explain ty == None
diff --git a/src/librustc_mir/interpret/intern.rs b/src/librustc_mir/interpret/intern.rs
index 606f506..0031dbc4 100644
--- a/src/librustc_mir/interpret/intern.rs
+++ b/src/librustc_mir/interpret/intern.rs
@@ -57,6 +57,8 @@
/// `mode` is the mode of the environment where we found this pointer.
/// `mutablity` is the mutability of the place to be interned; even if that says
/// `immutable` things might become mutable if `ty` is not frozen.
+/// `ty` can be `None` if there is no potential interior mutability
+/// to account for (e.g. for vtables).
fn intern_shallow<'rt, 'mir, 'tcx>(
ecx: &'rt mut CompileTimeEvalContext<'mir, 'tcx>,
leftover_allocations: &'rt mut FxHashSet<AllocId>,
@@ -97,6 +99,7 @@
// read-only memory, and also by Miri when evluating other constants/statics that
// access this one.
if mode == InternMode::Static {
+ // When `ty` is `None`, we assume no interior mutability.
let frozen = ty.map_or(true, |ty| ty.is_freeze(
ecx.tcx.tcx,
ecx.param_env,
