Remove the lightbeam backend (#3390)

This commit removes the Lightbeam backend from Wasmtime as per [RFC 14].
This backend hasn't received maintenance in quite some time, and as [RFC
14] indicates this doesn't meet the threshold for keeping the code
in-tree, so this commit removes it.

A fast "baseline" compiler may still be added in the future. The
addition of such a backend should be in line with [RFC 14], though, with
the principles we now have for stable releases of Wasmtime. I'll close
out Lightbeam-related issues once this is merged.

[RFC 14]: https://github.com/bytecodealliance/rfcs/pull/14

crates/wasmtime/Cargo.toml

@@ -59,9 +59,6 @@ default = ['async', 'cache', 'wat', 'jitdump', 'parallel-compilation', 'cranelif
 # precompiled WebAssembly modules.
 cranelift = ["wasmtime-cranelift"]
 
-# Deprecated, does not actually do anything any more.
-lightbeam = []
-
 # Enables support for the `perf` jitdump profiler
 jitdump = ["wasmtime-jit/jitdump"]
 

crates/wasmtime/src/config.rs

@@ -1333,12 +1333,6 @@ impl Config {
 fn compiler_builder(strategy: Strategy) -> Result<Box<dyn CompilerBuilder>> {
     match strategy {
         Strategy::Auto | Strategy::Cranelift => Ok(wasmtime_cranelift::builder()),
-        #[cfg(feature = "lightbeam")]
-        Strategy::Lightbeam => unimplemented!(),
-        #[cfg(not(feature = "lightbeam"))]
-        Strategy::Lightbeam => {
-            anyhow::bail!("lightbeam compilation strategy wasn't enabled at compile time");
-        }
     }
 }
 
@@ -1430,20 +1424,13 @@ pub enum Strategy {
     /// `wasmtime` crate itself should make the decision about what the best
     /// code generator for a wasm module is.
     ///
-    /// Currently this always defaults to Cranelift, but the default value will
+    /// Currently this always defaults to Cranelift, but the default value may
     /// change over time.
     Auto,
 
     /// Currently the default backend, Cranelift aims to be a reasonably fast
     /// code generator which generates high quality machine code.
     Cranelift,
-
-    /// A single-pass code generator that is faster than Cranelift but doesn't
-    /// produce as high-quality code.
-    ///
-    /// To successfully pass this argument to [`Config::strategy`] the
-    /// `lightbeam` feature of this crate must be enabled.
-    Lightbeam,
 }
 
 /// Possible optimization levels for the Cranelift codegen backend.

crates/wasmtime/src/module/serialization.rs

@@ -805,26 +805,6 @@ mod test {
         Ok(())
     }
 
-    #[cfg(feature = "lightbeam")]
-    #[test]
-    fn test_compilation_strategy_mismatch() -> Result<()> {
-        let engine = Engine::default();
-        let module = Module::new(&engine, "(module)")?;
-
-        let mut serialized = SerializedModule::new(&module);
-        serialized.metadata.strategy = CompilationStrategy::Lightbeam;
-
-        match serialized.into_module(&engine) {
-            Ok(_) => unreachable!(),
-            Err(e) => assert_eq!(
-                e.to_string(),
-                "Module was compiled with strategy 'Cranelift'",
-            ),
-        }
-
-        Ok(())
-    }
-
     #[test]
     fn test_tunables_int_mismatch() -> Result<()> {
         let engine = Engine::default();