Safe Transmute: Fix ICE (due to UnevaluatedConst)

This patch updates the code that looks at the `Assume` type when evaluating if
transmutation is possible. An ICE was being triggered in the case that the
`Assume` parameter contained an unevaluated const (in this test case, due to a
function with missing parameter names).

Fixes #110892

1 files changed, 12 insertions, 1 deletions

diff --git a/compiler/rustc_transmute/src/lib.rs b/compiler/rustc_transmute/src/lib.rs
index 34ad6bd8c69..05ad4a4a12a 100644
--- a/compiler/rustc_transmute/src/lib.rs
+++ b/compiler/rustc_transmute/src/lib.rs
@@ -78,6 +78,7 @@ mod rustc {
     use rustc_middle::ty::ParamEnv;
     use rustc_middle::ty::Ty;
     use rustc_middle::ty::TyCtxt;
+    use rustc_middle::ty::ValTree;
 
     /// The source and destination types of a transmutation.
     #[derive(TypeVisitable, Debug, Clone, Copy)]
@@ -148,7 +149,17 @@ mod rustc {
             );
 
             let variant = adt_def.non_enum_variant();
-            let fields = c.to_valtree().unwrap_branch();
+            let fields = match c.try_to_valtree() {
+                Some(ValTree::Branch(branch)) => branch,
+                _ => {
+                    return Some(Self {
+                        alignment: true,
+                        lifetimes: true,
+                        safety: true,
+                        validity: true,
+                    });
+                }
+            };
 
             let get_field = |name| {
                 let (field_idx, _) = variant