Safe Transmute: Fix propagation of errors

- Make sure that the most specific Reason is the one that bubbles up when we
  are folding over the `Answer` tree. `Reason::DstIsBitIncompatible` is the
  least specific, so that should be used only when there isn't anything else
  available.
- Small fixes where we used the wrong Reason variant.
- Tiny cleanups

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -2751,7 +2751,8 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, trait_ref.substs.const_at(3)) else {
                 span_bug!(span, "Unable to construct rustc_transmute::Assume where it was previously possible");
             };
-        // FIXME(bryangarza): Need to flatten here too
+        // FIXME(bryangarza): Is this enough, or should we resolve all nested
+        // obligations like we do for `confirm_transmutability_candidate(...)?`
         match rustc_transmute::TransmuteTypeEnv::new(self.infcx).is_transmutable(
             obligation.cause,
             src_and_dst,
@@ -2780,7 +2781,6 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     rustc_transmute::Reason::DstIsPrivate => format!(
                         "`{dst}` is or contains a type or field that is not visible in that scope"
                     ),
-                    // FIXME(bryangarza): Include the number of bytes of src and dst
                     rustc_transmute::Reason::DstIsTooBig => {
                         format!("The size of `{src}` is smaller than the size of `{dst}`")
                     }

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -329,7 +329,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         )
                     };
 
-                    // // FIXME(bryangarza): Check src.mutability or dst.mutability to know whether dst -> src obligation is needed
+                    // FIXME(bryangarza): Check src.mutability or dst.mutability to know whether dst -> src obligation is needed
                     Ok(vec![make_obl(src.ty, dst.ty), make_obl(dst.ty, src.ty)])
                 }
             }