Rollup merge of #86113 - the8472:doctest-lld, r=Mark-Simulacrum

build doctests with lld if use-lld = true

results when running `./x.py test library/core --doc --stage 0`:

```
# OLD
test result: FAILED. 2844 passed; 6 failed; 28 ignored; 0 measured; 0 filtered out; finished in 21.13s

# NEW
test result: FAILED. 2844 passed; 6 failed; 28 ignored; 0 measured; 0 filtered out; finished in 11.92s
```

4 files changed, 27 insertions, 16 deletions

diff --git a/src/bootstrap/bin/rustdoc.rs b/src/bootstrap/bin/rustdoc.rs
index cba17c8e608..e4396d53016 100644
--- a/src/bootstrap/bin/rustdoc.rs
+++ b/src/bootstrap/bin/rustdoc.rs
@@ -41,7 +41,12 @@ fn main() {
         cmd.arg(arg);
     }
     if env::var_os("RUSTDOC_FUSE_LD_LLD").is_some() {
-        cmd.arg("-Clink-args=-fuse-ld=lld");
+        cmd.arg("-Clink-arg=-fuse-ld=lld");
+        if cfg!(windows) {
+            cmd.arg("-Clink-arg=-Wl,/threads:1");
+        } else {
+            cmd.arg("-Clink-arg=-Wl,--threads=1");
+        }
     }
 
     // Needed to be able to run all rustdoc tests.
diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
index 06f8bf89dae..bc499fdba59 100644
--- a/src/bootstrap/builder.rs
+++ b/src/bootstrap/builder.rs
@@ -1142,6 +1142,7 @@ impl<'a> Builder<'a> {
         }
         if self.is_fuse_ld_lld(compiler.host) {
             cargo.env("RUSTC_HOST_FUSE_LD_LLD", "1");
+            cargo.env("RUSTDOC_FUSE_LD_LLD", "1");
         }
 
         if let Some(target_linker) = self.linker(target) {
@@ -1151,6 +1152,9 @@ impl<'a> Builder<'a> {
         if self.is_fuse_ld_lld(target) {
             rustflags.arg("-Clink-args=-fuse-ld=lld");
         }
+        self.lld_flags(target).for_each(|flag| {
+            rustdocflags.arg(&flag);
+        });
 
         if !(["build", "check", "clippy", "fix", "rustc"].contains(&cmd)) && want_rustdoc {
             cargo.env("RUSTDOC_LIBDIR", self.rustc_libdir(compiler));
diff --git a/src/bootstrap/lib.rs b/src/bootstrap/lib.rs
index 21c7dd11d24..347236c655a 100644
--- a/src/bootstrap/lib.rs
+++ b/src/bootstrap/lib.rs
@@ -923,6 +923,21 @@ impl Build {
         self.config.use_lld && !target.contains("msvc")
     }
 
+    fn lld_flags(&self, target: TargetSelection) -> impl Iterator<Item = String> {
+        let mut options = [None, None];
+
+        if self.config.use_lld {
+            if self.is_fuse_ld_lld(target) {
+                options[0] = Some("-Clink-arg=-fuse-ld=lld".to_string());
+            }
+
+            let threads = if target.contains("windows") { "/threads:1" } else { "--threads=1" };
+            options[1] = Some(format!("-Clink-arg=-Wl,{}", threads));
+        }
+
+        std::array::IntoIter::new(options).flatten()
+    }
+
     /// Returns if this target should statically link the C runtime, if specified
     fn crt_static(&self, target: TargetSelection) -> Option<bool> {
         if target.contains("pc-windows-msvc") {
diff --git a/src/bootstrap/test.rs b/src/bootstrap/test.rs
index cc7c143d474..70cf1b105c0 100644
--- a/src/bootstrap/test.rs
+++ b/src/bootstrap/test.rs
@@ -1131,19 +1131,6 @@ struct Compiletest {
     compare_mode: Option<&'static str>,
 }
 
-impl Compiletest {
-    fn add_lld_flags(builder: &Builder<'_>, target: TargetSelection, flags: &mut Vec<String>) {
-        if builder.config.use_lld {
-            if builder.is_fuse_ld_lld(target) {
-                flags.push("-Clink-arg=-fuse-ld=lld".to_string());
-            }
-
-            let threads = if target.contains("windows") { "/threads:1" } else { "--threads=1" };
-            flags.push(format!("-Clink-arg=-Wl,{}", threads));
-        }
-    }
-}
-
 impl Step for Compiletest {
     type Output = ();
 
@@ -1289,12 +1276,12 @@ note: if you're sure you want to do this, please open an issue as to why. In the
 
         let mut hostflags = flags.clone();
         hostflags.push(format!("-Lnative={}", builder.test_helpers_out(compiler.host).display()));
-        Self::add_lld_flags(builder, compiler.host, &mut hostflags);
+        hostflags.extend(builder.lld_flags(compiler.host));
         cmd.arg("--host-rustcflags").arg(hostflags.join(" "));
 
         let mut targetflags = flags;
         targetflags.push(format!("-Lnative={}", builder.test_helpers_out(target).display()));
-        Self::add_lld_flags(builder, target, &mut targetflags);
+        targetflags.extend(builder.lld_flags(target));
         cmd.arg("--target-rustcflags").arg(targetflags.join(" "));
 
         cmd.arg("--docck-python").arg(builder.python());