bootstrap: introduce rust-lld cfg for rustc

Author: lqd

src/bootstrap/compile.rs

@@ -1000,6 +1000,10 @@ pub fn rustc_cargo_env(
         cargo.rustflag("--cfg=parallel_compiler");
         cargo.rustdocflag("--cfg=parallel_compiler");
     }
+    if builder.config.lld_enabled {
+        cargo.rustflag("--cfg=rust_lld");
+        cargo.rustdocflag("--cfg=rust_lld");
+    }
     if builder.config.rust_verify_llvm_ir {
         cargo.env("RUSTC_VERIFY_LLVM_IR", "1");
     }

src/bootstrap/lib.rs

@@ -169,6 +169,10 @@ 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), "rust_lld", None),
+    (Some(Mode::ToolRustc), "rust_lld", None),
+    (Some(Mode::Codegen), "rust_lld", None),
 ];
 
 /// A structure representing a Rust compiler.