Rollup merge of #101038 - RalfJung:interning-alignment, r=oli-obk

no alignment check during interning

This should fix https://github.com/rust-lang/rust/issues/101034
r? `@oli-obk`

Unfortunately we don't have a self-contained testcase for this problem. I am not sure how it can be triggered...

3 files changed, 22 insertions, 3 deletions

diff --git a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
index e13ad1c95bd..4601914c25f 100644
--- a/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
+++ b/compiler/rustc_const_eval/src/const_eval/eval_queries.rs
@@ -74,7 +74,9 @@ fn eval_body_using_ecx<'mir, 'tcx>(
             None => InternKind::Constant,
         }
     };
+    ecx.machine.check_alignment = false; // interning doesn't need to respect alignment
     intern_const_alloc_recursive(ecx, intern_kind, &ret)?;
+    // we leave alignment checks off, since this `ecx` will not be used for further evaluation anyway
 
     debug!("eval_body_using_ecx done: {:?}", *ret);
     Ok(ret)
diff --git a/compiler/rustc_const_eval/src/const_eval/machine.rs b/compiler/rustc_const_eval/src/const_eval/machine.rs
index 2a460c74b3d..6c1e61fccca 100644
--- a/compiler/rustc_const_eval/src/const_eval/machine.rs
+++ b/compiler/rustc_const_eval/src/const_eval/machine.rs
@@ -89,10 +89,10 @@ pub struct CompileTimeInterpreter<'mir, 'tcx> {
     /// exhaustion error.
     ///
     /// Setting this to `0` disables the limit and allows the interpreter to run forever.
-    pub steps_remaining: usize,
+    pub(super) steps_remaining: usize,
 
     /// The virtual call stack.
-    pub(crate) stack: Vec<Frame<'mir, 'tcx, AllocId, ()>>,
+    pub(super) stack: Vec<Frame<'mir, 'tcx, AllocId, ()>>,
 
     /// We need to make sure consts never point to anything mutable, even recursively. That is
     /// relied on for pattern matching on consts with references.
@@ -103,7 +103,7 @@ pub struct CompileTimeInterpreter<'mir, 'tcx> {
     pub(super) can_access_statics: bool,
 
     /// Whether to check alignment during evaluation.
-    check_alignment: bool,
+    pub(super) check_alignment: bool,
 }
 
 impl<'mir, 'tcx> CompileTimeInterpreter<'mir, 'tcx> {
diff --git a/src/test/ui/consts/extra-const-ub/issue-101034.rs b/src/test/ui/consts/extra-const-ub/issue-101034.rs
new file mode 100644
index 00000000000..e0de705c488
--- /dev/null
+++ b/src/test/ui/consts/extra-const-ub/issue-101034.rs
@@ -0,0 +1,17 @@
+// check-pass
+// compile-flags: -Zextra-const-ub-checks
+
+#[repr(packed)]
+pub struct Foo {
+    bar: u8,
+    baa: [u32; 1],
+}
+
+const FOOMP: Foo = Foo {
+    bar: 0,
+    baa: [69; 1],
+};
+
+fn main() {
+    let _val = FOOMP;
+}