bug") due to previous error; 1 warning emitted
Error: building at STEP "RUN cargo build": while running runtime: exit status 101

Same error happens for Podman and Docker, so I assume it's a problem with cargo not buildah nor buildx.

The problem does not happen if not using Dockerfile:

# this works!
docker run --rm -v $PWD:/src:ro rust:1.71.0-slim-bullseye sh -ec '
mkdir /app
cd /app
cp -v /src/Cargo.lock /src/Cargo.toml .
mkdir src && touch src/lib.rs
cargo build
cp -rvf /src/src src
cargo build
'

A workaround is to create the dummy project with a main.rs instead:

FROM rust:1.71.0-slim-bullseye

RUN cargo new --bin /app
WORKDIR /app
COPY Cargo.lock Cargo.toml ./
RUN cargo build
COPY src src
RUN cargo build

Steps

No response

Possible Solution(s)

No response

Notes

No response

Version

No response

[weihanglo]

Thanks for the report. I believe this is due to docker COPY not preserving mtime, while Cargo's build cache system currently relies on mtime.

As a workaround, touching src/lib.rs works:

  FROM rust:1.71.0-slim-bullseye

  WORKDIR /app
  COPY Cargo.lock Cargo.toml ./
  RUN mkdir src && touch src/lib.rs
  RUN cargo build
  COPY src src
+ RUN touch src/lib.rs
  RUN cargo build

Personally I may recommend some third party plugin at this moment if you have a complex project with docker (cargo-chef specifically). Some prior discussions and tracking issues:

• cargo build --dependencies-only #2644
• Cache usage meta tracking issue #7150

I'll close this in favor of the above issues. Let us know if it is wrong to close this.