Move some const asserts to runtime asserts (bytecodealliance#1894)

Helps to address [builds with `-Zrandomize-layout`][comment]

[comment]: bytecodealliance#1750 (comment)

Author: alexcrichton

crates/wasmparser/src/validator/component_types.rs

@@ -476,9 +476,10 @@ pub struct ComponentDefinedTypeId {
     alias_id: u32,
 }
 
-const _: () = {
+#[test]
+fn assert_defined_type_small() {
     assert!(core::mem::size_of::<ComponentDefinedTypeId>() <= 8);
-};
+}
 
 impl TypeIdentifier for ComponentDefinedTypeId {
     type Data = ComponentDefinedType;

crates/wasmparser/src/validator/operators.rs

@@ -252,9 +252,10 @@ enum MaybeType<T = ValType> {
 // The validator is pretty performance-sensitive and `MaybeType` is the main
 // unit of storage, so assert that it doesn't exceed 4 bytes which is the
 // current expected size.
-const _: () = {
+#[test]
+fn assert_maybe_type_small() {
     assert!(core::mem::size_of::<MaybeType>() == 4);
-};
+}
 
 impl core::fmt::Display for MaybeType {
     fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {

crates/wasmparser/src/validator/types.rs

@@ -89,6 +89,7 @@ macro_rules! define_type_id {
             }
         }
 
+
         // The size of type IDs was seen to have a large-ish impact in #844, so
         // this assert ensures that it stays relatively small.
         const _: () = {
@@ -106,9 +107,10 @@ pub struct CoreTypeId {
     index: u32,
 }
 
-const _: () = {
+#[test]
+fn assert_core_type_id_small() {
     assert!(core::mem::size_of::<CoreTypeId>() <= 4);
-};
+}
 
 impl TypeIdentifier for CoreTypeId {
     type Data = SubType;

crates/wast/src/core/expr.rs

@@ -1208,11 +1208,12 @@ instructions! {
 // since big `*.wat` files will have a lot of these. This is a small ratchet to
 // make sure that this enum doesn't become larger than it already is, although
 // ideally it also wouldn't be as large as it is now.
-const _: () = {
+#[test]
+fn assert_instruction_not_too_large() {
     let size = std::mem::size_of::<Instruction<'_>>();
     let pointer = std::mem::size_of::<u64>();
     assert!(size <= pointer * 11);
-};
+}
 
 impl<'a> Instruction<'a> {
     pub(crate) fn needs_data_count(&self) -> bool {