Add Hash derive

.gitignore

@@ -16,3 +16,6 @@ gh-pages
 # git files
 .swp
 /tags
+
+# RustRover/Other jetbrains IDE files
+.idea/

CHANGELOG.md

@@ -5,6 +5,14 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## Unreleased
+
+### Added
+ - Add `Hash` derive similar to `std`'s one, but considering generics correctly,
+   and supporting custom hash functions per field or skipping fields.
+   ([#532](https://github.com/JelteF/derive_more/pull/532))
+
+
 ## 2.1.1 - 2025-12-22
 
 ### Fixed

Cargo.toml

@@ -65,6 +65,7 @@ eq = ["derive_more-impl/eq"]
 error = ["derive_more-impl/error"]
 from = ["derive_more-impl/from"]
 from_str = ["derive_more-impl/from_str"]
+hash = ["derive_more-impl/hash"]
 index = ["derive_more-impl/index"]
 index_mut = ["derive_more-impl/index_mut"]
 into = ["derive_more-impl/into"]
@@ -93,6 +94,7 @@ full = [
     "error",
     "from",
     "from_str",
+    "hash",
     "index",
     "index_mut",
     "into",
@@ -180,6 +182,16 @@ name = "from_str"
 path = "tests/from_str.rs"
 required-features = ["from_str"]
 
+[[test]]
+name = "hash"
+path = "tests/hash.rs"
+required-features = ["hash"]
+
+[[test]]
+name = "hash_and_eq"
+path = "tests/hash_and_eq.rs"
+required-features = ["hash", "eq"]
+
 [[test]]
 name = "index_mut"
 path = "tests/index_mut.rs"

README.md

@@ -125,6 +125,8 @@ These are traits that can be used for operator overloading.
     `ShrAssign` and `ShlAssign`
 11. [`Eq`], [`PartialEq`]
 
+### Other traits
+1. [`Hash`]
 
 ### Static methods
 
@@ -264,6 +266,8 @@ Changing [MSRV] (minimum supported Rust version) of this crate is treated as a *
 [`Eq`]: https://docs.rs/derive_more/latest/derive_more/derive.Eq.html
 [`PartialEq`]: https://docs.rs/derive_more/latest/derive_more/derive.PartialEq.html
 
+[`Hash`]: https://docs.rs/derive_more/latest/derive_more/derive.Hash.html
+
 [`Constructor`]: https://docs.rs/derive_more/latest/derive_more/derive.Constructor.html
 [`IsVariant`]: https://docs.rs/derive_more/latest/derive_more/derive.IsVariant.html
 [`Unwrap`]: https://docs.rs/derive_more/latest/derive_more/derive.Unwrap.html

examples/deny_missing_docs.rs

@@ -4,7 +4,7 @@
 #![allow(dead_code)] // for illustration purposes
 
 use derive_more::{
-    Add, AddAssign, Constructor, Deref, DerefMut, Display, From, FromStr, Index,
+    Add, AddAssign, Constructor, Deref, DerefMut, Display, From, FromStr, Hash, Index,
     IndexMut, Into, IsVariant, Mul, MulAssign, Not, TryInto,
 };
 
@@ -18,6 +18,7 @@ fn main() {}
     Display,
     From,
     FromStr,
+    Hash,
     Into,
     Mul,
     MulAssign,

impl/Cargo.toml

@@ -61,6 +61,7 @@ eq = ["syn/extra-traits", "syn/visit"]
 error = ["syn/extra-traits"]
 from = ["syn/extra-traits"]
 from_str = ["syn/full", "syn/visit", "dep:convert_case"]
+hash = ["syn/extra-traits", "syn/visit"]
 index = []
 index_mut = []
 into = ["syn/extra-traits", "syn/visit-mut"]
@@ -88,6 +89,7 @@ full = [
     "error",
     "from",
     "from_str",
+    "hash",
     "index",
     "index_mut",
     "into",

impl/doc/hash.md

@@ -0,0 +1,266 @@
+# Using `#[derive(Hash)]`
+
+Deriving `Hash` works by hashing values according to their type structure.
+
+## Structural hashing
+
+Deriving `Hash` for enums/structs works in a similar way to the one in `std`,
+by hashing all the available fields, but, in contrast:
+1. Does not overconstrain generic parameters.
+2. Allows to ignore fields, whole structs or enum variants via `#[hash(skip)]` attribute.
+3. Allows to use a custom hash function for a field via `#[hash(with(function))]` attribute.
+
+### Structs
+
+For structs all the available fields are hashed.
+
+```rust
+# use std::marker::PhantomData;
+# use derive_more::Hash;
+#
+trait Trait {
+    type Assoc;
+}
+impl<T: ?Sized> Trait for T {
+    type Assoc = u8;
+}
+
+#[derive(Debug, Hash)]
+struct Foo<A, B, C: Trait + ?Sized> {
+    a: A,
+    b: PhantomData<B>,
+    c: C::Assoc,
+}
+
+#[derive(Debug)]
+struct NoHash;
+```
+
+This generates code equivalent to:
+
+```rust
+# use std::marker::PhantomData;
+# use derive_more::core::hash::{Hash, Hasher};
+#
+# trait Trait {
+#     type Assoc;
+# }
+# impl<T: ?Sized> Trait for T {
+#     type Assoc = u8;
+# }
+#
+# struct Foo<A, B, C: Trait + ?Sized> {
+#     a: A,
+#     b: PhantomData<B>,
+#     c: C::Assoc,
+# }
+#
+impl<A, B, C: Trait + ?Sized> Hash for Foo<A, B, C>
+where
+    A: Hash,
+    PhantomData<B>: Hash, // `B: Hash` is generated by `std` instead
+    C::Assoc: Hash,       // `C: Hash` is generated by `std` instead
+{
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        match self {
+            Self { a: self_0, b: self_1, c: self_2 } => {
+                self_0.hash(state);
+                self_1.hash(state);
+                self_2.hash(state);
+            }
+        }
+    }
+}
+```
+
+### Enums
+
+For enums the discriminant is hashed first, followed by the fields.
+
+```rust
+# use std::marker::PhantomData;
+# use derive_more::Hash;
+#
+# trait Trait {
+#     type Assoc;
+# }
+# impl<T: ?Sized> Trait for T {
+#     type Assoc = u8;
+# }
+#
+#[derive(Debug, Hash)]
+enum Foo<A, B, C: Trait + ?Sized> {
+    A(A),
+    B { b: PhantomData<B> },
+    C(C::Assoc),
+}
+#
+# #[derive(Debug)]
+# struct NoHash;
+```
+
+This generates code equivalent to:
+
+```rust
+# use std::marker::PhantomData;
+# use derive_more::core::hash::{Hash, Hasher};
+#
+# trait Trait {
+#     type Assoc;
+# }
+# impl<T: ?Sized> Trait for T {
+#     type Assoc = u8;
+# }
+#
+# enum Foo<A, B, C: Trait + ?Sized> {
+#     A(A),
+#     B { b: PhantomData<B> },
+#     C(C::Assoc),
+# }
+#
+impl<A, B, C: Trait + ?Sized> Hash for Foo<A, B, C>
+where
+    A: Hash,
+    PhantomData<B>: Hash, // `B: Hash` is generated by `std` instead
+    C::Assoc: Hash,       // `C: Hash` is generated by `std` instead
+{
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        std::mem::discriminant(self).hash(state);
+        match self {
+            Self::A(self_0) => { self_0.hash(state); }
+            Self::B { b: self_0 } => { self_0.hash(state); }
+            Self::C(self_0) => { self_0.hash(state); }
+        }
+    }
+}
+```
+
+### Ignoring
+
+The `#[hash(skip)]` attribute can be used to ignore fields, a whole struct or enum variants in the expansion.
+
+Note that if you also implement the `Eq` or `PartialEq` traits, fields marked with `#[eq(skip)]` or `#[partial_eq(skip)]` 
+will be ignored during hashing. This is done so that this property holds:
+
+```txt
+k1 == k2 -> hash(k1) == hash(k2)
+```
+That is [expected](https://doc.rust-lang.org/std/hash/trait.Hash.html#hash-and-eq) from `Hash` implementations.
+
+```rust
+# use derive_more::Hash;
+#
+#[derive(Debug)]
+struct NoHash; // doesn't implement `Hash`
+
+#[derive(Debug, Hash)]
+struct Foo {
+    num: i32,
+    #[hash(skip)] // or #[hash(ignore)]
+    ignored: f32,
+}
+
+#[derive(Debug, Hash)]
+// Makes all fields of this struct being ignored.
+#[hash(skip)] // or #[hash(ignore)]
+struct Bar(f32, NoHash);
+
+#[derive(Debug, Hash)]
+enum Enum {
+    Foo(i32, #[hash(skip)] NoHash),
+    #[hash(skip)]
+    Bar(NoHash),
+    Baz,
+}
+```
+
+This generates code equivalent to:
+
+```rust
+# use derive_more::core::hash::{Hash, Hasher};
+# struct NoHash;
+#
+# struct Foo { num: i32, ignored: f32 }
+#
+impl Hash for Foo {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        match self {
+            Self { num: self_0, .. } => { self_0.hash(state); }
+        }
+    }
+}
+
+# struct Bar(i32, NoHash);
+#
+impl Hash for Bar {
+    fn hash<H: Hasher>(&self, _state: &mut H) {}
+}
+
+# enum Enum {
+#     Foo(i32, NoHash),
+#     Bar(NoHash),
+#     Baz,
+# }
+#
+impl Hash for Enum {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        std::mem::discriminant(self).hash(state);
+        match self {
+            Self::Foo(self_0, _) => { self_0.hash(state); }
+            Self::Bar(_) => {}
+            Self::Baz => {}
+        }
+    }
+}
+```
+
+### Custom hash function
+
+The `#[hash(with(function))]` attribute can be used to specify a custom hash function for a field.
+The function must have the signature `fn<H: Hasher>(value: &FieldType, state: &mut H)`.
+
+```rust
+# use derive_more::Hash;
+mod my_hash {
+    use core::hash::Hasher;
+
+    pub fn hash_abs<H: Hasher>(value: &i32, state: &mut H) {
+        state.write_i32(value.abs());
+    }
+}
+
+#[derive(Hash)]
+struct Foo {
+    #[hash(with(my_hash::hash_abs))]
+    value: i32,
+}
+```
+
+This generates code equivalent to:
+
+```rust
+# use derive_more::core::hash::{Hash, Hasher};
+# mod my_hash {
+#     use derive_more::core::hash::Hasher;
+#
+#     pub fn hash_abs<H: Hasher>(value: &i32, state: &mut H) {
+#         state.write_i32(value.abs());
+#     }
+# }
+#
+# struct Foo {
+#     value: i32,
+# }
+#
+impl Hash for Foo {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        match self {
+            Self { value: self_0 } => {
+                my_hash::hash_abs(self_0, state);
+            }
+        }
+    }
+}
+```
+
+This is useful for types that don't implement `Hash` but can be hashed in a custom way, or when you need different hashing behavior than the default.