Only build rust_test_helpers for ui test suite

Author: jieyouxu

src/bootstrap/src/core/build_steps/test.rs

@@ -1624,21 +1624,13 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
             builder.tool_exe(Tool::RunMakeSupport);
         }
 
-        // Also provide `rust_test_helpers` for the host.
-        builder.ensure(TestHelpers { target: compiler.host });
-
         // ensure that `libproc_macro` is available on the host.
         if suite == "mir-opt" {
             builder.ensure(compile::Std::new(compiler, compiler.host).is_for_mir_opt_tests(true));
         } else {
             builder.ensure(compile::Std::new(compiler, compiler.host));
         }
 
-        // As well as the target
-        if suite != "mir-opt" {
-            builder.ensure(TestHelpers { target });
-        }
-
         let mut cmd = builder.tool_cmd(Tool::Compiletest);
 
         if suite == "mir-opt" {
@@ -1804,11 +1796,22 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
         }
 
         let mut hostflags = flags.clone();
-        hostflags.push(format!("-Lnative={}", builder.test_helpers_out(compiler.host).display()));
         hostflags.extend(linker_flags(builder, compiler.host, LldThreads::No, compiler.stage));
 
         let mut targetflags = flags;
-        targetflags.push(format!("-Lnative={}", builder.test_helpers_out(target).display()));
+
+        // Provide `rust_test_helpers`. This is only used in `ui` tests.
+        //
+        // FIXME(jieyouxu): consider splitting `ui` tests into those that need `rust_test_helpers`
+        // vs those that don't. Otherwise, e.g. when cross-ing to `wasm32-unknown-unknown` this
+        // introduces annoyances.
+        if suite == "ui" {
+            builder.ensure(TestHelpers { target: compiler.host });
+            builder.ensure(TestHelpers { target });
+            hostflags
+                .push(format!("-Lnative={}", builder.test_helpers_out(compiler.host).display()));
+            targetflags.push(format!("-Lnative={}", builder.test_helpers_out(target).display()));
+        }
 
         for flag in hostflags {
             cmd.arg("--host-rustcflags").arg(flag);