Add field magic support

deku-derive/src/lib.rs

@@ -502,6 +502,9 @@ struct FieldData {
 
     /// seek from start position
     seek_from_start: Option<TokenStream>,
+
+    /// magic value that needs to appear before field
+    magic: Option<syn::LitByteStr>,
 }
 
 impl FieldData {
@@ -547,6 +550,7 @@ impl FieldData {
             seek_from_current: receiver.seek_from_current?,
             seek_from_end: receiver.seek_from_end?,
             seek_from_start: receiver.seek_from_start?,
+            magic: receiver.magic,
         };
 
         FieldData::validate(&data)?;
@@ -980,6 +984,10 @@ struct DekuFieldReceiver {
     /// seek from start position
     #[darling(default = "default_res_opt", map = "map_litstr_as_tokenstream")]
     seek_from_start: Result<Option<TokenStream>, ReplacementError>,
+
+    /// magic value that needs to appear before field
+    #[darling(default)]
+    magic: Option<syn::LitByteStr>,
 }
 
 /// Receiver for the variant-level attributes inside a enum

deku-derive/src/macros/deku_read.rs

@@ -4,6 +4,7 @@ use darling::ast::{Data, Fields};
 use darling::ToTokens;
 use proc_macro2::TokenStream;
 use quote::quote;
+use syn::{Ident, LitByteStr};
 
 use crate::macros::{
     gen_ctx_types_and_arg, gen_field_args, gen_internal_field_ident, gen_internal_field_idents,
@@ -495,20 +496,24 @@ fn emit_enum(input: &DekuData) -> Result<TokenStream, syn::Error> {
 fn emit_magic_read(input: &DekuData) -> TokenStream {
     let crate_ = super::get_crate_name();
     if let Some(magic) = &input.magic {
-        quote! {
-            let __deku_magic = #magic;
+        emit_magic_read_lit(&crate_, magic)
+    } else {
+        quote! {}
+    }
+}
 
-            for __deku_byte in __deku_magic {
-                let __deku_read_byte = u8::from_reader_with_ctx(__deku_reader, ())?;
-                if *__deku_byte != __deku_read_byte {
-                    extern crate alloc;
-                    use alloc::borrow::Cow;
-                    return Err(::#crate_::DekuError::Parse(Cow::from(format!("Missing magic value {:?}", #magic))));
-                }
+fn emit_magic_read_lit(crate_: &Ident, magic: &LitByteStr) -> TokenStream {
+    quote! {
+        let __deku_magic = #magic;
+
+        for __deku_byte in __deku_magic {
+            let __deku_read_byte = u8::from_reader_with_ctx(__deku_reader, ())?;
+            if *__deku_byte != __deku_read_byte {
+                extern crate alloc;
+                use alloc::borrow::Cow;
+                return Err(::#crate_::DekuError::Parse(Cow::from(format!("Missing magic value {:?}", #magic))));
             }
         }
-    } else {
-        quote! {}
     }
 }
 
@@ -738,6 +743,12 @@ fn emit_field_read(
         quote! {}
     };
 
+    let magic_read = if let Some(magic) = &f.magic {
+        emit_magic_read_lit(&crate_, magic)
+    } else {
+        quote! {}
+    };
+
     let field_read_func = if field_reader.is_some() {
         quote! { #field_reader? }
     } else {
@@ -947,6 +958,7 @@ fn emit_field_read(
 
     let field_read = quote! {
         #seek
+        #magic_read
         #pad_bits_before
 
         #bit_offset

deku-derive/src/macros/deku_write.rs

@@ -528,6 +528,13 @@ fn emit_field_write(
 ) -> Result<TokenStream, syn::Error> {
     let crate_ = super::get_crate_name();
     let field_endian = f.endian.as_ref().or(input.endian.as_ref());
+    let magic_write = if let Some(magic) = &f.magic {
+        quote! {
+            ::#crate_::DekuWriter::to_writer(#magic, __deku_writer, ())?;
+        }
+    } else {
+        quote! {}
+    };
 
     let seek = if let Some(num) = &f.seek_from_current {
         quote! {
@@ -705,6 +712,7 @@ fn emit_field_write(
 
     let field_write = quote! {
         #seek
+        #magic_write
         #pad_bits_before
 
         #bit_offset

src/attributes.rs

@@ -33,7 +33,7 @@ enum DekuEnum {
 | Attribute | Scope | Description
 |-----------|------------------|------------
 | [endian](#endian) | top-level, field | Set the endianness
-| [magic](#magic) | top-level | A magic value that must be present at the start of this struct/enum
+| [magic](#magic) | top-level, field | A magic value that must be present at the start of this struct/enum/field
 | [seek_from_current](#seek_from_current) | top-level, field | Sets the offset of reader and writer to the current position plus the specified number of bytes
 | [seek_from_end](#seek_from_end) | top-level, field | Sets the offset to the size of reader and writer plus the specified number of bytes
 | [seek_from_start](#seek_from_start) | top-level, field | Sets the offset of reader and writer to provided number of bytes
@@ -151,10 +151,10 @@ assert_eq!(&*data, value);
 # magic
 
 Sets a "magic" value that must be present in the data at the start of
-a struct/enum when reading, and that is written out of the start of
+a struct/enum or field when reading, and that is written out of the start of
 that type's data when writing.
 
-Example:
+Example (top-level):
 ```rust
 # use deku::prelude::*;
 # use std::convert::{TryInto, TryFrom};
@@ -177,6 +177,29 @@ let value: Vec<u8> = value.try_into().unwrap();
 assert_eq!(data, value);
 ```
 
+Example (field):
+```rust
+# use deku::prelude::*;
+# use std::convert::{TryInto, TryFrom};
+# #[derive(Debug, PartialEq, DekuRead, DekuWrite)]
+struct DekuTest {
+    #[deku(magic = b"deku")]
+    data: u8
+}
+
+let data: &[u8] = &[b'd', b'e', b'k', b'u', 50];
+
+let value = DekuTest::try_from(data).unwrap();
+
+assert_eq!(
+    DekuTest { data: 50 },
+    value
+);
+
+let value: Vec<u8> = value.try_into().unwrap();
+assert_eq!(data, value);
+```
+
 # seek_from_current
 
 Using the internal reader, seek to current position plus offset before reading field.

tests/test_magic.rs

@@ -72,3 +72,22 @@ fn test_magic_enum(input: &[u8]) {
     let ret_write: Vec<u8> = ret_read.try_into().unwrap();
     assert_eq!(ret_write, input)
 }
+
+#[rstest(input,
+    case(&hex!("64656b7500")),
+)]
+fn test_struct_magic_field(input: &[u8]) {
+    #[derive(PartialEq, Debug, DekuRead, DekuWrite)]
+    struct TestStruct {
+        #[deku(magic = b"deku")]
+        magic: u8,
+    }
+    let input = input.to_vec();
+
+    let ret_read = TestStruct::try_from(input.as_slice()).unwrap();
+
+    assert_eq!(TestStruct { magic: 0 }, ret_read);
+
+    let ret_write: Vec<u8> = ret_read.try_into().unwrap();
+    assert_eq!(ret_write, input)
+}