Don't panic if rustdoc is missing

[emilal]

I came here through a rav1e issue and a built issue when trying to build image in a minimal container that doesn't have rustdoc installed; specifically one based on cgr.dev/chainguard/rust:latest.

The error can be produced with the following steps:

1. cargo new broken
2. Add the following to Cargo.toml:

[dependencies.rav1e]
version = "0.7.1"
default-features = false

3. Create a Dockerfile like

FROM cgr.dev/chainguard/rust:latest
WORKDIR /work
COPY Cargo.toml .
COPY src/ src/
RUN cargo build

4. Run podman build -t broken . and receive output like

error: failed to run custom build command for rav1e v0.7.1

Caused by:
process didn't exit successfully: /work/target/debug/build/rav1e-630d2ba1731d92a3/build-script->build (exit status: 101)
--- stderr
thread 'main' panicked at /home/nonroot/.cargo/registry/src/index.crates.io-6f17d22bba15001f/rav1e-0.7.1/build.rs:250:29:
Failed to acquire build-time information: Os { code: 2, kind: NotFound, message: "No such file or directory" }
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace
warning: build failed, waiting for other jobs to finish...
Error: building at STEP "RUN cargo build": while running runtime: exit status 101

To see that the patch eliminates the issue, the steps are similar:

1. cargo new fixed
2. Add built with the patch applied somewhere reachable by the coming container build context, e.g. rsync -a /path/to/built/ fixed/
3. Add the dependency as in broken, and also

[patch.crates-io]
built = { path = "./built" }

4. Add it to the Dockerfile like

FROM cgr.dev/chainguard/rust:latest
WORKDIR /work
COPY Cargo.toml .
COPY src/ src/
COPY built/ built/
RUN cargo build

5. Run podman build -t fixed . and see that the build completes successfully.

I don't have any particular opinion about what built should output when rustdoc is missing; my goal here is to successfully build a minimal container image using the image crate.

[emilal]

(The default-features=false for rav1e is because otherwise the build will fail due to the image missing nasm, which is irrelevant here.)

[lukaslueg]

Thanks for the PR. I suggest to change the behavior here to .unwrap_or_default(), which will result in an empty string; "NOTFOUND" is (hopefully) not a valid binary name, and may fail in weird ways, depending on how the value is used.

Do you happen to know how the base-image is crafted? I fail to see how to install rustc without rustdoc using the rust-provided toolchains.

[emilal]

Updated. Though as far as I can tell here it's not the command itself we're changing, but in case running the command fails, I figured the companion to "The output of {rustdoc} -V" could be "NOT FOUND".

I don't know exactly what chainguard is doing to create the image, but it's supposed to be a rather minimal "distroless" image. It's also, according to the doc, supposed to contain rustdoc, so I guess I should send them a notice too. If it is intended, they should likely clarify that the sentence about including rustdoc only applies to the *-dev versions.