handle precompiled compiler more properly

Signed-off-by: onur-ozkan <work@onurozkan.dev>

Author: onur-ozkan

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

@@ -1991,12 +1991,12 @@ impl Step for Assemble {
             }
         }
 
-        let maybe_install_llvm_bitcode_linker = || {
+        let maybe_install_llvm_bitcode_linker = |compiler| {
             if builder.config.llvm_bitcode_linker_enabled {
                 trace!("llvm-bitcode-linker enabled, installing");
                 let llvm_bitcode_linker =
                     builder.ensure(crate::core::build_steps::tool::LlvmBitcodeLinker {
-                        compiler: target_compiler,
+                        compiler,
                         target: target_compiler.host,
                         extra_features: vec![],
                     });
@@ -2020,7 +2020,9 @@ impl Step for Assemble {
                 builder.info(&format!("Creating a sysroot for stage{stage} compiler (use `rustup toolchain link 'name' build/host/stage{stage}`)", stage=target_compiler.stage));
             }
 
-            maybe_install_llvm_bitcode_linker();
+            let mut precompiled_compiler = target_compiler;
+            precompiled_compiler.forced_compiler(true);
+            maybe_install_llvm_bitcode_linker(precompiled_compiler);
 
             return target_compiler;
         }
@@ -2203,7 +2205,7 @@ impl Step for Assemble {
             );
         }
 
-        maybe_install_llvm_bitcode_linker();
+        maybe_install_llvm_bitcode_linker(target_compiler);
 
         // Ensure that `libLLVM.so` ends up in the newly build compiler directory,
         // so that it can be found when the newly built `rustc` is run.

src/bootstrap/src/core/build_steps/tool.rs

@@ -319,18 +319,20 @@ pub(crate) fn get_tool_rustc_compiler(
     builder: &Builder<'_>,
     target_compiler: Compiler,
 ) -> Compiler {
-    if builder.download_rustc() && target_compiler.stage == 1 {
-        // We already have the stage 1 compiler, we don't need to cut the stage.
-        builder.compiler(target_compiler.stage, builder.config.build)
-    } else if target_compiler.is_forced_compiler() {
-        target_compiler
-    } else {
-        // Similar to `compile::Assemble`, build with the previous stage's compiler. Otherwise
-        // we'd have stageN/bin/rustc and stageN/bin/$rustc_tool be effectively different stage
-        // compilers, which isn't what we want. Rustc tools should be linked in the same way as the
-        // compiler it's paired with, so it must be built with the previous stage compiler.
-        builder.compiler(target_compiler.stage.saturating_sub(1), builder.config.build)
+    if target_compiler.is_forced_compiler() {
+        return target_compiler;
+    }
+
+    if builder.download_rustc() && target_compiler.stage > 0 {
+        // We already have the stage N compiler, we don't need to cut the stage.
+        return builder.compiler(target_compiler.stage, builder.config.build);
     }
+
+    // Similar to `compile::Assemble`, build with the previous stage's compiler. Otherwise
+    // we'd have stageN/bin/rustc and stageN/bin/$rustc_tool be effectively different stage
+    // compilers, which isn't what we want. Rustc tools should be linked in the same way as the
+    // compiler it's paired with, so it must be built with the previous stage compiler.
+    builder.compiler(target_compiler.stage.saturating_sub(1), builder.config.build)
 }
 
 /// Links a built tool binary with the given `name` from the build directory to the