add default_lld_opt_in_targets method and modified test accordingly

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

make the config lld test parameter smoother to work, and have no_llvm_config set even when target_config is not present

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};
@@ -1647,12 +1648,33 @@ mod snapshot {
                 .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 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();
@@ -1663,7 +1685,6 @@ mod snapshot {
                 .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=[alloc,core]
         ");

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

@@ -410,6 +410,31 @@ pub(crate) fn validate_codegen_backends(backends: Vec<String>, section: &str) ->
     backends
 }
 
+#[cfg(not(test))]
+fn default_lld_opt_in_targets() -> Vec<String> {
+    vec!["x86_64-unknown-linux-gnu".to_string()]
+}
+
+#[cfg(test)]
+thread_local! {
+    static TEST_LLD_OPT_IN_TARGETS: std::cell::RefCell<Option<Vec<String>>> = std::cell::RefCell::new(None);
+}
+
+#[cfg(test)]
+fn default_lld_opt_in_targets() -> Vec<String> {
+    TEST_LLD_OPT_IN_TARGETS.with(|cell| cell.borrow().clone()).unwrap_or_default()
+}
+
+#[cfg(test)]
+pub fn with_lld_opt_in_targets<R>(targets: Vec<String>, 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 {
     pub fn apply_rust_config(&mut self, toml_rust: Option<Rust>, warnings: Warnings) {
         let mut debug = None;
@@ -617,12 +642,13 @@ impl Config {
         //   thus, disabled
         // - similarly, lld will not be built nor used by default when explicitly asked not to, e.g.
         //   when the config sets `rust.lld = false`
-        if self.host_target.triple == "x86_64-unknown-linux-gnu" && self.hosts == [self.host_target]
+        if default_lld_opt_in_targets().contains(&self.host_target.triple.to_string())
+            && self.hosts == [self.host_target]
         {
             let no_llvm_config = self
                 .target_config
                 .get(&self.host_target)
-                .is_some_and(|target_config| target_config.llvm_config.is_none());
+                .is_none_or(|target_config| target_config.llvm_config.is_none());
             let enable_lld = self.llvm_from_ci || no_llvm_config;
             // Prefer the config setting in case an explicit opt-out is needed.
             self.lld_enabled = lld_enabled.unwrap_or(enable_lld);