bootstrap: introduce rust-lld cfg for rustc

This is used to notify we want to use rust-lld as the default linker in
a target.

Author: lqd

src/bootstrap/src/core/build_steps/compile.rs

@@ -1129,6 +1129,12 @@ pub fn rustc_cargo_env(
         cargo.env("CFG_DEFAULT_LINKER", s);
     }
 
+    // Enable rustc's cfgs for `rust-lld` when requested.
+    if builder.config.lld_enabled {
+        cargo.rustflag("--cfg=use_rust_lld");
+        cargo.rustdocflag("--cfg=use_rust_lld");
+    }
+
     if builder.config.rust_verify_llvm_ir {
         cargo.env("RUSTC_VERIFY_LLVM_IR", "1");
     }

src/bootstrap/src/lib.rs

@@ -114,6 +114,10 @@ const EXTRA_CHECK_CFGS: &[(Option<Mode>, &str, Option<&[&'static str]>)] = &[
     // Needed to avoid the need to copy windows.lib into the sysroot.
     (Some(Mode::Rustc), "windows_raw_dylib", None),
     (Some(Mode::ToolRustc), "windows_raw_dylib", None),
+    // If rustc wants to use rust-lld as the default linker in a target spec.
+    (Some(Mode::Rustc), "use_rust_lld", None),
+    (Some(Mode::ToolRustc), "use_rust_lld", None),
+    (Some(Mode::Codegen), "use_rust_lld", None),
 ];
 
 /// A structure representing a Rust compiler.