From 20719767bc04ab72ae1c1d0751caff960b984ac9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jakub=20Ber=C3=A1nek?= <berykubik@gmail.com>
Date: Sat, 5 Jul 2025 10:55:12 +0200
Subject: [PATCH] Use stage auto-bump when cross-checking on stage 1

---
 src/bootstrap/src/core/build_steps/check.rs | 22 +++++++++++----------
 src/bootstrap/src/core/builder/tests.rs     |  9 ++-------
 2 files changed, 14 insertions(+), 17 deletions(-)

diff --git a/src/bootstrap/src/core/build_steps/check.rs b/src/bootstrap/src/core/build_steps/check.rs
index 555a02ac5872..40632f35bd77 100644
--- a/src/bootstrap/src/core/build_steps/check.rs
+++ b/src/bootstrap/src/core/build_steps/check.rs
@@ -1,7 +1,5 @@
 //! Implementation of compiling the compiler and standard library, in "check"-based modes.
 
-use build_helper::exit;
-
 use crate::core::build_steps::compile::{
     add_to_sysroot, run_cargo, rustc_cargo, rustc_cargo_env, std_cargo, std_crates_for_run_make,
 };
@@ -264,19 +262,23 @@ fn prepare_compiler_for_check(
             build_compiler
         }
         Mode::ToolRustc | Mode::Codegen => {
+            // FIXME: this is a hack, see description of Mode::Rustc below
+            let stage = if host == target { builder.top_stage - 1 } else { builder.top_stage };
             // When checking tool stage N, we check it with compiler stage N-1
-            let build_compiler = builder.compiler(builder.top_stage - 1, host);
+            let build_compiler = builder.compiler(stage, host);
             builder.ensure(Rustc::new(builder, build_compiler, target));
             build_compiler
         }
         Mode::Rustc => {
-            if builder.top_stage < 2 && host != target {
-                eprintln!("Cannot do a cross-compilation check of rustc on stage 1, use stage 2");
-                exit!(1);
-            }
-
-            // When checking the stage N compiler, we want to do it with the stage N-1 compiler
-            builder.compiler(builder.top_stage - 1, host)
+            // This is a horrible hack, because we actually change the compiler stage numbering
+            // here. If you do `x check --stage 1 --host FOO`, we build stage 1 host rustc,
+            // and use that to check stage 1 FOO rustc (which actually makes that stage 2 FOO
+            // rustc).
+            //
+            // FIXME: remove this and either fix cross-compilation check on stage 2 (which has a
+            // myriad of other problems) or disable cross-checking on stage 1.
+            let stage = if host == target { builder.top_stage - 1 } else { builder.top_stage };
+            builder.compiler(stage, host)
         }
         Mode::Std => {
             // When checking std stage N, we want to do it with the stage N compiler
diff --git a/src/bootstrap/src/core/builder/tests.rs b/src/bootstrap/src/core/builder/tests.rs
index f0dba1a85f34..a6bd531fd14b 100644
--- a/src/bootstrap/src/core/builder/tests.rs
+++ b/src/bootstrap/src/core/builder/tests.rs
@@ -1299,7 +1299,6 @@ mod snapshot {
         let ctx = TestCtx::new();
         insta::assert_snapshot!(
             ctx.config("check")
-                .stage(2)
                 .targets(&[TEST_TRIPLE_1])
                 .hosts(&[TEST_TRIPLE_1])
                 .render_steps(), @r"
@@ -1307,7 +1306,6 @@ mod snapshot {
         [build] rustc 0 <host> -> rustc 1 <host>
         [build] rustc 1 <host> -> std 1 <host>
         [build] rustc 1 <host> -> std 1 <target1>
-        [build] llvm <target1>
         [check] rustc 1 <host> -> rustc 2 <target1>
         [check] rustc 1 <host> -> Rustdoc 2 <target1>
         [check] rustc 1 <host> -> cranelift 2 <target1>
@@ -1318,12 +1316,9 @@ mod snapshot {
         [check] rustc 0 <host> -> MiroptTestTools 1 <target1>
         [check] rustc 1 <host> -> Rustfmt 2 <target1>
         [check] rustc 1 <host> -> rust-analyzer 2 <target1>
-        [build] rustc 1 <host> -> rustc 2 <host>
-        [build] rustc 2 <host> -> std 2 <host>
-        [build] rustc 2 <host> -> std 2 <target1>
-        [check] rustc 2 <host> -> TestFloatParse 3 <target1>
+        [check] rustc 1 <host> -> TestFloatParse 2 <target1>
         [check] rustc 0 <host> -> FeaturesStatusDump 1 <target1>
-        [check] rustc 2 <host> -> std 2 <target1>
+        [check] rustc 1 <host> -> std 1 <target1>
         ");
     }