Add support for data-carrying enums

The primary feature of this change is adding support for deriving
zerocopy's traits on data-carrying enums. Along with that, it has a few
other features:

- Adds support for enums with repr(C, int)
- Improves zero discriminant detection for enums with negative
  discriminants
- For development, adds a VSCode config so we can prevent rust-analyzer
  from choking on our tests with enums that have thousands of variants

.vscode/settings.json

@@ -0,0 +1,5 @@
+{
+    "rust-analyzer.check.extraEnv": {
+        "RUSTFLAGS": "--cfg rust_analyzer",
+    }
+}

src/macro_util.rs

@@ -274,7 +274,7 @@ macro_rules! align_of {
 #[doc(hidden)] // `#[macro_export]` bypasses this module's `#[doc(hidden)]`.
 #[macro_export]
 macro_rules! struct_has_padding {
-    ($t:ty, $($ts:ty),*) => {
+    ($t:ty, [$($ts:ty),*]) => {
         ::zerocopy::macro_util::core_reexport::mem::size_of::<$t>() > 0 $(+ ::zerocopy::macro_util::core_reexport::mem::size_of::<$ts>())*
     };
 }
@@ -294,11 +294,19 @@ macro_rules! struct_has_padding {
 #[doc(hidden)] // `#[macro_export]` bypasses this module's `#[doc(hidden)]`.
 #[macro_export]
 macro_rules! union_has_padding {
-    ($t:ty, $($ts:ty),*) => {
+    ($t:ty, [$($ts:ty),*]) => {
         false $(|| ::zerocopy::macro_util::core_reexport::mem::size_of::<$t>() != ::zerocopy::macro_util::core_reexport::mem::size_of::<$ts>())*
     };
 }
 
+#[doc(hidden)]
+#[macro_export]
+macro_rules! enum_has_padding {
+    ($t:ty, $disc:ty, $([$($ts:ty),*]),*) => {
+        false $(|| ::zerocopy::macro_util::core_reexport::mem::size_of::<$t>() != (::zerocopy::macro_util::core_reexport::mem::size_of::<$disc>() $(+ ::zerocopy::macro_util::core_reexport::mem::size_of::<$ts>())*))*
+    }
+}
+
 /// Does `t` have alignment greater than or equal to `u`?  If not, this macro
 /// produces a compile error. It must be invoked in a dead codepath. This is
 /// used in `transmute_ref!` and `transmute_mut!`.
@@ -728,7 +736,7 @@ mod tests {
             (#[$cfg:meta] ($($ts:ty),*) => $expect:expr) => {{
                 #[$cfg]
                 struct Test($(#[allow(dead_code)] $ts),*);
-                assert_eq!(struct_has_padding!(Test, $($ts),*), $expect);
+                assert_eq!(struct_has_padding!(Test, [$($ts),*]), $expect);
             }};
             (#[$cfg:meta] $(#[$cfgs:meta])* ($($ts:ty),*) => $expect:expr) => {
                 test!(#[$cfg] ($($ts),*) => $expect);
@@ -759,7 +767,7 @@ mod tests {
                 #[$cfg]
                 #[allow(unused)] // fields are never read
                 union Test{ $($fs: $ts),* }
-                assert_eq!(union_has_padding!(Test, $($ts),*), $expect);
+                assert_eq!(union_has_padding!(Test, [$($ts),*]), $expect);
             }};
             (#[$cfg:meta] $(#[$cfgs:meta])* {$($fs:ident: $ts:ty),*} => $expect:expr) => {
                 test!(#[$cfg] {$($fs: $ts),*} => $expect);