compiletest: Add a //@ needs-threads directive

This commit is extracted from #122036 and adds a new directive to the
`compiletest` test runner, `//@ needs-threads`. This is intended to
capture the need that a target must implement threading to execute a
specific test, typically one that uses `std::thread`. This is primarily
done for WebAssembly targets which currently do not have threads by
default. This enables transitioning a lot of `//@ ignore-wasm*`-style
ignores into a more self-documenting `//@ needs-threads` directive.
Additionally the `wasm32-wasi-preview1-threads` target, for example,
does actually have threads, but isn't tested in CI at this time. This
change enables running these tests for that target, but not other wasm
targets.

src/tools/compiletest/src/common.rs

@@ -451,6 +451,15 @@ impl Config {
         self.target_cfg().panic == PanicStrategy::Unwind
     }
 
+    pub fn has_threads(&self) -> bool {
+        // Wasm targets don't have threads unless `-threads` is in the target
+        // name, such as `wasm32-wasip1-threads`.
+        if self.target.starts_with("wasm") {
+            return self.target.contains("threads");
+        }
+        true
+    }
+
     pub fn has_asm_support(&self) -> bool {
         static ASM_SUPPORTED_ARCHS: &[&str] = &[
             "x86", "x86_64", "arm", "aarch64", "riscv32",

src/tools/compiletest/src/header.rs

@@ -787,6 +787,7 @@ const DIAGNOSTICS_DIRECTIVE_NAMES: &[&str] = &[
     "needs-sanitizer-shadow-call-stack",
     "needs-sanitizer-support",
     "needs-sanitizer-thread",
+    "needs-threads",
     "needs-unwind",
     "needs-xray",
     "no-prefer-dynamic",

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

@@ -84,6 +84,11 @@ pub(super) fn handle_needs(
             condition: config.run_enabled(),
             ignore_reason: "ignored when running the resulting test binaries is disabled",
         },
+        Need {
+            name: "needs-threads",
+            condition: config.has_threads(),
+            ignore_reason: "ignored on targets without threading support",
+        },
         Need {
             name: "needs-unwind",
             condition: config.can_unwind(),

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

@@ -592,3 +592,23 @@ fn ignore_mode() {
         assert!(!check_ignore(&config, &format!("//@ ignore-mode-{other}")));
     }
 }
+
+#[test]
+fn threads_support() {
+    let threads = [
+        ("x86_64-unknown-linux-gnu", true),
+        ("aarch64-apple-darwin", true),
+        ("wasm32-unknown-unknown", false),
+        ("wasm64-unknown-unknown", false),
+        #[cfg(not(bootstrap))]
+        ("wasm32-wasip1", false),
+        #[cfg(not(bootstrap))]
+        ("wasm32-wasip1-threads", true),
+        ("wasm32-wasi-preview1-threads", true),
+    ];
+    for (target, has_threads) in threads {
+        let config = cfg().target(target).build();
+        assert_eq!(config.has_threads(), has_threads);
+        assert_eq!(check_ignore(&config, "//@ needs-threads"), !has_threads)
+    }
+}