diff --git a/src/ci/docker/host-x86_64/x86_64-gnu-llvm-18/Dockerfile b/src/ci/docker/host-x86_64/x86_64-gnu-llvm-18/Dockerfile
index 7df32f18fdb3..3fe79c819fe6 100644
--- a/src/ci/docker/host-x86_64/x86_64-gnu-llvm-18/Dockerfile
+++ b/src/ci/docker/host-x86_64/x86_64-gnu-llvm-18/Dockerfile
@@ -59,6 +59,6 @@ COPY host-x86_64/dist-x86_64-linux/build-gccjit.sh /scripts/
 
 RUN /scripts/build-gccjit.sh /scripts
 
-COPY scripts/x86_64-gnu-llvm.sh /tmp/script.sh
 ARG SCRIPT_ARG
-ENV SCRIPT="/tmp/script.sh && ${SCRIPT_ARG}"
+COPY scripts/{SCRIPT_ARG} /tmp/script.sh
+ENV SCRIPT="/tmp/script.sh"
diff --git a/src/ci/docker/host-x86_64/x86_64-gnu-llvm-19/Dockerfile b/src/ci/docker/host-x86_64/x86_64-gnu-llvm-19/Dockerfile
index e6e6f3e779b7..d52adc89cabe 100644
--- a/src/ci/docker/host-x86_64/x86_64-gnu-llvm-19/Dockerfile
+++ b/src/ci/docker/host-x86_64/x86_64-gnu-llvm-19/Dockerfile
@@ -59,6 +59,6 @@ COPY host-x86_64/dist-x86_64-linux/build-gccjit.sh /scripts/
 
 RUN /scripts/build-gccjit.sh /scripts
 
-COPY scripts/x86_64-gnu-llvm.sh /tmp/script.sh
 ARG SCRIPT_ARG
-ENV SCRIPT="/tmp/script.sh && ${SCRIPT_ARG}"
+COPY scripts/{SCRIPT_ARG} /tmp/script.sh
+ENV SCRIPT="/tmp/script.sh"
diff --git a/src/ci/docker/scripts/x86_64-gnu-llvm1.sh b/src/ci/docker/scripts/x86_64-gnu-llvm1.sh
new file mode 100755
index 000000000000..7336b4ce37b6
--- /dev/null
+++ b/src/ci/docker/scripts/x86_64-gnu-llvm1.sh
@@ -0,0 +1,36 @@
+#!/bin/bash
+
+set -ex
+
+if [ "$READ_ONLY_SRC" = "0" ]; then
+    # `core::builder::tests::ci_rustc_if_unchanged_logic` bootstrap test ensures that
+    # "download-rustc=if-unchanged" logic don't use CI rustc while there are changes on
+    # compiler and/or library. Here we are adding a dummy commit on compiler and running
+    # that test to make sure we never download CI rustc with a change on the compiler tree.
+    echo "" >> ../compiler/rustc/src/main.rs
+    git config --global user.email "dummy@dummy.com"
+    git config --global user.name "dummy"
+    git add ../compiler/rustc/src/main.rs
+    git commit -m "test commit for rust.download-rustc=if-unchanged logic"
+    DISABLE_CI_RUSTC_IF_INCOMPATIBLE=0 ../x.py test bootstrap \
+        -- core::builder::tests::ci_rustc_if_unchanged_logic
+    # Revert the dummy commit
+    git reset --hard HEAD~1
+fi
+
+# Only run the stage 1 tests on merges, not on PR CI jobs.
+if [[ -z "${PR_CI_JOB}" ]]; then
+    ../x.py --stage 1 test
+      --skip tests
+      --skip coverage-map
+      --skip coverage-run
+      --skip library
+      --skip tidyselftest
+fi
+
+../x.py --stage 2 test
+  --skip tests
+  --skip coverage-map
+  --skip coverage-run
+  --skip library
+  --skip tidyselftest
diff --git a/src/ci/docker/scripts/x86_64-gnu-llvm2.sh b/src/ci/docker/scripts/x86_64-gnu-llvm2.sh
new file mode 100755
index 000000000000..a0d31d8d7ae6
--- /dev/null
+++ b/src/ci/docker/scripts/x86_64-gnu-llvm2.sh
@@ -0,0 +1,60 @@
+#!/bin/bash
+
+set -ex
+
+if [ "$READ_ONLY_SRC" = "0" ]; then
+    # `core::builder::tests::ci_rustc_if_unchanged_logic` bootstrap test ensures that
+    # "download-rustc=if-unchanged" logic don't use CI rustc while there are changes on
+    # compiler and/or library. Here we are adding a dummy commit on compiler and running
+    # that test to make sure we never download CI rustc with a change on the compiler tree.
+    echo "" >> ../compiler/rustc/src/main.rs
+    git config --global user.email "dummy@dummy.com"
+    git config --global user.name "dummy"
+    git add ../compiler/rustc/src/main.rs
+    git commit -m "test commit for rust.download-rustc=if-unchanged logic"
+    DISABLE_CI_RUSTC_IF_INCOMPATIBLE=0 ../x.py test bootstrap \
+        -- core::builder::tests::ci_rustc_if_unchanged_logic
+    # Revert the dummy commit
+    git reset --hard HEAD~1
+fi
+
+# Only run the stage 1 tests on merges, not on PR CI jobs.
+if [[ -z "${PR_CI_JOB}" ]]; then
+    ../x.py --stage 1 test
+      --skip compiler
+      --skip src
+
+    # Run the `mir-opt` tests again but this time for a 32-bit target.
+    # This enforces that tests using `// EMIT_MIR_FOR_EACH_BIT_WIDTH` have
+    # both 32-bit and 64-bit outputs updated by the PR author, before
+    # the PR is approved and tested for merging.
+    # It will also detect tests lacking `// EMIT_MIR_FOR_EACH_BIT_WIDTH`,
+    # despite having different output on 32-bit vs 64-bit targets.
+    ../x.py --stage 1 test tests/mir-opt --host='' --target=i686-unknown-linux-gnu
+
+    # Run `ui-fulldeps` in `--stage=1`, which actually uses the stage0
+    # compiler, and is sensitive to the addition of new flags.
+    ../x.py --stage 1 test tests/ui-fulldeps
+
+    # Rebuild the stdlib with the size optimizations enabled and run tests again.
+    RUSTFLAGS_NOT_BOOTSTRAP="--cfg feature=\"optimize_for_size\"" ../x.py --stage 1 test \
+        library/std library/alloc library/core
+fi
+
+../x.py --stage 2 test
+  --skip compiler
+  --skip src
+
+# Run the `mir-opt` tests again but this time for a 32-bit target.
+# This enforces that tests using `// EMIT_MIR_FOR_EACH_BIT_WIDTH` have
+# both 32-bit and 64-bit outputs updated by the PR author, before
+# the PR is approved and tested for merging.
+# It will also detect tests lacking `// EMIT_MIR_FOR_EACH_BIT_WIDTH`,
+# despite having different output on 32-bit vs 64-bit targets.
+../x --stage 2 test tests/mir-opt --host='' --target=i686-unknown-linux-gnu
+
+# Run the UI test suite again, but in `--pass=check` mode
+#
+# This is intended to make sure that both `--pass=check` continues to
+# work.
+../x.ps1 --stage 2 test tests/ui --pass=check --host='' --target=i686-unknown-linux-gnu
diff --git a/src/ci/github-actions/jobs.yml b/src/ci/github-actions/jobs.yml
index 4ce2c6872e7b..aafc13d2dff4 100644
--- a/src/ci/github-actions/jobs.yml
+++ b/src/ci/github-actions/jobs.yml
@@ -117,7 +117,7 @@ pr:
       ENABLE_GCC_CODEGEN: "1"
       # We are adding (temporarily) a dummy commit on the compiler
       READ_ONLY_SRC: "0"
-      DOCKER_SCRIPT: python3 ../x.py --stage 2 test --skip src/tools/tidy
+      DOCKER_SCRIPT: x86_64-gnu-llvm.sh
     <<: *job-linux-16c
   - image: x86_64-gnu-tools
     <<: *job-linux-16c
@@ -319,7 +319,7 @@ auto:
     env:
       RUST_BACKTRACE: 1
       IMAGE: x86_64-gnu-llvm-19
-      <<: *stage_2_test_set1
+      DOCKER_SCRIPT: x86_64-gnu-llvm1.sh
     <<: *job-linux-4c
 
   # Skip tests that run in x86_64-gnu-llvm-19-2
@@ -327,7 +327,7 @@ auto:
     env:
       RUST_BACKTRACE: 1
       IMAGE: x86_64-gnu-llvm-19
-      <<: *stage_2_test_set2
+      DOCKER_SCRIPT: x86_64-gnu-llvm2.sh
     <<: *job-linux-4c
 
   # The x86_64-gnu-llvm-18 job is split into multiple jobs to run tests in parallel.
@@ -337,7 +337,7 @@ auto:
       RUST_BACKTRACE: 1
       READ_ONLY_SRC: "0"
       IMAGE: x86_64-gnu-llvm-18
-      <<: *stage_2_test_set1
+      DOCKER_SCRIPT: x86_64-gnu-llvm1.sh
     <<: *job-linux-4c
 
   # Skip tests that run in x86_64-gnu-llvm-18-2
@@ -346,7 +346,7 @@ auto:
       RUST_BACKTRACE: 1
       READ_ONLY_SRC: "0"
       IMAGE: x86_64-gnu-llvm-18
-      <<: *stage_2_test_set2
+      DOCKER_SCRIPT: x86_64-gnu-llvm2.sh
     <<: *job-linux-4c
 
   - image: x86_64-gnu-nopt