add a feature flag and environment variable to enable santizers and fuzzing

crates/compiler/build/Cargo.toml

@@ -47,3 +47,5 @@ target-aarch64 = ["roc_gen_dev/target-aarch64"]
 target-x86 = []
 target-x86_64 = ["roc_gen_dev/target-x86_64"]
 target-wasm32 = []
+
+sanitizers = []

crates/compiler/build/src/program.rs

@@ -237,7 +237,80 @@ fn gen_from_mono_module_llvm<'a>(
 
     // annotate the LLVM IR output with debug info
     // so errors are reported with the line number of the LLVM source
-    let memory_buffer = if emit_debug_info {
+    let memory_buffer = if cfg!(feature = "sanitizers") && std::env::var("ROC_SANITIZERS").is_ok() {
+        let dir = tempfile::tempdir().unwrap();
+        let dir = dir.into_path();
+
+        let app_ll_file = dir.join("app.ll");
+        let app_bc_file = dir.join("app.bc");
+        let app_o_file = dir.join("app.o");
+
+        // write the ll code to a file, so we can modify it
+        module.print_to_file(&app_ll_file).unwrap();
+
+        // Apply coverage passes.
+        // Note, this is specifically tailored for `cargo afl` and afl++.
+        // It most likely will not work with other fuzzer setups without modification.
+        let mut passes = vec![];
+        let mut extra_args = vec![];
+        for sanitizer in std::env::var("ROC_SANITIZERS")
+            .unwrap()
+            .split(",")
+            .map(|x| x.trim())
+        {
+            match sanitizer {
+                "address" => passes.push("asan-module"),
+                "memory" => passes.push("msan-module"),
+                "thread" => passes.push("tsan-module"),
+                "fuzzer" => {
+                    passes.push("sancov-module");
+                    extra_args.extend_from_slice(&[
+                        "-sanitizer-coverage-level=3",
+                        "-sanitizer-coverage-prune-blocks=0",
+                        "-sanitizer-coverage-trace-pc-guard",
+                        // This can be used instead of the line above to enable working with `cargo fuzz` and libFuzzer.
+                        // "-sanitizer-coverage-inline-8bit-counters",
+                    ]);
+                }
+                _ => {}
+            }
+        }
+
+        use std::process::Command;
+        let mut opt = Command::new("opt");
+        opt.args([
+            app_ll_file.to_str().unwrap(),
+            "-o",
+            app_bc_file.to_str().unwrap(),
+        ])
+        .args(extra_args);
+        if !passes.is_empty() {
+            opt.arg(format!("-passes={}", passes.join(",")));
+        }
+        let opt = opt.output().unwrap();
+
+        assert!(opt.stderr.is_empty(), "{:#?}", opt);
+
+        // write the .o file. Note that this builds the .o for the local machine,
+        // and ignores the `target_machine` entirely.
+        //
+        // different systems name this executable differently, so we shotgun for
+        // the most common ones and then give up.
+        let bc_to_object = Command::new("llc")
+            .args(&[
+                "-relocation-model=pic",
+                "-filetype=obj",
+                app_bc_file.to_str().unwrap(),
+                "-o",
+                app_o_file.to_str().unwrap(),
+            ])
+            .output()
+            .unwrap();
+
+        assert!(bc_to_object.stderr.is_empty(), "{:#?}", bc_to_object);
+
+        MemoryBuffer::create_from_file(&app_o_file).expect("memory buffer creation works")
+    } else if emit_debug_info {
         module.strip_debug_info();
 
         let mut app_ll_dbg_file = PathBuf::from(roc_file_path);