Implement Inline Size Assertion Annotations

Implem the procedural macro `inline_assert_size_eq` to check the size of
a type at compile time. Additionally, add the test file
`inline_assert_size_eq_failed.rs` to ensure that the macro tirggers a
compile-time error when the size is not as expected
Fixes google#1329

tests/ui-nightly/inline_assert_size_eq_failed.rs

@@ -0,0 +1,9 @@
+extern crate zerocopy_derive;
+use zerocopy_derive::inline_assert_size_eq;
+
+#[inline_assert_size_eq(1)]
+struct TestSizeNeqStruct {
+    i: i32,
+}
+
+fn main() {}

tests/ui-nightly/inline_assert_size_eq_failed.stderr

@@ -0,0 +1,7 @@
+error[E0080]: evaluation of constant value failed
+ --> tests/ui-nightly/inline_assert_size_eq_failed.rs:4:1
+  |
+4 | #[inline_assert_size_eq(1)]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `0_usize - 1_usize`, which would overflow
+  |
+  = note: this error originates in the macro `static_assertions::const_assert` which comes from the expansion of the attribute macro `inline_assert_size_eq` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/ui-stable/inline_assert_size_eq_failed.rs

@@ -0,0 +1 @@
+../ui-nightly/inline_assert_size_eq_failed.rs

tests/ui-stable/inline_assert_size_eq_failed.stderr

@@ -0,0 +1,7 @@
+error[E0080]: evaluation of constant value failed
+ --> tests/ui-stable/inline_assert_size_eq_failed.rs:4:1
+  |
+4 | #[inline_assert_size_eq(1)]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `0_usize - 1_usize`, which would overflow
+  |
+  = note: this error originates in the macro `static_assertions::const_assert` which comes from the expansion of the attribute macro `inline_assert_size_eq` (in Nightly builds, run with -Z macro-backtrace for more info)

zerocopy-derive/src/lib.rs

@@ -55,6 +55,45 @@ macro_rules! try_or_print {
     };
 }
 
+/// Inline size assertion annotations
+/// Any non-generic type can be check if the size is as expected or not in compile time
+/// # Implementation
+/// ```
+/// # use zerocopy_derive::inline_assert_size_eq;
+/// # use static_assertions;
+/// #[inline_assert_size_eq(4)]
+/// struct MyStruct {
+///     val: i32
+/// }
+/// ```
+/// # Example that will fail
+/// ```compile_fail
+/// # use zero_derive::inline_assert_size_eq;
+/// # use static_assertions;
+/// #[inline_assert_size_eq(1)]
+/// struct MyStruct {
+///     val: i32
+/// }
+/// ```
+#[proc_macro_attribute]
+pub fn inline_assert_size_eq(
+    args: proc_macro::TokenStream,
+    input: proc_macro::TokenStream,
+) -> proc_macro::TokenStream {
+    let ast: Result<DeriveInput, _> = syn::parse(input.clone());
+    let expected_size: Result<usize, _> = args.to_string().trim().parse();
+    let mut ret: proc_macro::TokenStream = input;
+    if let (Ok(ast), Ok(size)) = (ast, expected_size) {
+        let name = &ast.ident;
+        ret = quote! {
+            #ast
+            static_assertions::const_assert!(core::mem::size_of::<#name>() == #size);
+        }
+        .into()
+    }
+    ret
+}
+
 // TODO(https://github.com/rust-lang/rust/issues/54140): Some errors could be
 // made better if we could add multiple lines of error output like this:
 //