Add check to ensure plugin version matches cargo version (#794)

This modifies Rojo's build script to throw a fit if we're building a
plugin with a semver incompatible version. In the process, it moves the
version of the plugin to a file named `Version.txt` that's parsed at
runtime. This should be minimally invasive but it's technically worse
for performance than the hardcoded table and string we had before.

This feels better than a CI check or just manually verifying because it
makes it physically impossible for us to forget since Rojo won't build
with it being wrong.

build.rs

@@ -7,6 +7,7 @@ use fs_err as fs;
 use fs_err::File;
 use maplit::hashmap;
 use memofs::VfsSnapshot;
+use semver::Version;
 
 fn snapshot_from_fs_path(path: &Path) -> io::Result<VfsSnapshot> {
     println!("cargo:rerun-if-changed={}", path.display());
@@ -43,6 +44,19 @@ fn main() -> Result<(), anyhow::Error> {
     let root_dir = env::var_os("CARGO_MANIFEST_DIR").unwrap();
     let plugin_root = PathBuf::from(root_dir).join("plugin");
 
+    let our_version = Version::parse(env::var_os("CARGO_PKG_VERSION").unwrap().to_str().unwrap())?;
+    let plugin_version =
+        Version::parse(fs::read_to_string(&plugin_root.join("Version.txt"))?.trim())?;
+
+    assert!(
+        our_version.major == plugin_version.major,
+        "plugin version does not match Cargo version"
+    );
+    assert!(
+        our_version.minor == plugin_version.minor,
+        "plugin version does not match Cargo version"
+    );
+
     let snapshot = VfsSnapshot::dir(hashmap! {
         "default.project.json" => snapshot_from_fs_path(&plugin_root.join("default.project.json"))?,
         "fmt" => snapshot_from_fs_path(&plugin_root.join("fmt"))?,
@@ -51,6 +65,7 @@ fn main() -> Result<(), anyhow::Error> {
         "rbx_dom_lua" => snapshot_from_fs_path(&plugin_root.join("rbx_dom_lua"))?,
         "src" => snapshot_from_fs_path(&plugin_root.join("src"))?,
         "Packages" => snapshot_from_fs_path(&plugin_root.join("Packages"))?,
+        "Version.txt" => snapshot_from_fs_path(&plugin_root.join("Version.txt"))?,
     });
 
     let out_path = Path::new(&out_dir).join("plugin.bincode");