Add CI workflow for verifying no_std compatibility.

.github/workflows/ci.yml

@@ -42,14 +42,36 @@ jobs:
       matrix:
         target:
           - wasm32-wasi
-
+          - thumbv7em-none-eabihf
     steps:
       - uses: actions/checkout@v4
+        with:
+          path: crate_root
+      # We use a synthetic crate to ensure no dev-dependencies are enabled, which can
+      # be incompatible with some of these targets.
+      - name: Create synthetic crate for testing
+        run: cargo init --lib ci-build
+      - name: Copy Rust version into synthetic crate
+        run: cp crate_root/rust-toolchain.toml ci-build/
+      - name: Copy patch directives into synthetic crate
+        run: |
+          echo "[patch.crates-io]" >> ./ci-build/Cargo.toml
+          cat ./crate_root/Cargo.toml | sed "0,/.\+\(patch.crates.\+\)/d" >> ./ci-build/Cargo.toml
+      - name: Add no_std pragma to lib.rs
+        run: |
+          echo "#![no_std]" > ./ci-build/src/lib.rs
+      - name: Add sapling-crypto as a dependency of the synthetic crate
+        working-directory: ./ci-build
+        run: cargo add --no-default-features --path ../crate_root
+      - name: Add lazy_static with the spin_no_std feature
+        working-directory: ./ci-build
+        run: cargo add lazy_static --features "spin_no_std"
       - name: Add target
+        working-directory: ./ci-build
         run: rustup target add ${{ matrix.target }}
-      - run: cargo fetch
-      - name: Build for ${{ matrix.target }} target
-        run: cargo build --verbose --no-default-features --target ${{ matrix.target }}
+      - name: Build for target
+        working-directory: ./ci-build
+        run: cargo build --verbose --target ${{ matrix.target }}
 
   bitrot:
     name: Bitrot check