add test for lld opt in and also add thread_local defined state to change opt in targets

Author: Shourya742

src/bootstrap/src/core/builder/tests.rs

@@ -642,6 +642,7 @@ mod snapshot {
     };
     use crate::core::builder::{Builder, Kind, StepDescription, StepMetadata};
     use crate::core::config::TargetSelection;
+    use crate::core::config::toml::rust::with_lld_opt_in_targets;
     use crate::utils::cache::Cache;
     use crate::utils::helpers::get_host_target;
     use crate::utils::tests::{ConfigBuilder, TestCtx};
@@ -1644,6 +1645,28 @@ mod snapshot {
         ");
     }
 
+    #[test]
+    fn test_lld_opt_in() {
+        let target: &'static str = Box::leak(Box::new(host_target()));
+        let slice: &'static [&'static str] = Box::leak(Box::new([target]));
+
+        with_lld_opt_in_targets(slice, || {
+            let ctx = TestCtx::new();
+
+            insta::assert_snapshot!(
+                ctx.config("doc")
+                    .path("core")
+                    .override_target_no_std(&host_target())
+                    .render_steps(), @r"
+            [build] llvm <host>
+            [build] rustc 0 <host> -> rustc 1 <host>
+            [build] rustc 0 <host> -> LldWrapper 1 <host>
+            [build] rustdoc 0 <host>
+            [doc] std 1 <host> crates=[core]
+            ");
+        });
+    }
+
     #[test]
     fn doc_library_no_std_target() {
         let ctx = TestCtx::new();

src/bootstrap/src/core/config/toml/rust.rs

@@ -413,9 +413,25 @@ pub(crate) fn validate_codegen_backends(backends: Vec<String>, section: &str) ->
 fn default_lld_opt_in_targets() -> &'static [&'static str] {
     &["x86_64-unknown-linux-gnu"]
 }
+
+#[cfg(test)]
+thread_local! {
+    static TEST_LLD_OPT_IN_TARGETS: std::cell::RefCell<Option<&'static [&'static str]>> = std::cell::RefCell::new(None);
+}
+
 #[cfg(test)]
 fn default_lld_opt_in_targets() -> &'static [&'static str] {
-    &[]
+    TEST_LLD_OPT_IN_TARGETS.with(|cell| cell.borrow().unwrap_or(&[]))
+}
+
+#[cfg(test)]
+pub fn with_lld_opt_in_targets<R>(targets: &'static [&'static str], f: impl FnOnce() -> R) -> R {
+    TEST_LLD_OPT_IN_TARGETS.with(|cell| {
+        let prev = cell.replace(Some(targets));
+        let result = f();
+        cell.replace(prev);
+        result
+    })
 }
 
 impl Config {