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/fuzzing/src/generators.rs

@@ -18,7 +18,6 @@ use arbitrary::{Arbitrary, Unstructured};
 /// testing between.
 #[derive(Arbitrary, Clone, Debug, PartialEq, Eq, Hash)]
 pub struct DifferentialConfig {
-    strategy: DifferentialStrategy,
     opt_level: OptLevel,
     force_jump_veneers: bool,
 }
@@ -26,10 +25,7 @@ pub struct DifferentialConfig {
 impl DifferentialConfig {
     /// Convert this differential fuzzing config into a `wasmtime::Config`.
     pub fn to_wasmtime_config(&self) -> anyhow::Result<wasmtime::Config> {
-        let mut config = crate::fuzz_default_config(match self.strategy {
-            DifferentialStrategy::Cranelift => wasmtime::Strategy::Cranelift,
-            DifferentialStrategy::Lightbeam => wasmtime::Strategy::Lightbeam,
-        })?;
+        let mut config = crate::fuzz_default_config(wasmtime::Strategy::Cranelift)?;
         config.cranelift_opt_level(self.opt_level.to_wasmtime());
         if self.force_jump_veneers {
             unsafe {
@@ -40,12 +36,6 @@ impl DifferentialConfig {
     }
 }
 
-#[derive(Arbitrary, Clone, Debug, PartialEq, Eq, Hash)]
-enum DifferentialStrategy {
-    Cranelift,
-    Lightbeam,
-}
-
 #[derive(Arbitrary, Clone, Debug, PartialEq, Eq, Hash)]
 enum OptLevel {
     None,

crates/fuzzing/src/oracles.rs

@@ -253,7 +253,7 @@ pub fn differential_execution(
     let configs: Vec<_> = match configs.iter().map(|c| c.to_wasmtime_config()).collect() {
         Ok(cs) => cs,
         // If the config is trying to use something that was turned off at
-        // compile time, eg lightbeam, just continue to the next fuzz input.
+        // compile time just continue to the next fuzz input.
         Err(_) => return,
     };