Test with -Zmiri-provenance-gc=1 on Linux

src/ci/docker/host-x86_64/x86_64-gnu-tools/checktools.sh

@@ -25,7 +25,18 @@ cat /tmp/toolstate/toolstates.json
 python3 "$X_PY" test --stage 2 check-tools
 python3 "$X_PY" test --stage 2 src/tools/clippy
 python3 "$X_PY" test --stage 2 src/tools/rustfmt
-python3 "$X_PY" test --stage 2 src/tools/miri
+
+# Testing Miri is a bit more complicated.
+# We set the GC interval to the shortest possible value (0 would be off) to increase the chance
+# that bugs which only surface when the GC runs at a specific time are more likely to cause CI to fail.
+# This significantly increases the runtime of our test suite, or we'd do this in PR CI too.
+set +u
+if [[ -z "${PR_CI_JOB}" ]]; then
+    MIRIFLAGS=-Zmiri-provenance-gc=1 python3 "$X_PY" test --stage 2 src/tools/miri
+else
+    python3 "$X_PY" test --stage 2 src/tools/miri
+fi
+set -u
 # We natively run this script on x86_64-unknown-linux-gnu and x86_64-pc-windows-msvc.
 # Also cover some other targets via cross-testing, in particular all tier 1 targets.
 export BOOTSTRAP_SKIP_TARGET_SANITY=1 # we don't need `cc` for these targets

src/tools/miri/tests/pass/0weak_memory_consistency.rs

@@ -1,4 +1,6 @@
-//@compile-flags: -Zmiri-ignore-leaks -Zmiri-disable-stacked-borrows
+//@compile-flags: -Zmiri-ignore-leaks -Zmiri-disable-stacked-borrows -Zmiri-provenance-gc=10000
+// This test's runtime explodes if the GC interval is set to 1 (which we do in CI), so we
+// override it internally back to the default frequency.
 
 // The following tests check whether our weak memory emulation produces
 // any inconsistent execution outcomes