Add Engine::precompile_compatibility_key

This method returns a key that can be used to index precompiled binaries
from one Engine instance that can be deserialized by another Engine
instance.

crates/wasmtime/src/engine.rs

@@ -239,6 +239,18 @@ impl Engine {
         Ok(mmap.to_vec())
     }
 
+    /// Returns a blob that can be used to check precompiled WebAssembly compatibility.
+    ///
+    /// The outputs of [`Engine::precompile_module`] and [`Engine::precompile_component`]
+    /// are compatible with a different [`Engine`] instance only if the two engines use
+    /// compatible [`Config`]s. If this blob matches between two [`Engine`]s then binaries
+    /// from one are guaranteed to deserialize in the other.
+    #[cfg(compiler)]
+    #[cfg_attr(nightlydoc, doc(cfg(feature = "cranelift")))] // see build.rs
+    pub fn precompile_compatibility_key(&self) -> impl AsRef<[u8]> {
+        serialization::build_compiler_info(&self)
+    }
+
     pub(crate) fn run_maybe_parallel<
         A: Send,
         B: Send,

crates/wasmtime/src/engine/serialization.rs

@@ -37,16 +37,8 @@ const VERSION: u8 = 0;
 /// Produces a blob of bytes by serializing the `engine`'s configuration data to
 /// be checked, perhaps in a different process, with the `check_compatible`
 /// method below.
-///
-/// The blob of bytes is inserted into the object file specified to become part
-/// of the final compiled artifact.
 #[cfg(compiler)]
-pub fn append_compiler_info(engine: &Engine, obj: &mut Object<'_>) {
-    let section = obj.add_section(
-        obj.segment_name(StandardSegment::Data).to_vec(),
-        obj::ELF_WASM_ENGINE.as_bytes().to_vec(),
-        SectionKind::ReadOnlyData,
-    );
+pub fn build_compiler_info(engine: &Engine) -> Vec<u8> {
     let mut data = Vec::new();
     data.push(VERSION);
     let version = match &engine.config().module_version {
@@ -62,6 +54,19 @@ pub fn append_compiler_info(engine: &Engine, obj: &mut Object<'_>) {
     data.push(version.len() as u8);
     data.extend_from_slice(version.as_bytes());
     bincode::serialize_into(&mut data, &Metadata::new(engine)).unwrap();
+    data
+}
+
+/// Inserts the "compiler info" from build_compiler_info into the object file
+/// specified to become part of the final compiled artifact.
+#[cfg(compiler)]
+pub fn append_compiler_info(engine: &Engine, obj: &mut Object<'_>) {
+    let section = obj.add_section(
+        obj.segment_name(StandardSegment::Data).to_vec(),
+        obj::ELF_WASM_ENGINE.as_bytes().to_vec(),
+        SectionKind::ReadOnlyData,
+    );
+    let data = build_compiler_info(engine);
     obj.set_section_data(section, data, 1);
 }