add variant for experimental UB (like Stacked Borrows)

Author: RalfJung

src/librustc/mir/interpret/error.rs

@@ -342,8 +342,10 @@ impl fmt::Debug for InvalidProgramInfo<'tcx> {
 
 #[derive(Clone, RustcEncodable, RustcDecodable, HashStable)]
 pub enum UndefinedBehaviorInfo {
-    /// Handle cases which for which we do not have a fixed variant.
+    /// Free-form case. Only for errors that are never caught!
     Ub(String),
+    /// Free-form case for experimental UB. Only for errors that are never caught!
+    UbExperimental(String),
     /// Unreachable code was executed.
     Unreachable,
 }
@@ -352,7 +354,7 @@ impl fmt::Debug for UndefinedBehaviorInfo {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         use UndefinedBehaviorInfo::*;
         match self {
-            Ub(ref msg) =>
+            Ub(msg) | UbExperimental(msg) =>
                 write!(f, "{}", msg),
             Unreachable =>
                 write!(f, "entered unreachable code"),
@@ -362,7 +364,7 @@ impl fmt::Debug for UndefinedBehaviorInfo {
 
 #[derive(Clone, RustcEncodable, RustcDecodable, HashStable)]
 pub enum UnsupportedOpInfo<'tcx> {
-    /// Handle cases which for which we do not have a fixed variant.
+    /// Free-form case. Only for errors that are never caught!
     Unsupported(String),
 
     // -- Everything below is not classified yet --