Add more things to the FIXMEs, per PR feedback

Author: scottmcm

Diff for: compiler/rustc_mir_transform/src/validate.rs

@@ -1135,6 +1135,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                     // FIXME: Add Checks for these
                     CastKind::PointerWithExposedProvenance | CastKind::PointerExposeProvenance => {}
                     CastKind::PointerCoercion(PointerCoercion::ReifyFnPointer) => {
+                        // FIXME: check signature compatibility.
                         check_kinds!(
                             op_ty,
                             "CastKind::{kind:?} input must be a fn item, not {:?}",
@@ -1147,7 +1148,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                         );
                     }
                     CastKind::PointerCoercion(PointerCoercion::UnsafeFnPointer) => {
-                        // FIXME: check safety
+                        // FIXME: check safety and signature compatibility.
                         check_kinds!(
                             op_ty,
                             "CastKind::{kind:?} input must be a fn pointer, not {:?}",
@@ -1160,7 +1161,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                         );
                     }
                     CastKind::PointerCoercion(PointerCoercion::ClosureFnPointer(..)) => {
-                        // FIXME: check safety & captures
+                        // FIXME: check safety, captures, and signature compatibility.
                         check_kinds!(
                             op_ty,
                             "CastKind::{kind:?} input must be a closure, not {:?}",
@@ -1199,7 +1200,8 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                         );
                     }
                     CastKind::PointerCoercion(PointerCoercion::Unsize) => {
-                        // FIXME: Add Checks for this
+                        // This is used for all `CoerceUnsized` types,
+                        // not just pointers/references, so is hard to check.
                     }
                     CastKind::IntToInt | CastKind::IntToFloat => {
                         let input_valid = op_ty.is_integral() || op_ty.is_char() || op_ty.is_bool();