Disallow seek attributes for DekuSize struct/enums and fields (#638)

* Add helper functions for detecting seek attributes

* Add compile-time error for seek attributes on DekuSize structs/enums

* Add compile-time error for seek attributes on DekuSize fields

* Remove unused import

Author: wcampbell0x2a

deku-derive/src/macros/deku_size.rs

@@ -3,6 +3,7 @@ use std::convert::TryFrom;
 use darling::ast::Data;
 use proc_macro2::TokenStream;
 use quote::quote;
+use syn::spanned::Spanned;
 
 use crate::{DekuData, DekuDataEnum, DekuDataStruct, FieldData};
 
@@ -45,6 +46,22 @@ fn calculate_fields_size<'a>(
     quote! { 0 #(+ #field_sizes)* }
 }
 
+/// Check if struct/enum has seek attributes
+fn has_seek_attributes(input: &DekuData) -> bool {
+    input.seek_rewind
+        || input.seek_from_current.is_some()
+        || input.seek_from_end.is_some()
+        || input.seek_from_start.is_some()
+}
+
+/// Check if field has seek attributes
+fn field_has_seek_attributes(field: &FieldData) -> bool {
+    field.seek_rewind
+        || field.seek_from_current.is_some()
+        || field.seek_from_end.is_some()
+        || field.seek_from_start.is_some()
+}
+
 /// Add DekuSize trait bounds to where clause for fields that need them
 fn add_field_bounds<'a>(
     where_clause: &mut Option<syn::WhereClause>,
@@ -102,13 +119,34 @@ fn calculate_discriminant_size(
 fn emit_struct(input: &DekuData) -> Result<TokenStream, syn::Error> {
     let crate_ = super::get_crate_name();
 
+    if has_seek_attributes(input) {
+        return Err(syn::Error::new(
+            input.ident.span(),
+            "DekuSize cannot be derived for types with seek attributes (seek_rewind, seek_from_current, seek_from_end, seek_from_start). Seek operations make size unpredictable.",
+        ));
+    }
+
     let DekuDataStruct {
         imp: _,
         wher: _,
         ident: _,
         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();
@@ -130,6 +168,13 @@ fn emit_struct(input: &DekuData) -> Result<TokenStream, syn::Error> {
 fn emit_enum(input: &DekuData) -> Result<TokenStream, syn::Error> {
     let crate_ = super::get_crate_name();
 
+    if has_seek_attributes(input) {
+        return Err(syn::Error::new(
+            input.ident.span(),
+            "DekuSize cannot be derived for types with seek attributes (seek_rewind, seek_from_current, seek_from_end, seek_from_start). Seek operations make size unpredictable.",
+        ));
+    }
+
     let DekuDataEnum {
         imp: _,
         wher: _,
@@ -140,6 +185,22 @@ 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

src/reader.rs

@@ -660,8 +660,6 @@ impl<R: Read + Seek> Reader<R> {
 #[cfg(test)]
 mod tests {
     use super::*;
-    #[cfg(all(feature = "alloc", feature = "bits"))]
-    use alloc::vec;
     use hexlit::hex;
     use no_std_io::io::Cursor;
 

tests/test_compile/cases/deku_size_seek_enum.rs

@@ -0,0 +1,12 @@
+use deku::prelude::*;
+
+#[derive(DekuRead, DekuWrite, DekuSize)]
+#[deku(id_type = "u8", seek_from_start = "5")]
+enum SeekEnum {
+    #[deku(id = "1")]
+    A,
+}
+
+fn main() {
+    let _ = SeekEnum::SIZE_BITS;
+}

tests/test_compile/cases/deku_size_seek_enum.stderr

@@ -0,0 +1,18 @@
+error: DekuSize cannot be derived for types with seek attributes (seek_rewind, seek_from_current, seek_from_end, seek_from_start). Seek operations make size unpredictable.
+ --> tests/test_compile/cases/deku_size_seek_enum.rs:5:6
+  |
+5 | enum SeekEnum {
+  |      ^^^^^^^^
+
+error[E0599]: no variant or associated item named `SIZE_BITS` found for enum `SeekEnum` in the current scope
+  --> tests/test_compile/cases/deku_size_seek_enum.rs:11:23
+   |
+ 5 | enum SeekEnum {
+   | ------------- variant or associated item `SIZE_BITS` not found for this enum
+...
+11 |     let _ = SeekEnum::SIZE_BITS;
+   |                       ^^^^^^^^^ variant or associated item not found in `SeekEnum`
+   |
+   = 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_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_struct.rs

@@ -0,0 +1,11 @@
+use deku::prelude::*;
+
+#[derive(DekuRead, DekuWrite, DekuSize)]
+#[deku(seek_from_start = "10")]
+struct SeekStruct {
+    field: u8,
+}
+
+fn main() {
+    let _ = SeekStruct::SIZE_BITS;
+}

tests/test_compile/cases/deku_size_seek_struct.stderr

@@ -0,0 +1,18 @@
+error: DekuSize cannot be derived for types with seek attributes (seek_rewind, seek_from_current, seek_from_end, seek_from_start). Seek operations make size unpredictable.
+ --> tests/test_compile/cases/deku_size_seek_struct.rs:5:8
+  |
+5 | struct SeekStruct {
+  |        ^^^^^^^^^^
+
+error[E0599]: no associated item named `SIZE_BITS` found for struct `SeekStruct` in the current scope
+  --> tests/test_compile/cases/deku_size_seek_struct.rs:10:25
+   |
+ 5 | struct SeekStruct {
+   | ----------------- associated item `SIZE_BITS` not found for this struct
+...
+10 |     let _ = SeekStruct::SIZE_BITS;
+   |                         ^^^^^^^^^ associated item not found in `SeekStruct`
+   |
+   = 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`