Add compile-time error for seek attributes on DekuSize fields

Author: wcampbell0x2a

deku-derive/src/macros/deku_size.rs

@@ -133,6 +133,18 @@ fn emit_struct(input: &DekuData) -> Result<TokenStream, syn::Error> {
         fields,
     } = DekuDataStruct::try_from(input)?;
 
+    for field in fields.iter().copied() {
+        if field_has_seek_attributes(field) {
+            let field_name = field.ident.as_ref()
+                .map(|i| i.to_string())
+                .unwrap_or_else(|| "unnamed field".to_string());
+            return Err(syn::Error::new(
+                field.ty.span(),
+                format!("DekuSize cannot be derived for types with seek attributes on field '{}'. Seek operations make size unpredictable.", field_name),
+            ));
+        }
+    }
+
     let size_calculation = calculate_fields_size(fields.iter().copied(), &crate_);
 
     let (imp_generics, ty_generics, where_clause) = input.generics.split_for_impl();
@@ -171,6 +183,20 @@ fn emit_enum(input: &DekuData) -> Result<TokenStream, syn::Error> {
         id_args: _,
     } = DekuDataEnum::try_from(input)?;
 
+    for variant in variants.iter() {
+        for field in variant.fields.iter() {
+            if field_has_seek_attributes(field) {
+                let field_name = field.ident.as_ref()
+                    .map(|i| i.to_string())
+                    .unwrap_or_else(|| "unnamed field".to_string());
+                return Err(syn::Error::new(
+                    field.ty.span(),
+                    format!("DekuSize cannot be derived for types with seek attributes on field '{}'. Seek operations make size unpredictable.", field_name),
+                ));
+            }
+        }
+    }
+
     let discriminant_size = calculate_discriminant_size(input, id, id_type, &crate_);
 
     let variant_sizes = variants

tests/test_compile/cases/deku_size_seek_field.rs

@@ -0,0 +1,12 @@
+use deku::prelude::*;
+
+#[derive(DekuRead, DekuWrite, DekuSize)]
+struct SeekFieldStruct {
+    offset: u8,
+    #[deku(seek_from_current = "*offset")]
+    field: u8,
+}
+
+fn main() {
+    let _ = SeekFieldStruct::SIZE_BITS;
+}

tests/test_compile/cases/deku_size_seek_field.stderr

@@ -0,0 +1,18 @@
+error: DekuSize cannot be derived for types with seek attributes on field 'field'. Seek operations make size unpredictable.
+ --> tests/test_compile/cases/deku_size_seek_field.rs:7:12
+  |
+7 |     field: u8,
+  |            ^^
+
+error[E0599]: no associated item named `SIZE_BITS` found for struct `SeekFieldStruct` in the current scope
+  --> tests/test_compile/cases/deku_size_seek_field.rs:11:30
+   |
+ 4 | struct SeekFieldStruct {
+   | ---------------------- associated item `SIZE_BITS` not found for this struct
+...
+11 |     let _ = SeekFieldStruct::SIZE_BITS;
+   |                              ^^^^^^^^^ associated item not found in `SeekFieldStruct`
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+   = note: the following trait defines an item `SIZE_BITS`, perhaps you need to implement it:
+           candidate #1: `deku::DekuSize`

tests/test_compile/cases/deku_size_seek_rewind.rs

@@ -0,0 +1,11 @@
+use deku::prelude::*;
+
+#[derive(DekuRead, DekuWrite, DekuSize)]
+struct SeekRewindStruct {
+    #[deku(seek_rewind)]
+    field: u8,
+}
+
+fn main() {
+    let _ = SeekRewindStruct::SIZE_BITS;
+}

tests/test_compile/cases/deku_size_seek_rewind.stderr

@@ -0,0 +1,18 @@
+error: DekuSize cannot be derived for types with seek attributes on field 'field'. Seek operations make size unpredictable.
+ --> tests/test_compile/cases/deku_size_seek_rewind.rs:6:12
+  |
+6 |     field: u8,
+  |            ^^
+
+error[E0599]: no associated item named `SIZE_BITS` found for struct `SeekRewindStruct` in the current scope
+  --> tests/test_compile/cases/deku_size_seek_rewind.rs:10:31
+   |
+ 4 | struct SeekRewindStruct {
+   | ----------------------- associated item `SIZE_BITS` not found for this struct
+...
+10 |     let _ = SeekRewindStruct::SIZE_BITS;
+   |                               ^^^^^^^^^ associated item not found in `SeekRewindStruct`
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+   = note: the following trait defines an item `SIZE_BITS`, perhaps you need to implement it:
+           candidate #1: `deku::DekuSize`