From 0079da4862cb02a99c5def36a79949de583a9aee Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jakub=20Ber=C3=A1nek?= <berykubik@gmail.com>
Date: Tue, 19 Aug 2025 23:36:43 +0200
Subject: [PATCH 1/2] Add snapshot tests for stage 3 compiler builds

---
 src/bootstrap/src/core/builder/tests.rs | 76 +++++++++++++++++++++++++
 1 file changed, 76 insertions(+)

diff --git a/src/bootstrap/src/core/builder/tests.rs b/src/bootstrap/src/core/builder/tests.rs
index f4266a6085bfb..85c7f46099fee 100644
--- a/src/bootstrap/src/core/builder/tests.rs
+++ b/src/bootstrap/src/core/builder/tests.rs
@@ -672,6 +672,82 @@ mod snapshot {
         ");
     }
 
+    #[test]
+    fn build_compiler_stage_3() {
+        let ctx = TestCtx::new();
+        insta::assert_snapshot!(
+            ctx.config("build")
+                .path("compiler")
+                .stage(3)
+                .render_steps(), @r"
+        [build] llvm <host>
+        [build] rustc 0 <host> -> rustc 1 <host>
+        [build] rustc 1 <host> -> std 1 <host>
+        [build] rustc 1 <host> -> rustc 2 <host>
+        [build] rustc 2 <host> -> std 2 <host>
+        [build] rustc 2 <host> -> rustc 3 <host>
+        ");
+    }
+
+    #[test]
+    fn build_compiler_stage_3_cross() {
+        let ctx = TestCtx::new();
+        insta::assert_snapshot!(
+            ctx.config("build")
+                .path("compiler")
+                .hosts(&[TEST_TRIPLE_1])
+                .stage(3)
+                .render_steps(), @r"
+        [build] llvm <host>
+        [build] llvm <target1>
+        [build] rustc 0 <host> -> rustc 1 <host>
+        [build] rustc 1 <host> -> std 1 <host>
+        [build] rustc 1 <host> -> rustc 2 <host>
+        [build] rustc 1 <host> -> std 1 <target1>
+        [build] rustc 2 <host> -> std 2 <target1>
+        [build] rustc 2 <host> -> rustc 3 <target1>
+        ");
+    }
+
+    #[test]
+    fn build_compiler_stage_3_full_bootstrap() {
+        let ctx = TestCtx::new();
+        insta::assert_snapshot!(
+            ctx.config("build")
+                .path("compiler")
+                .stage(3)
+                .args(&["--set", "build.full-bootstrap=true"])
+                .render_steps(), @r"
+        [build] llvm <host>
+        [build] rustc 0 <host> -> rustc 1 <host>
+        [build] rustc 1 <host> -> std 1 <host>
+        [build] rustc 1 <host> -> rustc 2 <host>
+        [build] rustc 2 <host> -> std 2 <host>
+        [build] rustc 2 <host> -> rustc 3 <host>
+        ");
+    }
+
+    #[test]
+    fn build_compiler_stage_3_cross_full_bootstrap() {
+        let ctx = TestCtx::new();
+        insta::assert_snapshot!(
+            ctx.config("build")
+                .path("compiler")
+                .stage(3)
+                .hosts(&[TEST_TRIPLE_1])
+                .args(&["--set", "build.full-bootstrap=true"])
+                .render_steps(), @r"
+        [build] llvm <host>
+        [build] llvm <target1>
+        [build] rustc 0 <host> -> rustc 1 <host>
+        [build] rustc 1 <host> -> std 1 <host>
+        [build] rustc 1 <host> -> rustc 2 <host>
+        [build] rustc 2 <host> -> std 2 <target1>
+        [build] rustc 2 <host> -> std 2 <host>
+        [build] rustc 2 <host> -> rustc 3 <target1>
+        ");
+    }
+
     #[test]
     fn build_compiler_codegen_backend() {
         let ctx = TestCtx::new();

From f254075e95a074b47307b7ffada62a907cb27c22 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jakub=20Ber=C3=A1nek?= <berykubik@gmail.com>
Date: Wed, 20 Aug 2025 08:43:57 +0200
Subject: [PATCH 2/2] Disable rustc uplifting during cross-compilation

---
 src/bootstrap/src/core/build_steps/compile.rs | 14 +++++---------
 src/bootstrap/src/core/builder/tests.rs       |  1 +
 2 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/src/bootstrap/src/core/build_steps/compile.rs b/src/bootstrap/src/core/build_steps/compile.rs
index 74a8cea1ebe77..f22d40e021ecb 100644
--- a/src/bootstrap/src/core/build_steps/compile.rs
+++ b/src/bootstrap/src/core/build_steps/compile.rs
@@ -1047,22 +1047,18 @@ impl Step for Rustc {
 
         // If we are building a stage3+ compiler, and full bootstrap is disabled, and we have a
         // previous rustc available, we will uplift a compiler from a previous stage.
+        // We do not allow cross-compilation uplifting here, because there it can be quite tricky
+        // to figure out which stage actually built the rustc that should be uplifted.
         if build_compiler.stage >= 2
             && !builder.config.full_bootstrap
-            && (target == builder.host_target || builder.hosts.contains(&target))
+            && target == builder.host_target
         {
             // Here we need to determine the **build compiler** that built the stage that we will
             // be uplifting. We cannot uplift stage 1, as it has a different ABI than stage 2+,
             // so we always uplift the stage2 compiler (compiled with stage 1).
             let uplift_build_compiler = builder.compiler(1, build_compiler.host);
-            let msg = if uplift_build_compiler.host == target {
-                format!("Uplifting rustc (stage2 -> stage{stage})")
-            } else {
-                format!(
-                    "Uplifting rustc (stage2:{} -> stage{stage}:{target})",
-                    uplift_build_compiler.host
-                )
-            };
+
+            let msg = format!("Uplifting rustc from stage2 to stage{stage})");
             builder.info(&msg);
 
             // Here the compiler that built the rlibs (`uplift_build_compiler`) can be different
diff --git a/src/bootstrap/src/core/builder/tests.rs b/src/bootstrap/src/core/builder/tests.rs
index 85c7f46099fee..a7db96055e663 100644
--- a/src/bootstrap/src/core/builder/tests.rs
+++ b/src/bootstrap/src/core/builder/tests.rs
@@ -705,6 +705,7 @@ mod snapshot {
         [build] rustc 1 <host> -> rustc 2 <host>
         [build] rustc 1 <host> -> std 1 <target1>
         [build] rustc 2 <host> -> std 2 <target1>
+        [build] rustc 2 <host> -> std 2 <host>
         [build] rustc 2 <host> -> rustc 3 <target1>
         ");
     }