2 files changed, 33 insertions, 15 deletions
diff --git a/src/bootstrap/src/core/build_steps/test.rs b/src/bootstrap/src/core/build_steps/test.rs
index 248d831b6e3..47b0637538b 100644
--- a/src/bootstrap/src/core/build_steps/test.rs
+++ b/src/bootstrap/src/core/build_steps/test.rs
@@ -1657,8 +1657,8 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
         // ensure that `libproc_macro` is available on the host.
         builder.ensure(compile::Std::new(compiler, compiler.host));
 
-        // As well as the target, except for plain wasm32, which can't build it
-        if suite != "mir-opt" && !target.contains("wasm") && !target.contains("emscripten") {
+        // As well as the target
+        if suite != "mir-opt" {
             builder.ensure(TestHelpers { target });
         }
 
@@ -2511,16 +2511,7 @@ fn prepare_cargo_test(
     dylib_path.insert(0, PathBuf::from(&*builder.sysroot_libdir(compiler, target)));
     cargo.env(dylib_path_var(), env::join_paths(&dylib_path).unwrap());
 
-    if target.contains("emscripten") {
-        cargo.env(
-            format!("CARGO_TARGET_{}_RUNNER", envify(&target.triple)),
-            builder.config.nodejs.as_ref().expect("nodejs not configured"),
-        );
-    } else if target.starts_with("wasm32") {
-        let node = builder.config.nodejs.as_ref().expect("nodejs not configured");
-        let runner = format!("{} {}/src/etc/wasm32-shim.js", node.display(), builder.src.display());
-        cargo.env(format!("CARGO_TARGET_{}_RUNNER", envify(&target.triple)), &runner);
-    } else if builder.remote_tested(target) {
+    if builder.remote_tested(target) {
         cargo.env(
             format!("CARGO_TARGET_{}_RUNNER", envify(&target.triple)),
             format!("{} run 0", builder.tool_exe(Tool::RemoteTestClient).display()),
diff --git a/src/bootstrap/src/lib.rs b/src/bootstrap/src/lib.rs
index 6520b7ed089..9cbd4d367f0 100644
--- a/src/bootstrap/src/lib.rs
+++ b/src/bootstrap/src/lib.rs
@@ -1361,9 +1361,36 @@ impl Build {
     /// An example of this would be a WebAssembly runtime when testing the wasm
     /// targets.
     fn runner(&self, target: TargetSelection) -> Option<String> {
-        let target = self.config.target_config.get(&target)?;
-        let runner = target.runner.as_ref()?;
-        Some(runner.to_owned())
+        let configured_runner =
+            self.config.target_config.get(&target).and_then(|t| t.runner.as_ref()).map(|p| &**p);
+        if let Some(runner) = configured_runner {
+            return Some(runner.to_owned());
+        }
+
+        if target.starts_with("wasm") && target.contains("wasi") {
+            self.default_wasi_runner()
+        } else {
+            None
+        }
+    }
+
+    /// When a `runner` configuration is not provided and a WASI-looking target
+    /// is being tested this is consulted to prove the environment to see if
+    /// there's a runtime already lying around that seems reasonable to use.
+    fn default_wasi_runner(&self) -> Option<String> {
+        let mut finder = crate::core::sanity::Finder::new();
+
+        // Look for Wasmtime, and for its default options be sure to disable
+        // its caching system since we're executing quite a lot of tests and
+        // ideally shouldn't pollute the cache too much.
+        if let Some(path) = finder.maybe_have("wasmtime") {
+            if let Ok(mut path) = path.into_os_string().into_string() {
+                path.push_str(" run -C cache=n --dir .");
+                return Some(path);
+            }
+        }
+
+        None
     }
 
     /// Returns the root of the "rootfs" image that this target will be using,