update std uplifting algorithm

Signed-off-by: onur-ozkan <[email protected]>

Author: onur-ozkan

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

@@ -185,8 +185,20 @@ impl Step for Std {
 
         let mut target_deps = builder.ensure(StartupObjects { compiler, target });
 
-        let compiler_to_use = builder.compiler_for(compiler.stage, compiler.host, target);
-        if compiler_to_use != compiler {
+        let mut compiler_to_use = builder.compiler_for(compiler.stage, compiler.host, target);
+
+        // We only need to build std twice; therefore,
+        // - If stage 2 std is built, the artifacts are fully up to date. For the next stages, uplift the artifacts instead of compiling std again.
+        // - If `--build-std-on-stage0` was used it means std was already built on stage 0. So, if stage 1 is built, that means std is fully up to date.
+        //      For the next stages, uplift the artifacts instead of compiling std again.
+        if (compiler.stage > 2 || builder.config.build_std_on_stage0) && compiler_to_use != compiler
+        {
+            // If stage 2 std was built (not uplifted), uplift the artifacts from stage 2.
+            // (This is necessary; without this, they will be uplifted from stage 1).
+            if !builder.config.build_std_on_stage0 && compiler.stage > 2 {
+                compiler_to_use.stage = 2;
+            }
+
             builder.ensure(Std::new(compiler_to_use, target));
             let msg = if compiler_to_use.host == target {
                 format!(