Functionize check API script

Add functions to the `check-for-api-changes.sh` script.

Refactor only, no logic changes.

contrib/check-for-api-changes.sh

@@ -13,27 +13,51 @@ CMD="cargo +nightly public-api --simplified"
 # `sort -n -u` doesn't work for some reason.
 SORT="sort --numeric-sort"
 
-# cargo public-api uses nightly so the toolchain must be available.
-if ! cargo +nightly --version > /dev/null; then
-    echo "script requires a nightly toolchain to be installed (possibly >= nightly-2023-05-24)" >&2
-    exit 1
-fi
-
-pushd "$REPO_DIR" > /dev/null
-$CMD --no-default-features | $SORT | uniq > "$API_DIR/no-default-features.txt"
-$CMD | $SORT | uniq > "$API_DIR/default-features.txt"
-$CMD --no-default-features --features=alloc | $SORT | uniq > "$API_DIR/alloc.txt"
-$CMD --no-default-features --features=core2 | $SORT | uniq > "$API_DIR/core2.txt"
-$CMD --all-features | $SORT | uniq > "$API_DIR/all-features.txt"
-
-if [[ $(git status --porcelain api) ]]; then
-    echo "You have introduced changes to the public API, commit the changes to api/ currently in your working directory" >&2
+main() {
+    # cargo public-api uses nightly so the toolchain must be available.
+    if ! cargo +nightly --version > /dev/null; then
+        echo "script requires a nightly toolchain to be installed (possibly >= nightly-2023-05-24)" >&2
+        exit 1
+    fi
+
+    generate_api_files
+    check_for_changes
+}
+
+generate_api_files() {
+    pushd "$REPO_DIR" > /dev/null
+
+    $CARGO | $SORT | uniq > "$API_DIR/default-features.txt"
+
+    $CARGO --no-default-features | $SORT | uniq > "$API_DIR/no-default-features.txt"
+    $CARGO --no-default-features --features=alloc | $SORT | uniq > "$API_DIR/alloc.txt"
+    $CARGO --no-default-features --features=core2 | $SORT | uniq > "$API_DIR/core2.txt"
+
+    $CARGO --all-features | $SORT | uniq > "$API_DIR/all-features.txt"
+
     popd > /dev/null
-    exit 1
-else
-    echo "No changes to the current public API"
+}
+
+# Check if there are changes (dirty git index) to the `api/` directory.
+check_for_changes() {
+    pushd "$REPO_DIR" > /dev/null
+
+    if [[ $(git status --porcelain api) ]]; then
+        git diff --color=always
+
+        echo
+        echo "You have introduced changes to the public API, commit the changes to api/ currently in your working directory" >&2
+        exit 1
+
+    else
+        echo "No changes to the current public API"
+    fi
+
     popd > /dev/null
-fi
+}
 
+#
+# Main script
+#
+main "$@"
 exit 0
-