typeshaper

typeshaper lets you derive new struct types from existing ones in a single expression — omit fields, pick fields, merge two structs, make all fields optional, or restore them as required. Every generated type automatically receives conversion impls and can feed into further expressions.

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Have you ever written code like this?

pub struct User {
    pub id: u64,
    pub name: String,
    pub email: String,
    pub password_hash: String,
    pub role: String,
    pub active: bool,
    pub created_at: i64,
}

The API layer needs it, but password_hash must not be exposed, so you duplicate:

pub struct UserPublic {
    pub id: u64,
    pub name: String,
    pub email: String,
    // no password_hash
    pub role: String,
    pub active: bool,
    pub created_at: i64,
}

impl From<User> for UserPublic {
    fn from(u: User) -> Self {
        Self {
            id: u.id,
            name: u.name,
            email: u.email,
            role: u.role,
            active: u.active,
            created_at: u.created_at,
        }
    }
}

The search endpoint only needs id and name, so you duplicate again:

pub struct UserSummary {
    pub id: u64,
    pub name: String,
}
// ... another From ...

The patch endpoint requires all fields to be optional, so you duplicate once more:

pub struct UserPatch {
    pub id: Option<u64>,
    pub name: Option<String>,
    pub email: Option<String>,
    pub password_hash: Option<String>,
    // ...
}
// ... another From ...

Add one field to User and you must update UserPublic, UserPatch, UserSummary — structs, From impls, and every test you might have missed.

And that's just User. You still have Order, Product, Article, Comment…

---

A different approach

[dependencies]
typeshaper = "0.1"

use typeshaper::{TypeshaperExt, typeshaper, typex};

#[typeshaper]
#[derive(Debug, Clone)]
pub struct User {
    pub id: u64,
    pub name: String,
    pub email: String,
    pub password_hash: String,
    pub role: String,
    pub active: bool,
    pub created_at: i64,
}

// Remove two fields
typex!(#[derive(Debug, Clone)] pub UserPublic  = User - [password_hash, created_at]);

// Keep only two fields
typex!(#[derive(Debug, Clone)] pub UserSummary = User & [id, name]);

// Make all fields optional
typex!(#[derive(Debug, Clone)] pub UserPatch   = User?);

Conversions just work:

let user: User = /* from the database */;

let public:  UserPublic  = user.clone().project(); // drops password_hash, created_at
let summary: UserSummary = user.clone().project(); // only id and name
let patch    = UserPatch::from(user);              // all fields become Option

Add a field to User — the three typex!() lines stay unchanged, and the new field propagates automatically.

---

Going further: merge two sources into one

An order snapshot needs both user and address information:

#[typeshaper]
pub struct Address {
    pub street: String,
    pub city: String,
    pub country: String,
}

// Merge User and Address into a new type
typex!(#[derive(Debug, Clone)] pub OrderSnapshot = User + Address);

let snapshot = OrderSnapshot::from((user, address));
// or via .project() on a tuple:
let snapshot = (user, address).project::<OrderSnapshot>();

Keep only the fields that are in User but not in Address:

typex!(#[derive(Debug, Clone)] pub UserOnly = User % Address); // Diff

---

Expressions compose

// Remove password_hash, then make remaining fields optional
typex!(#[derive(Debug)] pub UserSafePatch = User - [password_hash]?);

// Remove password_hash, then pick summary fields
typex!(#[derive(Debug)] pub UserSafeDto = User - [password_hash] & [id, name, email]);

// Parentheses control associativity: Partial then Required (round-trip)
typex!(#[derive(Debug)] pub UserRestored = (User - [password_hash])?!);

---

Full patch round-trip

// Optional version for update endpoints
typex!(#[derive(Debug, Clone)] pub UserPatch    = User?);

// Restore to required after validation
typex!(#[derive(Debug, Clone)] pub UserVerified = UserPatch!);

// ---

let patch = UserPatch {
    name: Some("alice".into()),
    email: Some("new@example.com".into()),
    // other fields left as None — "no update"
    ..Default::default()
};

// Recover the fully-typed version if all fields are present
match UserVerified::try_from(patch) {
    Ok(verified) => { /* commit */ }
    Err(e)       => { /* report which field is missing */ }
}

---

Cross-crate: define models once

Define the domain model in one crate; derive views in another without copying structs:

// core-crate/src/lib.rs
#[typeshaper(export)]          // export generates a companion macro
#[derive(Debug, Clone)]
pub struct User { /* ... */ }
// automatically exports: pub macro typeshaper_import_User!()

// api-crate/src/lib.rs
use core_crate::{User, typeshaper_import_User};

typeshaper_import_User!();  // registers User's field metadata in this crate

// works exactly like a locally annotated type
typex!(#[derive(Debug, Clone)] pub UserPublic = User - [password_hash, created_at]);
typex!(#[derive(Debug, Clone)] pub UserPatch  = User?);

---

Re-export with new attributes

Sometimes you need to annotate a struct you didn't write — and cannot touch.

A typical case is FFI. The napi-rs framework requires #[napi] on every struct it exposes to Node.js. Your domain model lives in core-crate; the FFI crate must not modify it — adding attributes directly to someone else's package crosses crate boundaries and won't compile.

With typeshaper, a bare source expression (T with no operator) rebuilds the struct verbatim so you can attach new attributes:

// napi-crate/src/lib.rs
use core_crate::{User, typeshaper_import_User};

typeshaper_import_User!();

// Exact same fields as User — zero manual duplication
typex!(#[napi] pub UserNapi = User);

// Drop sensitive fields first, then expose via napi
typex!(#[napi] pub UserPublicNapi = User - [password_hash]);

The User → UserNapi conversion impl is generated automatically. The domain struct stays clean; the FFI crate owns the annotation.

The same pattern applies whenever an attribute can't live in the source crate: #[repr(C)] for C FFI, #[pyclass] for PyO3, a custom #[derive] from a third-party crate, and so on.

---

Generic types

When a source struct has type parameters, you must declare them explicitly in typex!() — on the target name and on each generic source node. This is intentional: implicit inheritance would silently produce the wrong struct when multiple type parameters come from different sources.

Basic type parameter

#[typeshaper]
#[derive(Debug, Clone)]
pub struct Wrapper<T> {
    pub inner: T,
    pub label: String,
    pub count: usize,
}

// <T> is declared on both the target name and the source node
typex!(#[derive(Debug, Clone)] pub WrapperNoLabel<T>  = Wrapper<T> - [label]);
typex!(#[derive(Debug, Clone)] pub WrapperPartial<T>  = Wrapper<T>?);
typex!(#[derive(Debug, Clone)] pub WrapperRequired<T> = WrapperPartial<T>!);

let w = Wrapper { inner: 42u32, label: "hi".into(), count: 3 };
let no_label: WrapperNoLabel<u32> = w.project();

Multiple type parameters (Merge)

When merging two generic types, the target declares all parameters; each source node uses its own:

#[typeshaper]
pub struct Person<T> { pub name: T, pub age: u8 }

#[typeshaper]
pub struct Addr<U> { pub city: U, pub zip: String }

// T comes from Person, U comes from Addr — both declared on the target
typex!(#[derive(Debug)] pub PersonWithAddr<T, U> = Person<T> + Addr<U>);

let full = PersonWithAddr::from((person, addr));

Inline trait bounds and where clauses

typex!(pub PrintableValue<T: std::fmt::Display + Clone> = Printable<T> - [note]);

typex!(pub ConstrainedData<T> where T: Clone + PartialEq = Constrained<T> - [meta]);

Lifetime parameters

#[typeshaper]
pub struct Borrowed<'a> { pub name: &'a str, pub value: u32 }

typex!(pub BorrowedName<'a> = Borrowed<'a> & [name]);

Cross-crate generic types

Generic parameter metadata is encoded in the companion macro and fully restored on import:

// core-crate
#[typeshaper(export)]
pub struct GenericModel<T> { pub id: u64, pub payload: T, pub hidden: bool }

// app-crate
typeshaper_import_GenericModel!();

typex!(#[derive(Debug)] pub ModelPublic<T> = GenericModel<T> - [hidden]);
typex!(#[derive(Debug)] pub ModelDraft<T>  = GenericModel<T>?);

Compile-error guard: forgetting type parameters is caught at compile time:

typex!(Bad = Wrapper - [label]);
//          ^^^^^^^ error: type `Wrapper` has generic parameters;
//                  declare them explicitly, e.g. `Target<T> = Wrapper<T>`

---

Reference

Installation

[dependencies]
typeshaper = "0.1"

Source annotation: #[typeshaper]

Add once to a source struct. Field metadata is written to the compile-time registry; the struct itself is left unchanged.

Form	Effect
#[typeshaper]	Use within the same crate
#[typeshaper(export)]	Use within the same crate + generates typeshaper_import_T!() for other crates

#[typeshaper]
#[derive(Debug, Clone, PartialEq)]
pub struct User {
    pub id: u64,
    pub name: String,
    pub age: u8,
    pub email: String,
}

#[typeshaper] stacks on top of any other attributes without affecting existing behavior.

---

Operator reference

Syntax	Name	Meaning	Generated impl
T	Rebuild	Copy all fields unchanged; apply new attributes	TypeshaperInto<Target> for T
T - [f1, f2]	Omit	Remove listed fields	TypeshaperInto<Target> for T
T & [f1, f2]	Pick	Keep only listed fields	TypeshaperInto<Target> for T
A + B	Merge	Combine all fields of A and B (no duplicates)	From<(A, B)> for Target + TypeshaperInto<Target> for (A, B)
T?	Partial	Wrap every field in Option<_>	From<T> for Target
T!	Required	Unwrap Option<_> from a Partial type	TryFrom<T> for Target (or From<T> when the source has no Option fields)
A % B	Diff	Fields present in A but absent in B (matched on both field name and type)	TypeshaperInto<Target> for A

Composition rules

Operators are left-associative; parentheses change precedence:

// User - [age] & [id, name]  means  (User - [age]) & [id, name]
typex!(Dto      = User - [age] & [id, name]);

// Parentheses make the right side evaluate first
typex!(Full     = User + (Badge - [label]));

// Postfix chaining
typex!(Draft    = User - [password_hash]?);
typex!(Roundtrip = (User - [password_hash])?!);

---

typex!() syntax

typex!( [#[attr...]]  [vis]  TargetName[<Params>] [where ...]  =  Expr );

• Attributes (optional): placed before TargetName, forwarded verbatim to the generated struct; multiple attributes can be stacked. typex!() never adds any #[derive] on its own.
• Visibility (optional): pub, pub(crate), pub(super), etc. Defaults to private when omitted — the generated struct is only accessible within the same module. Write explicit pub for types intended to be used outside the declaring module.
• TargetName: the name of the generated struct; also registered in the compile-time table so it can be used as a source in subsequent typex!() calls.
• <Params> (optional): explicit generic or lifetime parameters for the target type — required when any source in Expr is a generic type. Inline bounds (T: Clone + Debug) and separate where clauses are both accepted.
• Expr: a type-algebra expression — see the table above. Each source node that refers to a generic type must carry matching type arguments: Source<T>, Source<'a>, etc.

typex!(
    #[derive(Debug, Clone, PartialEq)]
    #[serde(rename_all = "camelCase")]
    pub UserPublicDto = User & [id, name, email]
);

---

Conversion methods

TypeshaperExt is blanket-implemented for all types; the target is inferred from the binding:

let public: UserPublic = user.project();   // equivalent to user.typeshaper_into()

Merge uses tuple From or .project(), Partial uses From, Required uses TryFrom:

let snapshot = OrderSnapshot::from((user, address));
let snapshot = (user, address).project::<OrderSnapshot>(); // also works
let draft    = UserPatch::from(user);
let verified = UserVerified::try_from(draft)?;

---

Cross-crate usage

Exporting crate

// core-crate/src/lib.rs
use typeshaper::typeshaper;

#[typeshaper(export)]
#[derive(Debug, Clone)]
pub struct User {
    pub id: u64,
    pub name: String,
    pub role: String,
    pub active: bool,
}
// automatically exports: pub macro typeshaper_import_User!()

Importing crate

// app-crate/src/lib.rs
use typeshaper::typex;
use core_crate::{User, typeshaper_import_User};

typeshaper_import_User!();  // call once at module top-level

typex!(#[derive(Debug, Clone)] UserPublic = User - [role, active]);
typex!(#[derive(Debug, Clone)] UserPatch  = User?);

Multiple types each get their own companion macro; cross-crate Merge and Diff are fully supported:

use core_crate::{Address, typeshaper_import_Address};

typeshaper_import_User!();
typeshaper_import_Address!();

typex!(#[derive(Debug, Clone)] OrderSnapshot = User + Address);
typex!(#[derive(Debug, Clone)] UserOnly      = User % Address);

	Same crate	Cross-crate
Source annotation	#[typeshaper]	#[typeshaper(export)]
Caller prerequisite	none	typeshaper_import_T!()
typex!() syntax	identical	identical

---

Supported operations

• Omit — T - [fields]
• Pick — T & [fields]
• Merge — A + B
• Partial — T?
• Required — T!
• Diff — A % B
• Expression composition and chaining
• Attribute forwarding
• Cross-crate export / import
• Generics, lifetimes, and trait bounds — explicit type parameters required

---

License

Licensed under either of Apache License, Version 2.0 or MIT license at your option.