Implement //@ needs-target-std directive in compiletest

To support tests to condition their (potentially cross-compile)
execution based on whether the target supports std.

Author: jieyouxu

src/tools/compiletest/src/directive-list.rs

@@ -166,6 +166,7 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "needs-subprocess",
     "needs-symlink",
     "needs-target-has-atomic",
+    "needs-target-std",
     "needs-threads",
     "needs-unwind",
     "needs-wasmtime",

src/tools/compiletest/src/header/needs.rs

@@ -174,6 +174,11 @@ pub(super) fn handle_needs(
             condition: config.with_std_debug_assertions,
             ignore_reason: "ignored if std wasn't built with debug assertions",
         },
+        Need {
+            name: "needs-target-std",
+            condition: build_helper::targets::target_supports_std(&config.target),
+            ignore_reason: "ignored if target does not support std",
+        },
     ];
 
     let (name, rest) = match ln.split_once([':', ' ']) {

src/tools/compiletest/src/header/tests.rs

@@ -945,3 +945,14 @@ fn test_ignore_auxiliary() {
     let config = cfg().build();
     assert!(check_ignore(&config, "//@ ignore-auxiliary"));
 }
+
+#[test]
+fn test_needs_target_std() {
+    // Cherry-picks two targets:
+    // 1. `x86_64-unknown-none`: Tier 2, intentionally never supports std.
+    // 2. `x86_64-unknown-linux-gnu`: Tier 1, always supports std.
+    let config = cfg().target("x86_64-unknown-none").build();
+    assert!(check_ignore(&config, "//@ needs-target-std"));
+    let config = cfg().target("x86_64-unknown-linux-gnu").build();
+    assert!(!check_ignore(&config, "//@ needs-target-std"));
+}