Error out when self-contained LLD is requested, but it is not provided

src/bootstrap/config.rs

@@ -1498,6 +1498,16 @@ impl Config {
             }
             set(&mut config.lld_mode, rust.lld_mode);
             set(&mut config.lld_enabled, rust.lld);
+
+            if matches!(config.lld_mode, LldMode::SelfContained)
+                && !config.lld_enabled
+                && flags.stage.unwrap_or(0) > 0
+            {
+                panic!(
+                    "Self-contained LLD was requested with `rust.use-lld`, but it is not be available (`rust.lld` is false)."
+                );
+            }
+
             set(&mut config.llvm_tools_enabled, rust.llvm_tools);
             config.rustc_parallel = rust.parallel_compiler.unwrap_or(false);
             config.rustc_default_linker = rust.default_linker;