Do not optimize out SwitchInt before borrowck, or if Zmir-preserve-ub

3 files changed, 30 insertions, 1 deletions

diff --git a/src/tools/miri/src/lib.rs b/src/tools/miri/src/lib.rs
index 03f76cfa652..6f227e04d00 100644
--- a/src/tools/miri/src/lib.rs
+++ b/src/tools/miri/src/lib.rs
@@ -169,7 +169,7 @@ pub const MIRI_DEFAULT_ARGS: &[&str] = &[
     "-Zalways-encode-mir",
     "-Zextra-const-ub-checks",
     "-Zmir-emit-retag",
-    "-Zmir-keep-place-mention",
+    "-Zmir-preserve-ub",
     "-Zmir-opt-level=0",
     "-Zmir-enable-passes=-CheckAlignment,-CheckNull",
     // Deduplicating diagnostics means we miss events when tracking what happens during an
diff --git a/src/tools/miri/tests/fail/read_from_trivial_switch.rs b/src/tools/miri/tests/fail/read_from_trivial_switch.rs
new file mode 100644
index 00000000000..d34b1cd5820
--- /dev/null
+++ b/src/tools/miri/tests/fail/read_from_trivial_switch.rs
@@ -0,0 +1,14 @@
+// Ensure that we don't optimize out `SwitchInt` reads even if that terminator
+// branches to the same basic block on every target, since the operand may have
+// side-effects that affect analysis of the MIR.
+//
+// See <https://github.com/rust-lang/miri/issues/4237>.
+
+use std::mem::MaybeUninit;
+
+fn main() {
+    let uninit: MaybeUninit<i32> = MaybeUninit::uninit();
+    let bad_ref: &i32 = unsafe { uninit.assume_init_ref() };
+    let &(0 | _) = bad_ref;
+    //~^ ERROR: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
+}
diff --git a/src/tools/miri/tests/fail/read_from_trivial_switch.stderr b/src/tools/miri/tests/fail/read_from_trivial_switch.stderr
new file mode 100644
index 00000000000..6b3d4539b96
--- /dev/null
+++ b/src/tools/miri/tests/fail/read_from_trivial_switch.stderr
@@ -0,0 +1,15 @@
+error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
+  --> tests/fail/read_from_trivial_switch.rs:LL:CC
+   |
+LL |     let &(0 | _) = bad_ref;
+   |         ^^^^^^^^ using uninitialized data, but this operation requires initialized memory
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `main` at tests/fail/read_from_trivial_switch.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+