compiletest: Add support for //@ needs-env-vars

Author: Zalathar

src/doc/rustc-dev-guide/src/tests/directives.md

@@ -158,6 +158,7 @@ settings:
 
 - `needs-asm-support` — ignores if it is running on a target that doesn't have
   stable support for `asm!`
+- `needs-env-vars` - ignores if the target doesn't support environment variables
 - `needs-profiler-runtime` — ignores the test if the profiler runtime was not
   enabled for the target
   (`build.profiler = true` in rustc's `bootstrap.toml`)

src/tools/compiletest/src/common.rs

@@ -520,6 +520,16 @@ impl Config {
             || self.target_cfg().os == "emscripten";
         !unsupported_target
     }
+
+    pub fn has_env_vars(&self) -> bool {
+        // As with some of the other target properties, we don't have a principled
+        // way to know whether the target supports environment variables, so we
+        // just hard-code a reasonable approximation.
+
+        let target = self.target_cfg();
+        let no_env = matches!(target.arch.as_str(), "wasm32" | "wasm64") && target.os == "unknown";
+        !no_env
+    }
 }
 
 /// Known widths of `target_has_atomic`.

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

@@ -135,6 +135,7 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "needs-deterministic-layouts",
     "needs-dlltool",
     "needs-dynamic-linking",
+    "needs-env-vars",
     "needs-enzyme",
     "needs-force-clang-based-tests",
     "needs-git-hash",

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-env-vars",
+            condition: config.has_env_vars(),
+            ignore_reason: "ignored on targets without environment variables",
+        },
     ];
 
     let (name, rest) = match ln.split_once([':', ' ']) {