From e1c60a6de83f3ea2adff5bd2b4f436537502bfaf Mon Sep 17 00:00:00 2001
From: "google-labs-jules[bot]"
 <161369871+google-labs-jules[bot]@users.noreply.github.com>
Date: Mon, 9 Feb 2026 21:09:38 +0000
Subject: [PATCH] Run Miri tests in parallel in CI

Refactors the Miri test execution in `.github/workflows/ci.yml` to run
Stacked Borrows and Tree Borrows tests in parallel.

This is achieved by:
1.  Launching both `cargo miri nextest` commands as background jobs.
2.  Assigning a unique `CARGO_TARGET_DIR` to each job (`target/by-toolchain/$TOOLCHAIN/miri-stacked` and `...-tree`) to prevent build directory lock contention.
3.  Redirecting stdout/stderr to log files to avoid interleaved output.
4.  Waiting for both jobs to complete, printing their logs, and propagating exit codes.

This change significantly reduces the wall-clock time for the Miri CI step.

Co-authored-by: joshlf <1046063+joshlf@users.noreply.github.com>
---
 .github/workflows/ci.yml | 56 +++++++++++++++++++++++++++++++++++++---
 1 file changed, 52 insertions(+), 4 deletions(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3ed8893ea6..e851f2f1f8 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -468,16 +468,64 @@ jobs:
 
         # Run under both the stacked borrows model (default) and under the tree 
         # borrows model to ensure we're compliant with both.
-        for EXTRA_FLAGS in "" "-Zmiri-tree-borrows"; do
-          MIRIFLAGS="$MIRIFLAGS $EXTRA_FLAGS" ./cargo.sh +$TOOLCHAIN \
+        #
+        # We run these in the background to allow them to run in parallel.
+        #
+        # We need to manually set `CARGO_TARGET_DIR` for each job to avoid
+        # conflicts since they will be running at the same time.
+        BASE_TARGET_DIR="target/by-toolchain/${TOOLCHAIN}/miri"
+
+        (
+          MIRIFLAGS="$MIRIFLAGS" \
+          CARGO_TARGET_DIR="${BASE_TARGET_DIR}-stacked" \
+          ./cargo.sh +$TOOLCHAIN \
             miri nextest run \
             --test-threads "$THREADS" \
             --package $CRATE \
             --target $TARGET \
-            $FEATURES
-        done
+            $FEATURES \
+          > miri-stacked.log 2>&1
+        ) &
+        PID_STACKED=$!
+
+        (
+          MIRIFLAGS="$MIRIFLAGS -Zmiri-tree-borrows" \
+          CARGO_TARGET_DIR="${BASE_TARGET_DIR}-tree" \
+          ./cargo.sh +$TOOLCHAIN \
+            miri nextest run \
+            --test-threads "$THREADS" \
+            --package $CRATE \
+            --target $TARGET \
+            $FEATURES \
+          > miri-tree.log 2>&1
+        ) &
+        PID_TREE=$!
+
+        # Wait for both jobs to finish.
+        # We turn off `set -e` so that `wait` doesn't cause the script to exit
+        # if one of the jobs fails.
+        set +e
+        wait $PID_STACKED
+        CODE_STACKED=$?
+        wait $PID_TREE
+        CODE_TREE=$?
+        set -e
+
+        echo "Output from Stacked Borrows Miri tests:"
+        cat miri-stacked.log
+        echo "Output from Tree Borrows Miri tests:"
+        cat miri-tree.log
 
         mv .cargo/config.toml.bak .cargo/config.toml
+
+        if [ $CODE_STACKED -ne 0 ]; then
+          echo "Stacked Borrows Miri tests failed with exit code $CODE_STACKED"
+          exit $CODE_STACKED
+        fi
+        if [ $CODE_TREE -ne 0 ]; then
+          echo "Tree Borrows Miri tests failed with exit code $CODE_TREE"
+          exit $CODE_TREE
+        fi
       # Only nightly has a working Miri, so we skip installing on all other
       # toolchains.
       #