Safe Transmute: Fix ICE (due to UnevaluatedConst)

compiler/rustc_middle/src/ty/consts.rs

@@ -294,6 +294,14 @@ impl<'tcx> Const<'tcx> {
         Self::from_bits(tcx, n as u128, ParamEnv::empty().and(tcx.types.usize))
     }
 
+    /// Attempts to convert to a `ValTree`
+    pub fn try_to_valtree(self) -> Option<ty::ValTree<'tcx>> {
+        match self.kind() {
+            ty::ConstKind::Value(valtree) => Some(valtree),
+            _ => None,
+        }
+    }
+
     #[inline]
     /// Attempts to evaluate the given constant to bits. Can fail to evaluate in the presence of
     /// generics (or erroneous code) or if the value can't be represented as bits (e.g. because it

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

tests/ui/transmutability/issue-110892.rs

@@ -0,0 +1,40 @@
+// check-fail
+#![feature(generic_const_exprs, transmutability)]
+#![allow(incomplete_features)]
+
+mod assert {
+    use std::mem::{Assume, BikeshedIntrinsicFrom};
+
+    pub fn is_transmutable<
+        Src,
+        Dst,
+        Context,
+        const ASSUME_ALIGNMENT: bool,
+        const ASSUME_LIFETIMES: bool,
+        const ASSUME_SAFETY: bool,
+        const ASSUME_VALIDITY: bool,
+    >()
+    where
+        Dst: BikeshedIntrinsicFrom<
+            Src,
+            Context,
+            { from_options(ASSUME_ALIGNMENT, ASSUME_LIFETIMES, ASSUME_SAFETY, ASSUME_VALIDITY) }
+        >,
+    {}
+
+    // This should not cause an ICE
+    const fn from_options(
+        , //~ ERROR expected parameter name, found `,`
+        , //~ ERROR expected parameter name, found `,`
+        , //~ ERROR expected parameter name, found `,`
+        , //~ ERROR expected parameter name, found `,`
+    ) -> Assume {} //~ ERROR mismatched types
+}
+
+fn main() {
+    struct Context;
+    #[repr(C)] struct Src;
+    #[repr(C)] struct Dst;
+
+    assert::is_transmutable::<Src, Dst, Context, false, false, { true }, false>();
+}

tests/ui/transmutability/issue-110892.stderr

@@ -0,0 +1,36 @@
+error: expected parameter name, found `,`
+  --> $DIR/issue-110892.rs:27:9
+   |
+LL |         ,
+   |         ^ expected parameter name
+
+error: expected parameter name, found `,`
+  --> $DIR/issue-110892.rs:28:9
+   |
+LL |         ,
+   |         ^ expected parameter name
+
+error: expected parameter name, found `,`
+  --> $DIR/issue-110892.rs:29:9
+   |
+LL |         ,
+   |         ^ expected parameter name
+
+error: expected parameter name, found `,`
+  --> $DIR/issue-110892.rs:30:9
+   |
+LL |         ,
+   |         ^ expected parameter name
+
+error[E0308]: mismatched types
+  --> $DIR/issue-110892.rs:31:10
+   |
+LL |     const fn from_options(
+   |              ------------ implicitly returns `()` as its body has no tail or `return` expression
+...
+LL |     ) -> Assume {}
+   |          ^^^^^^ expected `Assume`, found `()`
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0308`.