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 +1,10 @@
+#![allow(unused)]
+extern crate zerocopy_derive;
+extern crate static_assertions;
+use zerocopy_derive::*;
+use static_assertions;
+
+#[zerocopy_derive::inline_assert_size_eq(1)]
+struct test_size_neq_struct {
+    i: i8,
+}

tests/ui-nightly/inline_assert_size_eq_failed.stderr

@@ -0,0 +1,24 @@
+error[E0254]: the name `static_assertions` is defined multiple times
+ --> tests/ui-stable/inline_assert_size_eq_failed.rs:5:5
+  |
+3 | extern crate static_assertions;
+  | ------------------------------- previous import of the extern crate `static_assertions` here
+4 | use zerocopy_derive::*;
+5 | use static_assertions;
+  |     ^^^^^^^^^^^^^^^^^ `static_assertions` reimported here
+  |
+  = note: `static_assertions` must be defined only once in the type namespace of this module
+
+warning: type `test_size_neq_struct` should have an upper camel case name
+ --> tests/ui-stable/inline_assert_size_eq_failed.rs:8:8
+  |
+8 | struct test_size_neq_struct {
+  |        ^^^^^^^^^^^^^^^^^^^^ help: convert the identifier to upper camel case: `TestSizeNeqStruct`
+  |
+  = note: `#[warn(non_camel_case_types)]` on by default
+
+error[E0601]: `main` function not found in crate `$CRATE`
+  --> tests/ui-stable/inline_assert_size_eq_failed.rs:10:2
+   |
+10 | }
+   |  ^ consider adding a `main` function to `$DIR/tests/ui-stable/inline_assert_size_eq_failed.rs`

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 +1,24 @@
+error[E0254]: the name `static_assertions` is defined multiple times
+ --> tests/ui-stable/inline_assert_size_eq_failed.rs:5:5
+  |
+3 | extern crate static_assertions;
+  | ------------------------------- previous import of the extern crate `static_assertions` here
+4 | use zerocopy_derive::*;
+5 | use static_assertions;
+  |     ^^^^^^^^^^^^^^^^^ `static_assertions` reimported here
+  |
+  = note: `static_assertions` must be defined only once in the type namespace of this module
+
+warning: type `test_size_neq_struct` should have an upper camel case name
+ --> tests/ui-stable/inline_assert_size_eq_failed.rs:8:8
+  |
+8 | struct test_size_neq_struct {
+  |        ^^^^^^^^^^^^^^^^^^^^ help: convert the identifier to upper camel case: `TestSizeNeqStruct`
+  |
+  = note: `#[warn(non_camel_case_types)]` on by default
+
+error[E0601]: `main` function not found in crate `$CRATE`
+  --> tests/ui-stable/inline_assert_size_eq_failed.rs:10:2
+   |
+10 | }
+   |  ^ consider adding a `main` function to `$DIR/tests/ui-stable/inline_assert_size_eq_failed.rs`

zerocopy-derive/src/lib.rs

@@ -55,6 +55,43 @@ 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();
+    if let (Ok(ast), Ok(size)) = (ast, expected_size) {
+        let name  =&ast.ident;
+        quote! {
+            #ast
+            static_assertions::const_assert!(core::mem::size_of::<#name>() == #size);
+        }.into()
+    }
+    else {
+        input
+    }
+
+}
+
 // 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:
 //