limitations

Author: wanguardd

module/core/former/limitations.md

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+# Former Macro: Architectural Limitations Analysis
+
+This document provides a systematic analysis of the 4 fundamental limitations preventing certain tests from being enabled in the Former crate. Each limitation is **experimentally verified** and characterized using the Target Type Classification framework from the specification.
+
+## Target Type Classification Context
+
+According to the Former specification, the macro operates on two fundamental **Target Type Categories**:
+- **Structs** - Regular Rust structs with named fields
+- **Enums** - Rust enums with variants, subdivided by **Variant Structure Types** (Unit, Tuple, Named)
+
+Each limitation affects these target types differently, as detailed in the analysis below.
+
+## 1. Generic Enum Parsing Limitation ✅ TESTED
+
+### Limitation Characteristics
+- **Scope**: Enum Target Type Category only (Structs unaffected)
+- **Severity**: Complete blocking - no generic enums supported
+- **Behavioral Categories Affected**: All enum formers (Unit/Tuple/Named Variant Formers)
+- **Variant Structure Types Affected**: All (Unit, Tuple, Named variants)
+- **Root Cause**: Macro parser architecture limitation
+- **Workaround Availability**: Full (concrete type replacement)
+- **Future Compatibility**: Possible (requires major rewrite)
+
+**What it means**: The macro cannot parse generic parameter syntax in enum declarations.
+
+### ❌ This Breaks:
+```rust
+#[derive(Former)]
+pub enum GenericEnum<T> {  // <-- The <T> part breaks the macro
+    Variant(T),
+}
+```
+**Verified Error**: `expected '::' found '>'` - macro parser fails on generic syntax
+
+### ✅ This Works:
+```rust
+#[derive(Former)]
+pub enum ConcreteEnum {  // <-- No <T>, so it works
+    Variant(String),
+}
+// Usage: ConcreteEnum::variant()._0("hello".to_string()).form()
+```
+
+**The Technical Choice**: Simple token-based parser vs Full AST parser with generics
+
+**Trade-off Details**:
+- **Current approach**: Fast compilation, simple implementation
+- **Alternative approach**: Slow compilation, complex parser supporting generics
+- **Implementation cost**: Complete macro rewrite with full Rust AST parsing
+- **Performance impact**: Significant compilation time increase
+
+**Can Both Be Combined?** 🟡 **PARTIALLY**
+- Technically possible but requires rewriting the entire macro parser
+- Would need full Rust AST parsing instead of simple token matching
+- Trade-off: Fast builds vs Generic enum support
+
+---
+
+## 2. Lifetime Constraint Limitation ✅ VERIFIED IN CODE
+
+### Limitation Characteristics
+- **Scope**: Both Target Type Categories (Structs and Enums)
+- **Severity**: Fundamental blocking - no lifetime parameters supported
+- **Behavioral Categories Affected**: All Former types with lifetime parameters
+- **Variant Structure Types Affected**: N/A (applies to type-level generics)
+- **Root Cause**: Rust language constraint (trait objects + lifetimes)
+- **Workaround Availability**: Partial (owned data only)
+- **Future Compatibility**: Impossible (fundamental Rust limitation)
+
+**What it means**: Rust's memory safety rules fundamentally prevent borrowed data in Former storage due to trait object lifetime requirements.
+
+### ❌ This Breaks:
+```rust
+// From parametrized_dyn_manual.rs:210 - real example
+impl<'callback> StoragePreform for StylesFormerStorage<'callback> {
+    fn preform(self) -> Self::Preformed {
+        // ERROR E0521: borrowed data escapes outside of method
+        (&PhantomData::<&'callback dyn FilterCol>).maybe_default()
+        // `'callback` must outlive `'static`
+    }
+}
+```
+
+### ✅ This Works:
+```rust
+#[derive(Former)]
+pub struct OwnedStruct {
+    owned_data: String,         // <-- Owned data is fine
+    numbers: Vec<i32>,          // <-- Owned collections work
+    static_ref: &'static str    // <-- Static references work
+}
+```
+
+**The Technical Choice**: Trait object compatibility with memory safety vs Complex lifetime support
+
+**Trade-off Details**:
+- **Current approach**: Memory safety + trait objects work reliably
+- **Alternative approach**: Complex lifetime tracking in all generated code
+- **Fundamental constraint**: Trait objects require `'static` bounds for type erasure
+- **Rust limitation**: Cannot allow borrowed data to escape method boundaries
+
+**Can Both Be Combined?** 🔴 **NO**
+- This is a hard Rust language constraint, not our design choice
+- Trait objects fundamentally require `'static` bounds
+- Even perfect implementation cannot overcome Rust's type system rules
+
+---
+
+## 3. Trait Conflict Limitation ✅ TESTED
+
+### Limitation Characteristics
+- **Scope**: Enum Target Type Category only (multi-variant enums)
+- **Severity**: Selective blocking - single-variant enums work fine
+- **Behavioral Categories Affected**: Mixed enum scenarios (Complex Scenario Formers)
+- **Variant Structure Types Affected**: All when combined in single enum
+- **Root Cause**: Duplicate trait implementation generation
+- **Workaround Availability**: Full (single variant per enum)
+- **Future Compatibility**: Possible (requires complex deduplication logic)
+
+**What it means**: The macro generates conflicting trait implementations when multiple enum variants require the same traits.
+
+### ❌ This Breaks:
+```rust
+#[derive(Former)]
+pub enum MultiVariantEnum {
+    VariantA { field: String },  // <-- Each variant tries to
+    VariantB { other: i32 },     // <-- generate the same traits
+    VariantC,                    // <-- causing conflicts
+}
+```
+**Verified Error E0119**: `conflicting implementations of trait EntityToStorage`
+
+### ✅ This Works:
+```rust
+#[derive(Former)]
+pub enum SingleVariantEnum {
+    OnlyVariant { field: String },  // <-- One variant = no conflicts
+}
+// Usage: SingleVariantEnum::only_variant().field("test".to_string()).form()
+```
+
+**The Technical Choice**: Simple per-enum trait generation vs Complex trait deduplication
+
+**Trade-off Details**:
+- **Current approach**: Simple code generation, one trait impl per enum
+- **Alternative approach**: Sophisticated trait deduplication with variant-specific logic
+- **Implementation complexity**: Exponential increase in generated code complexity
+- **Maintenance burden**: Much harder to debug and maintain complex generation
+
+**Can Both Be Combined?** 🟡 **YES, BUT VERY COMPLEX**
+- Technically possible with sophisticated trait merging logic
+- Requires tracking implementations across all variants
+- Major increase in macro complexity and maintenance burden
+- Cost/benefit analysis favors current simple approach
+
+---
+
+## Comprehensive Limitations Matrix
+
+| Limitation | Target Type Scope | Severity Level | Behavioral Categories | Future Fix | Workaround | Decision Impact |
+|------------|------------------|----------------|----------------------|-----------|------------|----------------|
+| **Generic Parsing** | Enums only | Complete blocking | All enum formers | 🟡 Possible (major rewrite) | ✅ Concrete types | High - affects API design |
+| **Lifetime Constraints** | Structs + Enums | Fundamental blocking | All with lifetimes | 🔴 Impossible (Rust constraint) | 🟡 Owned data only | Critical - shapes data patterns |
+| **Trait Conflicts** | Multi-variant enums | Selective blocking | Complex scenarios | 🟡 Possible (complex logic) | ✅ Single variants | Medium - affects enum design |
+
+### Key Decision-Making Insights
+
+**Architectural Impact Ranking**:
+1. **Lifetime Constraints** - Most critical, shapes fundamental data patterns
+2. **Generic Parsing** - High impact on API flexibility and user experience
+3. **Trait Conflicts** - Medium impact, affects complex enum design strategies
+4. **Compile-fail Tests** - Low impact, testing methodology only
+
+**Workaround Effectiveness**:
+- ✅ **Full workarounds available**: Generic Parsing, Trait Conflicts
+- 🟡 **Partial workarounds**: Lifetime Constraints (owned data patterns)
+- ❌ **No workarounds needed**: Compile-fail Tests (working as intended)
+
+**Engineering Trade-offs**:
+- **Generic Parsing**: Simple parser vs Universal enum support
+- **Lifetime Constraints**: Memory safety vs Flexible borrowing patterns
+- **Trait Conflicts**: Simple generation vs Complex multi-variant enums
+- **Compile-fail Tests**: Error validation vs Maximum passing test count