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.

Author: alexcrichton

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(),

tests/codegen/cffi/c-variadic.rs

@@ -1,4 +1,4 @@
-//@ ignore-wasm32-bare compiled with panic=abort by default
+//@ needs-unwind
 //@ compile-flags: -C no-prepopulate-passes -Copt-level=0
 //
 

tests/ui/abi/extern/extern-call-deep2.rs

@@ -1,6 +1,6 @@
 //@ run-pass
 #![allow(unused_must_use)]
-//@ ignore-emscripten no threads support
+//@ needs-threads
 #![feature(rustc_private)]
 
 extern crate libc;

tests/ui/abi/extern/extern-call-scrub.rs

@@ -4,7 +4,7 @@
 // make sure the stack pointers are maintained properly in both
 // directions
 
-//@ ignore-emscripten no threads support
+//@ needs-threads
 #![feature(rustc_private)]
 
 extern crate libc;

tests/ui/abi/foreign/foreign-call-no-runtime.rs

@@ -1,5 +1,5 @@
 //@ run-pass
-//@ ignore-emscripten no threads support
+//@ needs-threads
 
 #![feature(rustc_private)]
 

tests/ui/box/unit/unique-send-2.rs

@@ -1,6 +1,6 @@
 //@ run-pass
 #![allow(unused_must_use)]
-//@ ignore-emscripten no threads support
+//@ needs-threads
 
 use std::sync::mpsc::{channel, Sender};
 use std::thread;

tests/ui/codegen/init-large-type.rs

@@ -7,8 +7,7 @@
 // optimisation.
 
 //@ pretty-expanded FIXME #23616
-//@ ignore-emscripten no threads support
-
+//@ needs-threads
 #![feature(intrinsics)]
 
 use std::{mem, thread};

tests/ui/codegen/issue-28950.rs

@@ -1,5 +1,5 @@
 //@ run-pass
-//@ ignore-emscripten no threads
+//@ needs-threads
 //@ compile-flags: -O
 
 // Tests that the `vec!` macro does not overflow the stack when it is