icp-dev-env-rust: missing clang prevents cargo build --target wasm32-unknown-unknown for canisters with C dependencies #40

Problem

The icp-dev-env-rust image cannot build Rust canisters that have C/C++ FFI dependencies (e.g. secp256k1, ring, openssl) when cross-compiling to wasm32-unknown-unknown.

The root cause is that the cc Rust crate — used internally by secp256k1-sys and similar — selects clang as the C compiler when targeting wasm32-unknown-unknown. gcc has no wasm32 backend, so clang is required. The current image has neither clang nor llvm-ar installed.

Current Dockerfile

FROM rust:1.95.0-slim-bookworm
RUN apt-get -yq update && apt-get -yq upgrade && apt-get -yqq install --no-install-recommends \
    curl ca-certificates git jq make libdbus-1-3 && \
    apt-get autoremove -y && apt-get clean && rm -rf /var/lib/apt/lists/*
#                                                    ^^^^^^^^^^^^^^^^^^^
# Package lists are wiped after install (standard slim image practice).
# Any subsequent apt-get install in CI requires apt-get update first.

RUN rustup target add wasm32-unknown-unknown

Observed error

When running cargo build --target wasm32-unknown-unknown --release inside the container for any crate with a C dependency (bitcoin, secp256k1, ring, etc.):

error occurred in cc-rs: failed to find tool "clang": No such file or directory (os error 2)

Important: The @dfinity/rust icp-cli recipe runs cargo build --target wasm32-unknown-unknown --release verbatim with no C toolchain setup. So icp build and icp deploy fail identically for affected canisters.

Real-world impact

Discovered while migrating rust/threshold-schnorr in dfinity/examples to icp-cli. The canister uses bitcoin = "0.32" (pulls in secp256k1-sys). Neither cargo build nor icp build/icp deploy work in the container without clang.

Affected: any canister using bitcoin, secp256k1, k256, p256, ring, or any crate with a C build script targeting wasm32.

Current workaround

Add an explicit step before any build in affected CI workflows:

- name: Install clang
  run: apt-get update && apt-get install -y --no-install-recommends clang

apt-get update is required because the slim image wipes /var/lib/apt/lists/*. This adds ~30-60 s per CI job and must be repeated in every workflow.

Proposed fix

Add clang to the apt install step in rust/Dockerfile:

RUN apt-get -yq update && apt-get -yq upgrade && apt-get -yqq install --no-install-recommends \
    curl ca-certificates git jq make libdbus-1-3 clang && \
    apt-get autoremove -y && apt-get clean && rm -rf /var/lib/apt/lists/*

On Debian Bookworm, clang installs the current default LLVM release and creates /usr/bin/clang. No version pinning needed.

Tradeoff: ~80 MB image size increase. Once in the image, no per-workflow overhead, and all cargo and icp-cli workflows work uniformly for any Rust canister.