Vendor SPIRV-Headers grammar for git dependency builds

When spirv-tools-core is used as a cargo git dependency, submodules
are not cloned, so the grammar file at external/spirv-headers/ is
unavailable. Vendor a copy under grammar/ and update build.rs to
fall back to it when the submodule path does not exist.

Resolution order: SPIRV_HEADERS_PATH env var > submodule > vendored.

Author: LegNeato

rust/spirv-tools-core/build.rs

@@ -107,21 +107,28 @@ fn bit_enum_ident(enumerant: &str) -> String {
 
 fn main() {
     let manifest_dir = env::var("CARGO_MANIFEST_DIR").expect("CARGO_MANIFEST_DIR");
-    // Allow overriding the SPIRV-Headers path via environment variable for standalone builds.
-    // When used as a git dependency, the submodule isn't available, so users can set
-    // SPIRV_HEADERS_PATH to point to their local SPIRV-Headers checkout.
+    println!("cargo:rerun-if-env-changed=SPIRV_HEADERS_PATH");
+    // Resolve the grammar file with the following priority:
+    // 1. SPIRV_HEADERS_PATH env var (explicit override)
+    // 2. ../../external/spirv-headers/ (submodule, for in-tree builds)
+    // 3. grammar/ (vendored copy, for git dependency builds)
     let grammar_path = if let Ok(headers_path) = env::var("SPIRV_HEADERS_PATH") {
-        println!("cargo:rerun-if-env-changed=SPIRV_HEADERS_PATH");
         PathBuf::from(headers_path).join("include/spirv/unified1/spirv.core.grammar.json")
     } else {
-        PathBuf::from(&manifest_dir)
-            .join("../../external/spirv-headers/include/spirv/unified1/spirv.core.grammar.json")
+        let submodule_path = PathBuf::from(&manifest_dir)
+            .join("../../external/spirv-headers/include/spirv/unified1/spirv.core.grammar.json");
+        if submodule_path.exists() {
+            submodule_path
+        } else {
+            // Fall back to vendored grammar for git dependency builds
+            PathBuf::from(&manifest_dir).join("grammar/spirv.core.grammar.json")
+        }
     };
     let grammar_contents = fs::read_to_string(&grammar_path).unwrap_or_else(|_| {
         panic!(
             "failed to read spirv.core.grammar.json from {:?}. \
-            If building as a git dependency, set SPIRV_HEADERS_PATH to point to a \
-            SPIRV-Headers checkout.",
+            Set SPIRV_HEADERS_PATH to point to a SPIRV-Headers checkout, or ensure \
+            the vendored grammar/ directory is present.",
             grammar_path
         )
     });