Fix false negatives in dcou by using patched cargo

ci/buildkite-pipeline.sh

@@ -181,7 +181,9 @@ wait_step() {
 
 all_test_steps() {
   command_step checks1 "ci/docker-run-default-image.sh ci/test-checks.sh" 20 check
-  command_step checks2 "ci/docker-run-default-image.sh ci/test-dev-context-only-utils.sh check-bins" 15 check
+  command_step checks2b "ci/docker-run-default-image.sh ci/test-dev-context-only-utils.sh check-bins" 15 check
+  command_step checks2l "ci/docker-run-default-image.sh ci/test-dev-context-only-utils.sh check-lib" 15 check
+  command_step checks2bl "ci/docker-run-default-image.sh ci/test-dev-context-only-utils.sh check-bins-and-lib" 15 check
   command_step checks3 "ci/docker-run-default-image.sh ci/test-dev-context-only-utils.sh check-all-targets" 15 check
   command_step miri "ci/docker-run-default-image.sh ci/test-miri.sh" 5 check
   wait_step

scripts/check-dev-context-only-utils.sh

@@ -43,7 +43,7 @@ allowed="${allowed%,}"
 
 mode=${1:-full}
 case "$mode" in
-  tree | check-bins | check-all-targets | full)
+  tree | check-bins | check-lib | check-bins-and-lib | check-all-targets | full)
     ;;
   *)
     echo "$0: unrecognized mode: $mode";
@@ -156,8 +156,43 @@ fi
 # consistency with other CI steps and for the possibility of new similar lints.
 export RUSTFLAGS="-D warnings -Z threads=8 $RUSTFLAGS"
 
-if [[ $mode = "check-bins" || $mode = "full" ]]; then
-  _ cargo "+${rust_nightly}" hack check --bins
+if [[ $mode = "check-bins" || $mode = "check-lib" || $mode = "check-bins-and-lib" || $mode = "full" ]]; then
+  # Until https://github.com/rust-lang/cargo/pull/14163 is applied for our
+  # chosen nightly toolchain, dcou needs custom-built cargo to avoid
+  # false-negatives.
+
+  # This clone will fail every time we update our nightly toolchain because
+  # ${rust_nightly} is embedded in the branch name. This is intentional to
+  # force to re-cheery-pick the patch for the new nightly.
+  #
+  # Steps:
+  # 1. Clone the forked cargo repo.
+  # 2. Check out the commit hash printed by `/solana/cargo nightly --version
+  #    --verbose` _using the new toolchain_.
+  # 3. Cherry-pick old branch's tip commit titled as "Unify no-library-target
+  #    error into no-target warn".
+  # 4. Push new branch with its name updated with the new nightly version.
+  _ git clone --depth 1 --no-tags \
+    --branch "no-no-library-target-error-${rust_nightly}" \
+    https://github.com/anza-xyz/cargo.git cargo-for-dcou
+
+  # Now build the patched cargo
+  (
+    cd ./cargo-for-dcou/
+    _ cargo "+${rust_nightly}" build --release --bin cargo
+  )
+
+  # Use `cargo "+${rust_nightly}" hack ..` once we stop using custom-built one.
+  if [[ $mode = "check-bins" ]]; then
+    PATH="./cargo-for-dcou/target/release:$PATH" \
+      RUSTUP_TOOLCHAIN="${rust_nightly}" _ cargo hack check --bins
+  elif [[ $mode = "check-lib" ]]; then
+    PATH="./cargo-for-dcou/target/release:$PATH" \
+      RUSTUP_TOOLCHAIN="${rust_nightly}" _ cargo hack check --lib
+  else
+    PATH="./cargo-for-dcou/target/release:$PATH" \
+      RUSTUP_TOOLCHAIN="${rust_nightly}" _ cargo hack check --bins --lib
+  fi
 fi
 if [[ $mode = "check-all-targets" || $mode = "full" ]]; then
   _ cargo "+${rust_nightly}" hack check --all-targets