Fix the overflow issue for transmute_generic_consts

compiler/rustc_hir_typeck/src/intrinsicck.rs

@@ -82,8 +82,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // Try to display a sensible error with as much information as possible.
         let skeleton_string = |ty: Ty<'tcx>, sk| match sk {
-            Ok(SizeSkeleton::Known(size)) => format!("{} bits", size.bits()),
             Ok(SizeSkeleton::Pointer { tail, .. }) => format!("pointer to `{tail}`"),
+            Ok(SizeSkeleton::Known(size)) => {
+                if let Some(v) = u128::from(size.bytes()).checked_mul(8) {
+                    format!("{} bits", v)
+                } else {
+                    // `u128` should definitely be able to hold the size of different architectures
+                    // larger sizes should be reported as error `are too big for the current architecture`
+                    // otherwise we have a bug somewhere
+                    bug!("{:?} overflow for u128", size)
+                }
+            }
             Ok(SizeSkeleton::Generic(size)) => {
                 if let Some(size) = size.try_eval_target_usize(tcx, self.param_env) {
                     format!("{size} bytes")

tests/ui/const-generics/issue-112505-overflow.rs

@@ -0,0 +1,7 @@
+#![feature(transmute_generic_consts)]
+
+fn overflow(v: [[[u32; 8888888]; 9999999]; 777777777]) -> [[[u32; 9999999]; 777777777]; 239] {
+    unsafe { std::mem::transmute(v) } //~ ERROR cannot transmute between types of different sizes
+}
+
+fn main() { }

tests/ui/const-generics/issue-112505-overflow.stderr

@@ -0,0 +1,12 @@
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> $DIR/issue-112505-overflow.rs:4:14
+   |
+LL |     unsafe { std::mem::transmute(v) }
+   |              ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `[[[u32; 8888888]; 9999999]; 777777777]` (values of the type `[[[u32; 8888888]; 9999999]; 777777777]` are too big for the current architecture)
+   = note: target type: `[[[u32; 9999999]; 777777777]; 239]` (59484438436515561504 bits)
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0512`.